Article

Streamflow responses to the clearing of alien invasive trees from riparian zones at three sites in the Western Cape Province

July 1999
The Southern African Forestry Journal 185(185):1-7

July 1999
185(185):1-7

DOI:10.1080/10295925.1999.9631220

Authors:

University of British Columbia - Okanagan

The changes in streamflow following the removal of invasive wattle (Acacia mearnsii and A. longifolia) from riparian zones during the dry summer months in three small catchments in the Western Cape Province are described. Portable steel box weirs, with a 30° compound V-notch and equipped with Belfort water level recorders, were used to measure streamflow. Each of the three streams had a control catchment against which it was correlated during a pre-treatment period. The riparian zones of the treatment catchments were cleared after this period of calibration, and the response of stream-flow after treatment was recorded. In the three catchments there was a marked increase in streamflow after clearing of the riparian invasive vegetation. The streamflow increases in the three catchments were 8,8, 10,4 and 12 m³/day per ha cleared. The responses measured in these experiments are the result of changing from tall vegetation to minimal cover and represent a maximum response. Streamflow is expected to decrease again as vegetation regrows, but not to the levels that characterised the invaded site. The riparian areas should be kept under short indigenous vegetation, such as grass or fynbos, to sustain the long-term increases in streamflow.

Riparian vegetation and water yield: A synthesis

Article

Aug 2012
J HYDROL

Forested riparian zones perform numerous ecosystem functions, including the following: storing and fixing carbon; serving as wildlife habitats and ecological corridors; stabilizing streambanks; providing shade, organic matter, and food for streams and their biota; retaining sediments and filtering chemicals applied on cultivated/agricultural sites on upslope regions of the catchments. In this paper, we report a synthesis of a different feature of this type of vegetation, which is its effect on water yield. By synthesizing results from studies that used (i) the nested catchment and (ii) the paired catchment approaches, we show that riparian forests decrease water yield on a daily to annual basis. In terms of the treated area increases on average were 1.32 +/- 0.85 mm day(-1) and 483 +/- 309 mm yr(-1), respectively; n = 9. Similarly, riparian forest plantation or regeneration promoted reduced water yield (on average 1.25 +/- 0.34 mm day(-1) and 456 +/- 125 mm yr(-1) on daily and annual basis, respectively, when prorated to the catchment area subjected to treatment; n = 5). Although there are substantially fewer paired catchment studies assessing the effect of this vegetation type compared to classical paired catchment studies that manipulate the entire vegetation of small catchments, our results indicate the same trend. Despite the occurrence of many current restoration programs, measurements of the effect on water yield under natural forest restoration conditions are still lacking. We hope that presenting these gaps will encourage the scientific community to enhance the number of observations in these situations as well as produce more data from tropical regions.

Impacts of the Integrated Management of Invasive Weeds and Litter on Slope Hydrology in Eucalyptus Plantations in Central Yunnan, Southwest China

Article

Full-text available

May 2024

Background: The hydrological effects of invasive plant control in forestland have not been well studied in the past, and numerous scientific mysteries remain unsolved. The long-term suspension of the unsolved issues will unavoidably influence the sound growth and sustainable management of forest ecosystems. This study investigates the hydrological effects of controlling invasive weeds in forestland. The research aims to understand the impact of invasive weed control on soil and water loss. Methods: Conducted in Eucalyptus benthamii Maiden & Cambage plantations in Central Yunnan, SW China, which are invaded by alien weed Ageratina adenophora (Spreng.) R. M. King & H. Rob., four surface cover treatments were applied to study runoff and sediment yielding properties. The four surface cover treatments were weed harvesting and litter elimination (WH&LE), weed harvesting and litter retention (WH&LR), litter burning and weed renewal (LB&WR), and weed retention and litter retention (WR&LR). Essentially, WH&LE and LB&WR served as integrated management approaches for invasive weeds and litter, WH&LR was an independent weed control measure, and WR&LR served as a research control. Results: Runoff was significantly higher in the LB&WR plots (3.03 mm) compared to the WR&LR plots (1.48 mm) (p < 0.05). The WH&LE plots had higher runoff (2.39 mm) than the WR&LR plots (not statistically significant), while the WH&LR plots had less runoff (1.08 mm) than the WR&LR plots (not significant). Sediment yield was lower in the WH&LR plots (0.50 t/km2) than in the WR&LR plots (0.52 t/km2) (not significant), but significantly higher in the WH&LE plots (2.10 t/km2) and LB&WR plots (1.57 t/km2) than in the WR&LR plots (p < 0.05). Conclusions: Managing invasive weeds independently reduces the risk of soil and water loss, but combined management with litter can exacerbate the issue. Invasive weed control and litter management should be performed separately in slope plantations. This study provides a scientific basis for soil and water conservation, restoration and rehabilitation of plantation ecosystems.

Quantifying potential water savings from clearing invasive alien Eucalyptus camaldulensis using in situ and high resolution remote sensing data in the Berg River Catchment, Western Cape, South Africa

Article

Feb 2016
FOREST ECOL MANAG

Impacts of invading alien plant species on water flows at stand and catchment scales

Article

Full-text available

May 2015

There have been many studies of the diverse impacts of invasions by alien plants but few have assessed impacts on water resources. We reviewed the information on the impacts of invasions on surface runoff and groundwater resources at stand to catchment scales and covering a full annual cycle. Most of the research is South African so the emphasis is on South Africa's major invaders with data from commercial forest plantations where relevant. Catchment studies worldwide have shown that changes in vegetation structure and the physiology of the dominant plant species result in changes in surface runoff and groundwater discharge, whether they involve native or alien plant species. Where there is little change in vegetation structure (e.g. leaf area [index], height, rooting depth and seasonality) the effects of invasions generally are small or undetectable. In South Africa, the most important woody invaders typically are taller and deeper rooted than the native species. The impacts of changes in evaporation (and thus runoff) in dryland settings are constrained by water availability to the plants and, thus, by rainfall. Where the dryland invaders are evergreen and the native vegetation (grass) is seasonal the increases can reach 300-400 mm/yr. Where the native vegetation is evergreen (shrublands) the increases are about 200-300 mm/yr. Where water availability is greater (riparian settings or shallow water tables), invading tree water-use can reach 1.5-2.0 times that of the same species in a dryland setting. So riparian invasions have a much greater impact per unit area invaded than dryland invasions. The available data are scattered and incomplete, and there are many gaps and issues that must be addressed before a thorough understanding of the impacts at the site scale can be gained and used in extrapolating to watershed scales, and in converting changes in flows to water supply system yields. Published by Oxford University Press on behalf of the Annals of Botany Company.

Comparative water use of wattle thickets and indigenous plant communities at riparian sites in the Western Cape and KwaZulu-Natal

Article

Apr 2001
WATER SA

Large-scale funding by both the Government and the private sector continues in support of the Working-for-Water Programme, which is active in many regions of the country. One justification for this programme of alien tree removal is the streamflow enhancement that is believed to follow the replacement of dense stands of invasive trees by indigenous, largely herbaceous or shrub dominated plant communities. Often the densest stands of invader trees occur within riparian zones, where removal of trees in close proximity to stream channels is believed to strongly enhance streamflow. Few data are available, however, to support this assumption. Results from a number of research catchments have consistently shown that afforestation significantly decreases streamflow where the pre-afforestation vegetation was seasonally dormant mountain grassland or fynbos (Versfeld, 1994). The net difference in evapotranspiration (ET) between riparian thickets of alien trees and riparian fynbos may be quite different, due to the yearlong availability of soil water and enhanced plant growth in riparian zones. The water use of alien invasive trees in South Africa remains largely unknown, adding further uncertainty to the effect of alien removal on streamflow. This paper describes the results of a comparative study of annual ET between indigenous riparian plant communities and riparian wattle thickets (Acacia mearnsii() at four sites in the Western Cape and KwaZulu-Natal. The Bowen ratio energy balance (BREB) technique was used to record a 12-month record of 20 min evaporation rates from a fynbos riparian plant community in the Jonkershoek valley (Western Cape), and a grassland riparian community on the property Gilboa in the KwaZulu-Natal midlands. Closed-canopy, mature stands of self-established (A. mearnsii( in the Wellington and Groot Drakenstein areas of the Western Cape were selected to provide comparative transpiration data. The heat pulse velocity (HPV) technique was used to record hourly sap-flow rates in six sample trees representing the range of tree sizes at both wattle sites. Total daily sap flow in all sample trees experiencing adequate soil water availability was found to be very closely correlated to tree size and an index defined as the product of mean daily vapour pressure deficit (VPD) of the air and the number of daylight hours. These relationships were used to predict the water use of wattle thickets at Jonkershoek and Gilboa, using VPD and day-length data recorded at these sites. Published estimates of canopy rainfall interception were added to the sap flow (transpiration) component to yield a combined annual ET to compare to the BREB ET data. Table 1 summarises the annual evapotranspiration at each site. (table***) We conclude that the removal of riparian wattle and its replacement by indigenous herbaceous plants may indeed result in significant reductions in annual ET, and could very likely lead to streamflow enhancement. However, this study has clearly shown that annual ET varies considerably in different riparian plant communities, and that one must consider the structural and physiological characteristics of both the pre-clearing and post-clearing vegetation in order to predict the net change in ET. This conclusion supports an earlier view (Versfeld et al., 1998) that an improved methodology of general applicability is required to enhance the accuracy of water use predictions for a wide range of alien and indigenous plant communities. Such predictions are important to prioritise clearing operations in areas invaded by alien trees. WaterSA Vol.27(4) 2001: 529-538

Invasive plant species, a burning problem of the present-day World, their threats and mitigation measures

Article

Jan 2023

Invasive woody plants in the tropics: a delicate balance between control and harnessing potential benefits. A review

Article

Jun 2023

Invasive woody plants in the tropics: a delicate balance between control and harnessing potential benefts. A review

Article

May 2023
AGRON SUSTAIN DEV

The occurrence and spread of invasive woody species are a truly global phenomenon, but tropical regions seem to be particularly vulnerable due to high rates of soil degradation in combination with climate change, and limited resources for containment. There is increasing awareness that complete eradication programs are often not efective. The existence of many “controversial species,” i.e., species with both negative and positive impacts, renders decision-making processes for management exceedingly complex. By providing a very extensive overview of the current state of knowledge on impacts and containment strategies of invasive woody species, we aim to help underpin such decisions. We discuss both negative impacts and potential benefts of invasive woody species, focusing on the two most important ones, namely animal fodder production and the positive impacts on soil functioning and soil quality. Invasive woody species can positively impact livestock production (1) indirectly by improving pasture quality because of improved soil quality and functioning, and (2) directly by supplying a high-quality protein component for animal fodder. Invasive woody species increase soil carbon sequestration and nitrogen and phosphorus availability depending on the density of the invader, its capacity to fx nitrogen, the quantity and quality of its litter, and the direct interactions between its roots and the soil microbial community. The balance between potential benefts and risks depends to a large extent on the interaction with the local environment (climate, soil, vegetation, and animals) and the socioeconomic context of each region. When an invasion process starts because there is no local predator, then management can target eradication or very strict containment. If the invasion is the result of strong disturbance of the ecosystem, then intensive but well-thought management of the invasive species would be the choice to be made, as this may help to restore the ecosystem.

Evaluating the effectiveness of the Water Resources Master Plan in Campinas, Brazil

Article

Full-text available

Mar 2022

From 2013 to 2015, the Brazilian southeast region experienced the most severe and intense drought recorded, the Water Crisis. This outstanding drought led the government to make efforts toward an adequate water resources management. In Campinas, the third most populous city of the State of São Paulo, the government proposed the Campinas Water Resources Master Plan (CWRMP), the main objective of which was to ensure the quantity and quality of water, reducing water vulnerability in the municipality. In this study, we evaluated the effectiveness of this plan through the major guidelines into four actions: (1) increase the permeability rate and the soil infiltration capacity, (2) increase the vegetation cover, (3) improve the sanitation services, and (4) minimize the number of contaminated areas. For Actions 1 and 2, we inferred that the aforementioned parameters have increased after the CWRMP enactment. About the Action 3, we found that Campinas has historically provided good sanitation services to the population, regardless of the CWRMP promulgation. However, more improvements should be given to waste collection and recycling services. The underground water consumption also has to be regulated, as significant exploitation has no legal permit. Finally, considering Action 4, the number of contaminated areas was reduced in the available data period. Therefore, the CWRMP was considered a valuable initiative to support an integrated and sustainable use of water, improving the water resources management in Campinas. HIGHLIGHTS The 2013–2015 water crisis was the most severe drought recorded in the southeastern region of Brazil.; The CWRMP was enacted to ensure the water quantity and quality, reducing its vulnerability in Campinas.; The CWRMP actions have been effective to restore the local vegetation cover.; The CWRMP was a valuable initiative for the water resources management of Campinas.;

Landscape-scale hydrologic response of plant invasion relative to native vegetation in urban forests

Article

Aug 2021
SCI TOTAL ENVIRON

Large-scale invasion modifies watershed hydrology by changing surface runoff and lowering the seasonal availability of water to native plants. Due to costly field-based evapotranspiration (ET) measurements, which are highly localized and occasionally subject to instrument failure, landscape-scale water use assessments of invasive plants are infrequent. Therefore, the extent to which plant invaders alter water allocation between native and non-native vegetation in a given landscape is rarely accessed. We used a remote sensing-based ET modeling approach to measure the hydrologic response of an invasive shrub, Ligustrum sinense, across forests of the Charlotte Metropolitan Area, North Carolina. We hypothesized that this invader's widespread occurrence and dominant plant physiology significantly competes with native forests for water resources. We tested this hypothesis by comparing inter- and intra-annual variations in ET from invaded and uninvaded sites estimated using the surface-energy-balance system (SEBS) model and cloud-free Landsat images for the wettest (2003), driest (2007), and normal (2005 and 2011) water years. Our findings suggest that the water demand of L. sinense is higher than native forests (deciduous and evergreen) for most of the year except during the early spring and after high precipitation events. The daily ET flux of L. sinense was significantly different than evergreen vegetation during the driest year (2007) that, five years later (2011 - normal water year), was significantly different than both deciduous and evergreen vegetation types. This suggests that L. sinense consumes more water than native forest types, particularly during dry and normal precipitation years with increasing cover over time making it a strong competitor with native vegetation in urban forests for water resources. Therefore, accounting for the hydrologic response of invasive plants and potential water savings from their removal from forests, particularly in water-scarce regions, may enable land managers and decision-makers to prioritize areas for monitoring and control efforts.

Modeling the geographic spread and proliferation of invasive alien plants (IAPs) into new ecosystems using multi-source data and multiple predictive models in the Heuningnes catchment, South Africa

Article

Full-text available

Apr 2021

The geographic spread and proliferation of Invasive Alien Plants (IAPs) into new ecosystems requires accurate, constant, and frequent monitoring particularly under the changing climate to ensure the integrity and resilience of affected as well as vulnerable ecosystems. This study thus aimed to understand the distribution and shifts of IAPs and the factors influencing such distribution at the catchment scale to minimize their risks and impacts through effective management. Three machine learning Species Distribution Modeling (SDM) techniques, namely, Random Forest (RF), Maximum Entropy (MaxEnt), Boosted Regression Trees (BRT) and their respective ensemble model were used to predict the potential distribution of IAPs within the catchment. The current and future bioclimatic variables, environmental and Sentinel-2 Multispectral Instrument satellite data were used to fit the models to predict areas at risk of IAPs invasions in the Heuningnes catchment, South Africa. The present and two future climatic scenarios from the Community Climate System Model (CCSM4) were considered in modeling the potential distribution of these species. The two future scenarios represented the minimum and maximum atmospheric carbon Representative Concentration Pathways (RCP) 2.6 and 8.5 for 2050 (average for 2041–2060). The results show that IAPs are predicted to expand under the influence of climate change in the catchment. Concurrently, riparian zones, bare areas, and the native vegetation which is rich in biodiversity will greatly be affected. The mean diurnal range (Bio2), warmest quarter maximum temperature (Bio5), and the warmest quarter precipitation (Bio18) were the most important bioclimatic variables in modeling the spatial distribution of IAPs in the catchment. Comparatively, all the models were successful in predicting the potential distribution of IAPs for all the scenarios. The BRT, MaxEnt, and RF predicted the spatial distribution of IAPs with an Area Under Curve (AUC) of 0.89, 0.92, and 0.94, respectively. The study highlighted the importance of multi-source data and multiple predictive models in predicting the current and potential future IAP distribution. The results from this study provide baseline information for effective land management, planning, and continuous monitoring of the further spread of IAPs within the Heuningnes catchment.

Ecological restoration of ecosystems degraded by invasive alien plants in South African Fynbos: Is spontaneous succession a viable strategy?

Article

Full-text available

Jul 2020

Ecological restoration is a global imperative to reverse widespread habitat loss and degradation, including by invasive alien plants. In South Africa’s Core Cape Subregion, alien tree invasions are widespread and their control continues to be a major undertaking. As funding is limited, active restoration interventions are rarely implemented and the focus is on invader removal – the assumption being that ecosystems will self-repair. This paper reviews research findings from the past three decades to assess in which situations spontaneous succession is a viable strategy for restoring alien-invaded ecosystems. We found that ecosystems can self-repair, provided that key biotic and/or abiotic thresholds have not yet been crossed. Self-repair has been observed in many cases where dense invader stands with short residence times have been cleared and where diverse native plant growth forms survive, either in the above-ground vegetation or in soil seed banks. However, several factors influence this generalisation, including the identity of the invader, the ecosystem type, and the efficacy of alien control. Thresholds are crossed sooner with invasions of alien Acacia and Eucalyptus species than those of Hakea and Pinus species, resulting in lower potential for spontaneous recovery. Lowland fynbos ecosystems are less resilient to invasion, and have a lower capacity for self-repair, than mountain fynbos ecosystems. Poorly implemented alien plant control measures can result in a resurgence of the invader to the detriment of native species recovery. We outline some management principles for optimising spontaneous succession potential and integrating alien control and restoration interventions.

The effect of Acacia Mearnsii removal on water table fluctuations in the Tsomo Valley Eastern Cape of South Africa

Article

Full-text available

Mar 2010

A comparative assessment of water use by Acacia longifolia invasions occurring on hillslopes and riparian zones in the Cape Agulhas region of South Africa

Article

Oct 2018
Phys Chem Earth

The detrimental impacts of invasive alien plants on ecosystems and water resources have raised concerns in arid and semi-arid countries like South Africa where the average precipitation is approximately 500 mm/yr, which is below the world average of around 860 mm/yr. Several studies have examined the effects of invasive alien plants such as the Australian Acacias on the water resources. However, few studies have quantified the differences in water use between hillslope and riparian Acacia longifolia invasions. A. longifolia is one of the aggressive invader species in South Africa even on hillslopes that contribute substantially to runoff generation. Therefore, the encroachment of invasive alien plants has the potential to reduce runoff, thereby adversely affecting the available water downstream. This paper aims to; 1) compare transpiration rates of A. longifolia growing on hillslopes and along riparian areas, 2) establish the key drivers for water use by this species, and 3) estimate the hydrological impacts of the invasions at the catchment scale in the Heuningnes catchment, in the Western Cape Province of South Africa. Transpiration by the trees was measured using the heat pulse velocity sap flow method. Automatic weather stations and soil moisture sensors were used to monitor weather and soil water content variations at each site. The results showed that, at the stand level the riparian A. longifolia transpired two times more water (∼596 mm/yr) than on the hillslope (∼242 mm/yr). During years with above average rainfall, the water use rates by the invasions was estimated to be ∼579 mm/yr on the hillslope and could be as much as ∼1348 mm/yr at the riparian site. Thus, the hypothesis that riparian trees use more water than invasions on non-riparian areas was accepted in this study. At the catchment scale (740 km2), the estimated water use by the invasions was 20.5Mm3. Clearing of all the invasions in the study catchment is likely to make 17Mm3/yr of water available. Hence the clearing of A. longifolia along the riparian corridors should be prioritised as this will lead to water savings at the catchment scale.

Medium-term vegetation recovery after removal of invasive Eucalyptus camaldulensis stands along a South African river

Article

Aug 2018
S AFR J BOT

Effective ecological restoration requires detailed monitoring to determine the success achieved through different interventions in achieving objectives. In 2017, we resurveyed riparian sites along the Berg River in the Western Cape, South Africa, that have been cleared of invasive stands of Eucalyptus camaldulensis in 2010 using two clearing methods (fell-and-stackburn and fell-and-remove) and two restoration approaches: passive (where vegetation was allowed to recover without intervention) and active (assisted recovery). A significant increase in vegetation cover (P b .001) and diversity (P b .05) of native riparian species was recorded in passive restoration plots, but an increase in the cover of woody invasive alien plants was also observed. Only four of the nine native species that were planted to fast-track restoration were still present in the active restoration plots, but the abundance of these native species was significantly (P b .001) lower in 2017 than in 2011. We conclude that native vegetation recovery following E.camaldulensis removal seven years ago is following a positive recovery trajectory in both passive and active restoration sites, as shown by the increased occurrence of native trees and shrubs, e.g. Maytenus oleoides, Melianthus major and Searsia angustifolia which were not present before clearing. However, the reinvasion of cleared sites by woody invasive alien plants has the potential to slow down and potentially halt the recovery process. Further management interventions, e.g. removal of reinvading woody invasive alien plants, are required, emphasizing the sustained engagement to ensure restoration in these ecosystems.

Here to stay. Recent advances and perspectives about Acacia invasion in Mediterranean areas

Article

Full-text available

Jul 2017

The above- and belowground impacts due to Acacia invasions have been described in detail over the last 25 years. Future research should focus on the earlyd etection and prevention of new Acacia introductions and on a cost-effective and sustainable management of the novel ecosystems resulting from invasions.

Invasive alien trees and water resources in South Africa: Case studies of the costs and benefits of management

Article

Jan 2001
FOREST ECOL MANAG

River flow response to changes in vegetation cover in a South African fynbos catchment

Article

Full-text available

Jan 2009
WATER SA

Mountain fynbos catchments in the Western Cape region of South Africa are prone to substantial changes in land cover due to invasion by exotic tree species (and their clearing), fires, and vegetation response to inter-annual variations in rainfall. While small catchment experiments and modelling studies have pointed to reductions in river flow as catchment biomass increases, there is little empirical evidence of land cover change affecting river flow in large catchments that are important sources of water for the region. Monitoring changes in above-ground green biomass in multiple large catchments is challenging, but may be accomplished using a remotely sensed spectral vegetation index. It was hypothesised in this study that annual river yield (river flow as a fraction of rainfall) in the Molenaars catchment near Paarl, South Africa co-varies with an index of green vegetation cover derived from satellite data (the normalised difference vegetation index, NDVI). The catchment was parti- tioned into 'upland' and 'lowland' zones and the relationship between annual river yield and summer NDVI was determined for each zone over an 18-year period. There was a statistically significant negative linear relationship between annual river yield and the NDVI of the lowland zone when three outliers were excluded from the analysis. These outliers corresponded to periods with prolonged drought conditions when river yield appeared to be decoupled from vegetation water use in the lowland zone. There was no relationship between river yield and changes in the NDVI in the upland zone where plants were unlikely to have sustained access to adequate soil water for transpiration. The importance of considering the location of land cover changes in a catchment, and inadequacies in high-elevation measurements of rainfall in this mountainous region, were highlighted in the study.

Water yield in primary watersheds under grasslands and pine plantations in the hills of Córdoba (Argentina)

Article

Full-text available

Aug 2013

Vegetation changes can have a strong imprint on the hydrological cycle, affecting the magnitude and temporal distribution of stream and river flow. These effects gain special relevance in the mountain ranges of dry regions, which play a key role in water provision. This is the case of the eastern front of the Córdoba hills, where ~35.000 ha of pine plantations have replaced natural grasslands. We explored how this transformation has affected water yield in small watersheds. For this purpose we selected four pairs of neighboring watersheds occupied by natural grasslands and Pinus ellioti plantations (area: 27 to 143 ha; elevation: 1100 to 1750 m.a.s.l.). For all pairs, basal streamflow was measured through the tracer dilution method with seasonal frequency between May 2004 and January 2007, and in two of these pairs we performed a continuous monitoring using automatic level sensors during the transition between the dry and humid season of 2006-2007. On average, water yield in afforested watersheds was 48% less than in grassland watersheds (112 vs. 204 mm/year or 24 vs. 13% of precipitation inputs during the study period, P<0.05). Continuous measurements revealed higher base flows and little response to vegetation in high elevation and slope watersheds, and lower and more vegetation-sensitive base fows in low elevation and intermediate slope watersheds. The remote sensing characterization of watersheds based on a green index (NDVI) from MODIS suggested higher and more stable primary productivity and evapotranspiration under plantations compared to grasslands, with maximum contrasts taking place in winter. Currently the impact of plantations in the hills on water provision can be intense only at the level of primary watersheds, since those of larger order are only partially afforested. At the regional level is important to contemplate these effects to project how much, where and how will be planted, particularly in areas that supply water to the largest foci of hydroenergy and urban demand.

Native trees show conservative water use relative to invasive trees: Results from a removal experiment in a Hawaiian wet forest

Article

Full-text available

Jan 2014

While the supply of freshwater is expected to decline in many regions in the coming decades, invasive plant species, often ‘high water spenders’, are greatly expanding their ranges worldwide. In this study, we quantified the ecohydrological differences between native and invasive trees and also the effects of woody invasive removal on plot-level water use in a heavily invaded mono-dominant lowland wet tropical forest on the Island of Hawaii. We measured transpiration rates of co-occurring native and invasive tree species with and without woody invasive removal treatments. Twenty native Metrosideros polymorpha and 10 trees each of three invasive species, Cecropia obtusifolia, Macaranga mappa and Melastoma septemnervium, were instrumented with heat-dissipation sap-flux probes in four 100 m2 plots (two invaded, two removal) for 10 months. In the invaded plots, where both natives and invasives were present, Metrosideros had the lowest sap-flow rates per unit sapwood, but the highest sap-flow rates per whole tree, owing to its larger mean diameter than the invasive trees. Stand-level water use within the removal plots was half that of the invaded plots, even though the removal of invasives caused a small but significant increase in compensatory water use by the remaining native trees. By investigating the effects of invasive species on ecohydrology and comparing native vs. invasive physiological traits, we not only gain understanding about the functioning of invasive species, but we also highlight potential water-conservation strategies for heavily invaded mono-dominant tropical forests worldwide. Native-dominated forests free of invasive species can be conservative in overall water use, providing a strong rationale for the control of invasive species and preservation of native-dominated stands.

Stream flow unaffected by Eucalyptus plantation harvesting implicates water use by the native forest streamside reserve

Article

Full-text available

Mar 2015

Study region: Tropical Atlantic Forest region, Brazil. Study focus: The temporal and spatial dynamics of soil water, water table depth and stream flow in relation to precipitation and the harvesting and regrowth cycle of a Eucalyptus grandis hybrid plantation in a headwater catchment. This landscape contains a mosaic of eucalypt plantation grown for pulpwood on plateau tops and native forest reserves in gullies. Instead of harvesting the native forest to test this effect, we conducted a virtual experiment using a soil and hydrological model (HYDRUS). New hydrological insights: Plantation harvest had little effect on steam flow, despite a 6–11 m rise in water table level under the plantation area. This result suggests that the native forest reserve intercepted groundwater moving laterally between the plantation and the stream. Measured and simulated runoff coefficients were similarly low (5% and 3%, respectively), but simulated removal of the native forest led to an increase to 38%. Therefore, plantation management in this type of landscape is likely to have little impact on stream flows where there is an intact native rainforest reserve beside the stream.

Satellite-based annual evaporation estimates of invasive alien plant species and native vegetation in South Africa

Article

Full-text available

Jan 2014
WATER SA

In this study we assessed the impact that invasive alien plant species (IAPs), and the clearing thereof by the Working for Water (WFW) programme, have on total evaporation (ET) and the availability of water resources in two highly-invaded provinces of South Africa. The Surface Energy Balance Algorithm for Land (SEBAL) model, using MODIS satellite imagery, was used to estimate the annual total ET at 250 m pixel resolution. ET was estimated for 3 climatically different years for the Western Cape and KwaZulu-Natal. The average annual ET from areas under IAPs, native vegetation, exotic plantation forestry species and control (clearing) areas were compared. The ET of the 5 dominant IAPs (Acacia mearnsii, Acacia saligna, Eucalyptus spp., Hakea spp. and Pinus spp.) in the Western Cape province was 895 mm, which was significantly higher than the ET of most of the native vegetation (thicket 575 mm and fynbos 520 mm), but similar to the ET of dominant exotic plantation forestry species (805 mm). On average, the ET was reduced by 13% to 780 mm, following clearing. In KwaZulu-Natal Province, the ET of the 5 dominant IAPs (Acacia mearnsii, Chromoleana odorata, Eucalyptus spp., Lantana camara and Solanum mauritanium) was 875 mm, which was also higher than the ET of the native vegetation (thicket 755 mm, savanna 685 mm and grassland 640 mm). Following IAP control the ET was decreased by 6%, to 825 mm. This study has demonstrated that spatial ET data with GIS-information on land use can be used to assess the impact of IAPs, and clearing thereof, on water resources. We confirmed results from previous studies, which showed that ET from invaded areas exceeded that from native vegetation. The ET data needs further validation as validation appeared to be impossible. Our results are likely conservative since the majority of invaded areas considered in this analysis represent non-riparian areas. The impact of WFW control of densely-invaded riparian areas is likely more pronounced. We concluded that the clearing of IAPs by the WFW programme has a positive effect on the availability of water resources through a reduction in ET.

ARTICLE IN PRESS G Model

Article

Dec 2014

Study region Tropical Atlantic Forest region, Brazil. Study focus The temporal and spatial dynamics of soil water, water table depth and stream flow in relation to precipitation and the harvesting and regrowth cycle of a Eucalyptus grandis hybrid plantation in a headwater catchment. This landscape contains a mosaic of eucalypt plantation grown for pulpwood on plateau tops and native forest reserves in gullies. Instead of harvesting the native forest to test this effect, we conducted a virtual experiment using a soil and hydrological model (HYDRUS). New hydrological insights Plantation harvest had little effect on steam flow, despite a 6–11 m rise in water table level under the plantation area. This result suggests that the native forest reserve intercepted groundwater moving laterally between the plantation and the stream. Measured and simulated runoff coefficients were similarly low (5% and 3%, respectively), but simulated removal of the native forest led to an increase to 38%. Therefore, plantation management in this type of landscape is likely to have little impact on stream flows where there is an intact native rainforest reserve beside the stream. Keywords Eucalyptus plantation; Hydrology; Harvesting; Native forest; Stream flow; Modelling

Hawaiian ecohydrology: Water use of native and invasive trees in a lowland tropical rainforest

Conference Paper

Aug 2009

Background/Question/Methods While the supply of clean water in Hawaii is expected to decline in the coming decades, invasive plant species are greatly extending their ranges across the islands. To quantify the ecohydrological impacts of invasive species, we compared the water use of co-occurring native and invasive tree species in a heavily invaded forest on the Big Island of Hawaii. We also compared native tree water use in the presence and absence of invasive species. This study was based within the framework of an ongoing project investigating the impacts of invasive species on a lowland tropical rainforest ecosystem with respect to water, light, nutrients, and regeneration, where all invasive species were removed from four 10 x 10m plots, and four additional 10 x 10m plots were established as controls. We measured soil moisture and tree water use in two of the established removal plots and two of the control plots. Twenty native Metrosideros polymorpha and ten trees each of three invasive species: Macaranga mappa, Melastoma septemnervium, and Cecropia obtusifolia were instrumented with heat dissipation sap flow probes from February to December 2008. Twelve of the fifty trees were instrumented with additional probes to investigate the variation in sap flow with sapwood depth. Results/Conclusions Sap flow varied with sapwood depth for all tree species, and this information was subsequently used to model sap flow rates for the rest of the trees which had probes installed at a single depth. In the control plots, where both natives and invasives were present, preliminary data show that native M. polymorpha had the lowest sap flow rates per unit sapwood overall. Conversely, M. polymorpha had the highest sap flow rates per tree, likely because the native trees tended to be larger in diameter than the invasive trees. Stand level water use declined after invasive species were removed, which was confirmed by a substantial increase in soil moisture in the removal plots. There was, however, no discernible difference in native M. polymorpha sap flow in the presence vs. absence of invasive species, possibly because water is not a limiting resource for this native species, even during periods of drought. Alternatively, M. polymorpha species may be too slow growing and conservative in its resource use to respond in a measurable way to increases in water availability.

Rendimiento hídrico en cuencas primarias bajo pastizales y plantaciones de pino de las sierras de Córdoba (Argentina)

Article

Full-text available

Aug 2013

Los cambios en la cobertura vegetal pueden tener importantes efectos sobre el ciclo hidrológico, afectando la magnitud y distribución temporal del caudal de ríos y arroyos. Estos efectos cobran una relevancia especial en áreas montañosas de regiones secas dado su papel importante en la provisión de agua. Este es el caso de las laderas orientales de las sierras de Córdoba, en donde se establecieron ~35000 ha de plantaciones de pinos en reemplazo de pastizales naturales. Exploramos cómo esta transformación ha afectado el rendimiento hídrico de pequeñas cuencas serranas. Para ello seleccionamos cuatro pares de cuencas primarias ocupadas por pastizales naturales y plantaciones de Pinus ellioti (superficie: 27 a 143 ha; elevación: 1100 a 1750 m.s.n.m.). En todos estos pares de cuencas determinamos el caudal base de arroyos por dilución de un trazador salino con frecuencia estacional entre mayo de 2004 y enero de 2007, y en dos de ellos realizamos un seguimiento continuo del caudal con sensores automáticos durante la transición entre la estación seca y húmeda de 2006-2007. En promedio, el rendimiento hídrico de las cuencas forestadas fue 48% inferior al de las cuencas de pastizal (112 vs. 204 mm/año o 24 vs. 13% de la precipitación recibida, P

Impacts of invasive alien plants on water quality, with particular emphasis on South Africa

Article

Full-text available

Apr 2012
WATER SA

We review the current state of knowledge of quantified impacts of invasive alien plants on water quality, with a focus on South Africa. In South Africa, over 200 introduced plant species are regarded as invasive. Many of these species are particularly prominent in riparian ecosystems and their spread results in native species loss, increased biomass and fire intensity and consequent erosion, as well as decreased river flows. Research on the impact of invasive alien plants on water resources has historically focused on water quantity. However, although invasive alien plants also affect the quality of water, this aspect has not been well documented. Alien invasive plants increase evaporation rates, and reduce stream flow and dilution capacity. The biomass inputs of alien invasive plants, especially nitrogen fixers such as Acacia spp., alter nutrient cycles and can elevate nutrient concentrations in groundwater. Alien plant invasions alter the fire regimes in invaded areas by changing the size, distribution and plant chemistry of the biomass. More intense fires increase soil erosion and thereby decrease water quality. In contrast to riparian invasions, aquatic invasive plants have been more extensively studied in South Africa and their impacts on water quality have been relatively well monitored. Water quality in South Africa is rapidly deteriorating, and all factors that influence this deterioration need to be taken into account when formulating actions to address the problem. The changes in water quality brought about by alien plant invasions can exacerbate the already serious water quality problems.

Both complete clearing and thinning of invasive trees lead to short-term recovery of native riparian vegetation in the Western Cape, South Africa

Article

Apr 2013
APPL VEG SCI

QuestionsMost rivers in the Western Cape Province of South Africa are heavily invaded by alien trees, often resulting in profound changes in biodiversity and ecosystem functioning. Although large-scale management operations are underway to clear invasive trees and restore ecosystem function, little is known regarding native species recovery after alien clearing. Has Eucalyptus invasion along the Berg River altered the distribution and composition of native vegetation? How does the removal of invasive trees through complete clearing and thinning facilitate the recovery of native vegetation? LocationBerg River, Western Cape, South Africa. Methods We assessed the recovery of native vegetation after 4 yr of complete clearing of the invasive tree Eucalyptus camaldulensis (100% alien cover removal) and thinning (40–50% alien cover removal) along the Berg River in the Western Cape, South Africa. Native and alien plant cover, species richness and diversity were recorded on completely cleared and thinned sites and compared to natural (non-invaded control sites) and E. camaldulensis invaded sites. ResultsSpecies richness and diversity were significantly higher in both completely cleared and thinned sites compared to natural and invaded sites. Increases in species richness and diversity in completely cleared and thinned sites were a result of re-invasion by alien herbaceous and graminoid species, which have the potential to hinder native species recovery. Cover of native trees and shrubs was higher in both completely cleared and thinned sites compared to invaded sites. Species composition (relative cover) in completely cleared and thinned sites was similar to species composition in natural sites. Conclusions Both complete clearing and thinning methods promote indigenous vegetation recovery and a positive trajectory towards recovery of ecosystem structure and composition can be expected in future. To improve management operations, a four-stage thinning process that has the potential to facilitate native species recovery is suggested.

The effectiveness of active and passive restoration on recovery of indigenous vegetation in riparian zones in the Western Cape, South Africa: A preliminary assessment

Data

Full-text available

Sep 2013
S AFR J BOT

Riparian ecosystems in South Africa's fynbos biome are heavily invaded by alien woody plants. Although large-scale clearing of these species is underway, the assumption that native vegetation will self-repair after clearing has not been thoroughly tested. Understanding the processes that mediate the recruitment of native species fol-lowing clearing of invasive species is crucial for optimising restoration techniques. This study aimed to determine native species recovery patterns following implementation of different manage-ment interventions. We tested the influence of two clearing treatments ("fell & remove" and "fell & stack burn") on the outcomes of passive restoration (natural recovery of native riparian species) and active restoration (seed sowing and planting of cuttings) along the Berg River in the Western Cape. Under greenhouse conditions we in-vestigated seed viability and germination pre-treatments of selected native species. There was no recruitment of native species in sites that were not seeded (passive restoration sites), possibly be-cause of the dominance of alien herbaceous species and graminoids or the lack of native species in the soil-stored seed bank. Germination of our targeted native species in the field was low in both "fell & remove" and "fell & stack burn" treatments. However, "fell & stack burn" gave better germination for the species Searsia angustifolia, Leonotis leonurus and Melianthus major. Seedling survival in the field was significantly reduced in summer, with drought stress being the main cause for seedling mortality. Germination rates in the greenhouse were high, an indication that harvested seeds were viable. Most seeds germinated without germination pre-treatments. We conclude that failure of native seeds to germinate under field conditions, secondary invasion of alien herbs and graminoids, the lack of native species in the soil-stored seed bank, and dry summer conditions hamper seed-ling establishment and recovery on sites cleared of dense stands of alien trees. For active restoration to achieve its goals, effective recruitment and propagation strategies need to be established.

Comparison of water-use by alien invasive pine trees growing in riparian and non-riparian zones in the Western Cape Province, South Africa

Article

Apr 2013
FOREST ECOL MANAG

Self-established stands of alien invasive pine trees are common in many parts of South Africa and elsewhere. They mainly invade non-riparian settings but sometimes invade riparian habitats. There are clear visual differences in the physical attributes of trees that occupy riparian and non-riparian zones. We have little information whether the differences between trees growing in these contrasting habitats reflect their water-use. The goal of this study was to establish the water-use of alien invasive pines growing adjacent to and away from a perennial stream, and to determine the driving factors behind the variations. The study was conducted in a self-established 20-year-old mixed pine forest occupied by roughly equal proportions of Pinus pinaster and Pinus halepensis. Individual tree transpiration rates were measured using the heat pulse velocity (HPV) sap flow method. Evapotranspiration (ET) from entire stands was determined from the surface energy balance equation using sensible heat flux data collected using a boundary layer scintillometer and measurements of the available energy (net radiation – soil heat flux). A simple two-layer model in which the stand ET was calculated as the algebraic sum of the outputs from transpiration (E) and soil evaporation sub-models was evaluated at the two contrasting sites. Annual transpiration and ET rates were higher in the riparian zone at 980 and 1417 mm compared to 753 and 1190 mm, respectively in the non-riparian area. The model predicted stand transpiration fairly accurately for both sites (average R2 > 0.75), but was less accurate for evapotranspiration (average R2 < 0.70) due to the difficulties in simulating soil evaporation. No significant differences in sap velocities were found between trees at the two sites so the greater water-use of trees in the riparian zone was due to the larger basal area per stem. Based on the measured transpiration data we conclude that self-sown pine stands growing in riparian zones use at least 36% more water than those occurring in non-riparian settings justifying the high priority given to clearing invasive trees in riparian zones.

Water relations and the effects of clearing invasive Prosopis trees on groundwater in an arid environment in the Northern Cape, South Africa

Article

Mar 2013

Several Prosopis species have been introduced into South Africa in the last century and many of them have become invasive. This study investigates the water relations, effects of clearing, and the seasonal dynamics of groundwater use by invasive Prosopis trees. The trees were growing on deep sandy soils in the floodplain of an episodic river in the arid Northern Cape Province of South Africa. Data were collected on tree water uptake, evapotranspiration and water table depth over different seasons. Effects of tree clearing on groundwater were quantified by comparing data from a Prosopis invaded and an adjacent cleared area. Transpiration rates were less than 1.0 mm/d throughout the year and the trees showed structural and physiological adaptations to the combined low rainfall and low water holding capacity of the soils by developing very narrow sapwood areas and by closing their stomata. The trees abstracted groundwater as evidenced by the decline in borehole water levels in the Prosopis stand before the rainy season. Groundwater savings of up to 70 m3/month could be achieved in spring for each hectare of Prosopis cleared. The study suggests that clearing of invasive Prosopis would conserve groundwater in the arid parts of South Africa.

Conservation and the delivery of ecosystem services

Article

Full-text available

Jan 2009

Ecosystem services are the benefits people obtain from ecosystems, such as clean air, fresh water, and the pollination of crops. The aim of this literature review was to find empirical data illustrating the ways in which conservation land and conservation management activities affect ecosystem services. The widely-held belief that natural ecosystems-such as those found on conservation land in New Zealand-provide a range of ecosystem services is generally supported by the literature. International studies show that natural vegetation can decrease air pollution, regulate local air temperatures, improve water quality, reduce shallow soil erosion, and retain natural nutrient cycles. It can also be beneficial for pest control and pollination on agricultural land. Wetlands can improve water quality and can play a role in drought and flood mitigation. Seagrasses, saltmarsh vegetation, and mangroves can reduce the height and force of waves and play a role in flood protection. In addition, maintaining biodiversity preserves genetic libraries and future options for discoveries of valuable biological compounds. The few studies investigating the effects of conservation management activities on ecosystem services indicate that restoring vegetation can improve water quality and water storage functions, can reverse soil degradation on a local scale, and can restore plant-insect interactions. Additionally, removing some invasive plant species can increase water yield. Unfortunately, very few studies of ecosystem services have been conducted in New Zealand to date, and only some of the international results are likely to be applicable under New Zealand conditions, Accordingly, while conservation is probably beneficial for a range of ecosystem services in New Zealand, the scarcity of local data makes it difficult to ascertain where and when, and to what extent, the majority of those benefits transpire.

Are We Destroying Our Insurance Policy? The Effects of Alien Invasion and Subsequent Restoration

Book

Sep 2013

Securing sufficient and reliable water supply is a priority for many countries worldwide, but their efforts are hindered by widespread landscape degradation and uncertainty around future climate change. We used historical aerial photographs and mapping techniques to investigate how a South African landscape has changed over the past century. The Kromme River Catchment, a valuable water-providing catchment for the Nelson Mandela Bay metropolitan hub, has become heavily degraded. The floodplain wetlands, which historically occupied the entire valley floor, have been almost completely replaced by agriculture or invaded by the alien tree Acacia mearnsii. Some efforts have been made to restore the wetlands and control the invasive plants, but our results show that at the current rate of clearing it would take 30 years before A. mearnsii would be brought under control. We recommend that investment should be made, as a type of insurance for natural capital, in restoring resilience in important water-providing catchments to hedge against future climatic uncertainties.

Ecology and management of alien plant invasions in South African fynbos: Accommodating key complexities in objective decision making

Article

Aug 2009
BIOL CONSERV

Invasive alien trees and shrubs pose significant threats to biodiversity and ecosystem services in South African fynbos ecosystems. An ambitious initiative, the Working for Water program, commenced in 1995 to reduce the extent and impact of plant invasions. Despite substantial progress, the problem remains immense, and innovative ways of improving the efficiency of control operations are urgently needed. This study sought to develop a robust conceptual framework for effective management of the most important invasive alien plant (IAP) species. Two methods were applied in exploring the complexity of problems, thereby identifying appropriate response strategies. The DPSIR (Driving forces-Pressure-State-Impacts-Responses) framework and the Analytic Hierarchy Process (AHP) tool were used to design a strategy for prioritizing management actions. This strategy considers explicitly the most influential factors that determine the distribution, abundance, spread and impacts of IAPs. Efficient management of IAPs is constrained by multiple interacting environmental and socio-economic factors. Factors related to the fire-prone nature of the ecosystem and the characteristics of the invasive stands emerged as pivotal features for setting spatially-explicit priorities for management. Results of the analyses provide an objective and quantifiable perspective for improving the management efficiency. We conclude that considerable progress in controlling the spread of IAPs in fynbos ecosystems could be achieved by better coordination of management practices and by improving the quality of species distribution data.

Viability of investing in ecological infrastructure in South Africa’s water supply areas

Article

May 2024
S AFR J SCI

Ecological infrastructure (natural ecosystems that provide important services and save on built infrastructure costs) can have an important role in securing water supply, particularly in water-scarce areas, but this importance is not reflected in investment decisions, partly due to a lack of evidence. In South Africa, one of the main threats to water supply is the proliferation of woody invasive alien plants which significantly reduce stream flow and water yields. We used existing spatial data and estimates of the impact of woody invasive plants on flows and water yields and on restoration costs to analyse the viability of investing in ecological infrastructure at the scale of major water supply areas. The analysis involved comparison of the costs and effects on water yields of catchment restoration with those of planned built infrastructure interventions designed to meet increasing water demands in the medium to long term. The cost-effectiveness analysis used the unit reference value as a measure of comparison, which is based on the discounted flows of costs and water supplied over a defined time. Restoration could supply 24% of the combined yield of planned built infrastructure interventions by 2050, and is not only cost-effective but has the added advantage of a range of co-benefits delivered by improving ecosystem health. This finding suggests that investing in ecological infrastructure should be considered ahead of new built-infrastructure projects.

Invasion of natural grasslands by exotic trees increases flood risks in mountainous landscapes in South India

Article

Dec 2022
J HYDROL

Invasive trees in catchments around the world are a source of concern due to their hydrological and ecological impacts. A large number of studies have focused on their impact on dry-season stream discharge due to enhanced transpiration. The impact of invasive trees on stream discharge and flood risk during extreme rain events, which are becoming more frequent due to climate change, has not been addressed adequately. We examined the influence of land-cover, canopy cover, and other catchment morphological characteristics on stream discharges in hilly catchments during light, heavy, and extreme rain events. Three years of rainfall-runoff observations, between January 2014 and December 2016, were collected in eleven neighbouring mountainous catchments in Nilgiris, South India. Each catchment had a distinct land cover, namely shola forests, montane grasslands, and catchments invaded by wattle (Acacia mearnsii). Hourly rain intensities between the percentiles 25-90, 90-95 and over 95 were categorised as light, heavy and extreme respectively, and were used to study hourly peak stream discharge responses. We compared discharge between native grassland catchments and grassland catchments invaded by wattle. Discharge rates were corrected for the catchment area. We found that land cover and canopy-cover influenced the hydrologic response to extreme rain events. Regression models with flood event as the response variable suggested an increase in rainfall-runoff response with an increase in canopy cover. The maximum hourly discharge observed in the reference grassland catchment (0.00098 mm s-1) was lower than the maximum discharge observed in low density (0.00529 mm s-1) and high density (0.00497 mm s-1) wattle catchments. At higher discharges, the flood magnitude in wattle invaded catchments was much greater than that in the grassland catchment for a given flood frequency - indicating a higher risk of severe flooding in wattle-dominated catchments. We attribute the enhanced flood discharges to the effects of spreading wattle roots on the enhancement of rapid, shallow sub-surface flows in invaded catchments. Catchments dominated by shallow sub-surface flows are known to have reduced water retention times in response to high rain intensities. Antecedent moisture conditions and drainage density were other factors that influenced the local rainfall-runoff relationships. In the Western Ghats and other parts of India grasslands have been planted with exotic woody trees, and some have become invasive. Our study indicates that replacing grasslands with woody plants at landscape scales, could affect hydrological processes and could increase flood risks during the monsoon, especially in mountainous regions. Management of invaded catchments by regulation of wattle and restoration of semi-natural. grassland, combined with the installation of early warning systems in these impacted areas will help reduce flood risk during extreme rain under climate change.

Impacts of Plant Invasions on Terrestrial Water Flows in South Africa

Chapter

Full-text available

Mar 2020

Considerable advances have been made since the first estimates of the impacts of invasive alien plants on water resources in the early 1990s. A large body of evidence shows that invasive alien plants can increase transpiration and evaporation losses and thus reduce river flows and mean annual runoff. Riparian invasions, and those in areas where groundwater is accessible, have 1.2–2 times the impact of invasions in dryland areas. The magnitude of the impacts is directly related to differences between the invading species and the dominant native species in size, rooting depth and leaf phenology. Information on the impacts has been successfully used to compare the water use of invasive plants and different land cover classes, to quantify the water resource benefits of control measures, and to prioritise areas for control operations. Nationally, the impacts of invasive alien plants on surface water runoff are estimated at 1.44–2.44 billion m³ per year. The most affected primary catchments (>5% reduction in mean annual runoff) are located in the Western and Eastern Cape, and KwaZulu-Natal. If no remedial action is taken, reductions in surface water runoff could increase to 2.59–3.15 billion m³ per year, about 50% higher than current reductions. This review illustrates the importance of measuring water-use over as wide a range of species as possible, and combining this with information from remote sensing to extrapolate the results to landscapes and catchments. These methods will soon provide much more robust estimates of water use by alien plants at appropriate spatial and temporal scales. The results of these studies can be used in water supply system studies to estimate the impacts on the assured yields. This information can also be used by catchment water resource managers to guide decision-makers when prioritising areas for clearing and rehabilitation, and for targeting species for control measures.

Evaluating the environmental and social net-worth of controlling alien plant invasions in the Inkomati catchment, South Africa

Article

Full-text available

Jan 2020

The present study attempted to bridge some gaps in the existing literature evaluating the net economic worth of South Africa’s Working for Water (WfW) programme for eradicating invasive alien plant species (IAPS). Specifically, the study employed the social benefit-cost analysis (BCA) methodology to assess the impact of accounting for the opportunity cost of invested capital funds and treating labour as a social benefit on the social worthiness of the WfW programme in the Inkomati catchment. The study also used improved measures of the value of other ecosystem services, particularly the carbon sequestration values. Results of the social BCA provided strong empirical evidence in support of the continuation of IAPS eradication activities, as economically and socially worthwhile investment of the country’s resources. The programme generates higher net worth under lower rates of discounting future values. This confirms the importance of the water-saving benefits, which continue theoretically for ever, at zero cost to the society, as all direct and indirect financial costs cease upon completion of the eradication operations. The social net worth of the IAPS eradication programme obviously increases when expenditure on labour wages was considered a social benefit rather than a direct financial cost, even under strict project funding scenarios that require funding through private capital markets, i.e., paying commercial rates of interest. However, more strategic planning for the control of IAPS is critically important given their high cost. Challenges facing the sustainability of IAPS eradication programmes in the study area and South Africa include: raising sufficient funding from private and public sources, and introducing incentive systems to encourage higher collaboration and participation of private landowners in the currently primarily publicly driven IAPS eradication efforts. The study also suggests a number of policy and technological reforms to address the said challenges.

Impacts of alien plant invasions on water resources and yields from the Western Cape Water Supply System (WCWSS)

Article

Oct 2019
WATER SA

A key motivation for managing invasive alien plant (IAP) species is their impacts on streamflows, which, for the wetter half of South Africa, are about 970 m 3 •ha −1 •a −1 or 1 444 mill. m 3 •a −1 (2.9% of naturalised mean annual runoff), comparable to forest plantations. However, the implications of these reductions for the reliability of yields from large water supply systems are less well known. The impacts on yields from the WCWSS were modelled under three invasion scenarios: 'Baseline' invasions; increased invasions by 2045 under 'No management'; and under 'Effective control' (i.e. minimal invasions). Monthly streamflow reductions (SFRs) by invasions were simulated using the Pitman rainfall−runoff catchment model, with taxon-specific mean annual and low-flow SFR factors for dryland (upland) invasions and crop factors for riparian invasions. These streamflow reduction sequences were input into the WCWSS yield model and the model was run in stochastic mode for the three scenarios. The 98% assured total system yields were predicted to be ±580 million m 3 •a −1 under 'Effective control', compared with ±542 million m 3 •a −1 under 'Baseline' invasions and ±450 mill. m 3 •a −1 in 45 years' time with 'No management'. The 'Baseline' invasions already reduce the yield by 38 mill. m 3 •a −1 (two thirds of the capacity of the Wemmershoek Dam) and, in 45 years' time with no clearing, the reductions would increase to 130 mill. m 3 •a −1 (capacity of the Berg River Dam). Therefore IAP-related SFRs can have significant impacts on the yields of large, complex water supply systems. A key reason for this substantial impact on yields is that all the catchments in the WCWSS are invaded, and the invasions are increasing. Invasions also will cost more to clear in the future. So, the best option for all the water-users in the WCWSS is a combined effort to clear the catchments and protect their least expensive source of water.

Impacts of alien plant invasions on water resources and yields from the Western Cape Water Supply System (WCWSS)

Article

Full-text available

Oct 2019

A key motivation for managing invasive alien plant (IAP) species is their impacts on streamflows, which, for the wetter half of South Africa, are about 970 m3∙ha−1∙a−1 or 1 444 mill. m3∙a−1 (2.9% of naturalised mean annual runoff), comparable to forest plantations. However, the implications of these reductions for the reliability of yields from large water supply systems are less well known. The impacts on yields from the WCWSS were modelled under three invasion scenarios: ‘Baseline’ invasions; increased invasions by 2045 under ‘No management’; and under ‘Effective control’ (i.e. minimal invasions). Monthly streamflow reductions (SFRs) by invasions were simulated using the Pitman rainfall−runoff catchment model, with taxon-specific mean annual and low-flow SFR factors for dryland (upland) invasions and crop factors for riparian invasions. These streamflow reduction sequences were input into the WCWSS yield model and the model was run in stochastic mode for the three scenarios. The 98% assured total system yields were predicted to be ±580 million m3∙a−1 under ‘Effective control’, compared with ±542 million m3∙a−1 under ‘Baseline’ invasions and ±450 mill. m3∙a−1 in 45 years’ time with ‘No management’. The ‘Baseline’ invasions already reduce the yield by 38 mill. m3∙a−1 (two thirds of the capacity of the Wemmershoek Dam) and, in 45 years’ time with no clearing, the reductions would increase to 130 mill. m3∙a−1 (capacity of the Berg River Dam). Therefore IAP-related SFRs can have significant impacts on the yields of large, complex water supply systems. A key reason for this substantial impact on yields is that all the catchments in the WCWSS are invaded, and the invasions are increasing. Invasions also will cost more to clear in the future. So, the best option for all the water-users in the WCWSS is a combined effort to clear the catchments and protect their least expensive source of water.

How the Stabilizing Effect of Vegetation on a Slope Changes Over Time: A Review

Chapter

Apr 2014

The principles of slope stabilization through vegetation are well known but substantial uncertainty remains about its transient effects, for example that of a forest stand throughout its life cycle. This comprises direct impacts but also more indirect ones that influence soil development that can be important but also difficult to observe and quantify. Often these effects are ambiguous, having potentially a stabilizing or destabilizing influence on a slope under particular conditions (e.g., more structured soils leading to both rapid infiltration and drainage). The overall aim of this research is to review the changing (de-)stabilizing effect of forest stands on potentially unstable slopes. Here we focus on the combined hydrological and mechanical effect as a function of temporal forest stand dynamics. To this end, we reviewed the fragmented literature on the life cycle of stands with respect to their water use, or more specifically their role in the water cycle and with respect to the development of root strength, particularly root density, area and root strength. We highlight that the life cycle of a forest stand has clear influence on the stability of a slope. The review will also identify knowledge gaps and future research needs.

Sustained water yield in afforested catchments — the South African experience

Chapter

Jan 2000

South Africa is a relatively dry country, and is particularly dependent on limited areas of high-rainfall mountain catchments as reliable sources of streams and rivers. The natural vegetation within these catchments is mostly seasonally-dormant grassland or fynbos (sclerophyllous shrubland), but has largely been replaced by evergreen forest plantations during the course of this century. This change of land use has brought about declines in streamflow over large parts of the country, to the detriment of downstream users. Sustaining water yields from these catchments is a development imperative, in light of increasing demand for both water and forest products. This paper traces over six decades of research information, and the parallel development of legislation and policies, which has brought South Africa within sight of an effective integrated water resource management system. This experience may offer useful insights to the problem of sustaining water yields from afforested catchments in other water-scarce countries.

The economic consequences of the environmental impacts of alien plant invasions in South Africa

Chapter

May 2011

Plant invasion consequences on the functioning of fluvial riparian ecosystems

Article

Dec 2010

Marion Bottollier-Curtet

The objective of this work is to evaluate plant species origin importance for biological invasion consequences, through the analysis of the influence of native and exotic dominant plant species for ecosystem functioning. Five pairs of native dominant species (Agrostis stolonifera, Rubus caesius, Populus nigra, Urtica dioica et Salix alba) and exotic invasive species (Paspalum distichum, Fallopia japonica, Buddleja davidii, Impatiens glandulifera et Acer negundo) were compared for litter breakdown process and primary production. A more detailed analysis evaluates the consequences of A. negundo invasion for riparian forests. Our results indicate that the lack of coevolution between exotic species and the organisms of recipient areas has little implications for ecosystem functioning, even if exotic invasive species can be more efficient in some ecological processes. No general pattern can be drawn for functional consequences of native dominant species replacement by exotic invasive ones.

Measurement and modelling of evapotranspiration in three fynbos vegetation types

Article

Full-text available

Mar 2014
WATER SA

Many studies have investigated the water relations of indigenous plants in the fynbos shrublands of the Cape, South Africa. These have mainly focused on understanding the mechanisms by which individual plant species respond to droughts, the frequency and severity of which is expected to increase due to climate change. However, comparatively little information exists on the dynamics of water use by indigenous plants in the region, and, in particular, how water use varies seasonally and between sites. In this study we determined water use by 3 fynbos vegetation types growing at 4 different sites, namely: (i) lowland Atlantis Sand Plain fynbos growing on deep sandy soils, (ii) Kogelberg Sandstone fynbos growing in a riparian zone on deep alluvial soils, (iii) dryland Kogelberg Sandstone fynbos growing on shallow sandy soils at a montane site, and (iv) alluvial Swartland fynbos growing in clayey soils. Evapotranspiration (ET) was quantified at each site during specific periods using a boundary layer scintillometer and energy balance system. A simple dual source model in which the stand ET was calculated as the algebraic sum of outputs from soil evaporation and transpiration sub-models was used to scale up the ET measurements to annual values. The data showed large differences in ET depending on site characteristics and on plant attributes. Dense stands of riparian Sandstone Fynbos had an annual ET of 1 460 mm which exceeded the reference ET of 1 346 mm. Dryland Sandstone Fynbos used only 551 mm of water per year while the Sand Plain Fynbos’ annual ET was 1 031 mm, which was similar to the reference ET of 1 059 mm. We conclude that some indigenous plant species use large volumes of water which should be accounted for in, e.g., groundwater recharge estimates, and calculations of incremental water gains after clearing alien invasive plants, among other applications.

Measurement and modelling of evapotranspiration in three fynbos vegetation types

Article

Full-text available

Apr 2014

Sebinasi Dzikiti

ABSTRACT Many studies have investigated the water relations of indigenous plants in the fynbos shrublands of the Cape, South Africa. These have mainly focused on understanding the mechanisms by which individual plant species respond to droughts, the frequency and severity of which is expected to increase due to climate change. However, comparatively little information exists on the dynamics of water use by indigenous plants in the region, and, in particular, how water use varies seasonally and between sites. In this study we determined water use by 3 fynbos vegetation types growing at 4 different sites, namely: (i) lowland Atlantis Sand Plain fynbos growing on deep sandy soils, (ii) Kogelberg Sandstone fynbos growing in a riparian zone on deep alluvial soils, (iii) dryland Kogelberg Sandstone fynbos growing on shallow sandy soils at a montane site, and (iv) alluvial Swartland fynbos growing in clayey soils. Evapotranspiration (ET) was quantified at each site during specific periods using a boundary layer scintillometer and energy balance system. A simple dual source model in which the stand ET was calculated as the algebraic sum of outputs from soil evaporation and transpiration sub-models was used to scale up the ET measurements to annual values. The data showed large differences in ET depending on site characteristics and on plant attributes. Dense stands of riparian Sandstone Fynbos had an annual ET of 1 460 mm which exceeded the reference ET of 1 346 mm. Dryland Sandstone Fynbos used only 551 mm of water per year while the Sand Plain Fynbos’ annual ET was 1 031 mm, which was similar to the reference ET of 1 059 mm. We conclude that some indigenous plant species use large volumes of water which should be accounted for in, e.g., groundwater recharge estimates, and calculations of incremental water gains after clearing alien invasive plants, among other applications. Keywords: Evapotranspiration, fynbos, scintillometer, Western Cape

Evaluation of Mechanical Effect of Vegetation on Slope Stability in Terms of Root Evolution

Article

Apr 2012

Vegetation is planted to reduce probability of slope failure for its hydrological and mechanical effects. Root reinforcement provides additional cohesion to soil to protect slope from mass movement. The interaction between roots and soil in terms of root morphology was understood and quantified well with in situ and ex situ experiments and numerical models, especially for soil core samples. However, root morphology such as root density, root depth, root tensile strength and root orientation, which are the basis of the additional cohesion for soil, varies gradually during the life of vegetation. Furthermore, entire stand structure also changes because of individual growing that would affect root morphology within a stand. As a consequence, the mechanical effect of vegetation on slope stability should be investigated at different life stages at stand scale. The objective of this research is to understand and quantify the dynamic stabilizing mechanical effect of forest on potentially unstable slopes. To achieve this, we focus on the changes of slope stability with regard to variation in root morphology over the life cycle of stand. First, root evolutes as a result of stand density decrease with competition of bigger individual tree. Second, gaps get larger between them which could not protect soil loss. Root density, root depth and root tensile strength varies to meet the physiological demands of various growth stage of a forest stand. A spatially distributed, physical-based, dynamical model is employed to calculate the safety factor and probability of failure in a catchment on a daily time scale with root system dynamic evolution as a parameter. The results of the quantification of the mechanical effect of vegetation show there is a significantly influence by temporal and spatial distribution of stand root system and this provides reference for reliable management strategies at the scale of forest stands. Key words: slope stability, mechanical effect, root, vegetation

Guidelines for improved management of riparian zones invaded by alien plants in South Africa

Article

Full-text available

Jul 2008
S AFR J BOT

This paper reviews the results of recent research on riparian vegetation recovery following the clearance of invasive alien plants. In Fynbos, Grassland and Savanna Biomes, riparian ecosystems were found to have relatively-high ecological resilience to invasion by alien plants, except in some situations of closed alien stands (75–100% aerial cover). Where alien invasion is the primary disturbance at a site, and invasion intensity is low (

Linking research to management: Restoring fynbos riparian vegetation following alien plant invasion

Article

Full-text available

Apr 2008
S AFR J BOT

Changing Perspectives on Managing Biological Invasions: Insights from South Africa and the Working for Water programme

Chapter

Nov 2010

IntroductionBiological Invasions in South AfricaHistoric Approaches to Invasive Alien Plant ManagementA New Approach: The Birth of Working for WaterApproaches Adopted by Working for WaterFactors Promoting SuccessRemaining ProblemsConclusions AcknowledgementsReferences

The Economic Consequences of Alien Plant Invasions: Examples of Impacts and Approaches to Sustainable Management in South Africa

Article

Jun 2001

The invasion of natural ecosystems by alien plants is a serious environmental problem that threatens the sustainable use of benefits derived from such ecosytems. Most past studies in this field have focussed on the history, ecology and management of invasive alien species, and little work has been done on the economic aspects and consequences of invasions. This paper reviews what is known of the economic consequences of alien plant invasions in South Africa. These economic arguments have been used to successfully launch the largest environmental management programme in Africa. Ten million hectares of South Africa has been invaded by 180 alien species, but their impacts are not fully understood, although they are undoubtedly significant. The indications are that the total costs of these impacts are substantial. Selected studies show that invasions have reduced the value of fynbos ecosystems by over US$ 11.75 billion; that the total cost of invasion would be about US$ 3.2 billion on the Agulhas Plain alone; that the net present cost of invasion by black wattles amounts to US$ 1.4 billion; that invasions by red water fern have cost US$ 58 million; and that the cost to clear the alien plant invasions in South Africa is around US$ 1.2 billion. These few examples indicate that the economic consequences of invasions are huge. One of the unique aspects of invasive plant control programmes in South Africa has been the ability to leverage further benefits (mainly through employment) for the expensive control programmes from the government's poverty relief budget. This has made it possible to allocate substantial funding to a programme that would otherwise have struggled to obtain significant support. Biological control of invasive species also offers considerable benefits, but is often the subject of debate. We believe that, at least in the case of many invasive alien plant species in South Africa, biological control offers one of the best, and most cost-effective, interventions for addressing the problem.

Invasive Plants and Water Resources in the Western Cape Province, South Africa: Modelling the Consequences of a Lack of Management

Article

Feb 1996

1. The invasion of fynbos shrublands by woody weed species can reduce the water yield from catchment areas dramatically. We modelled the consequences of uncontrolled invasion on water yield using a geographical information system (Arc/Info). 2. Five important processes were recognized: the occurrence of fire; the spread and establishment of alien plants after fire; rainfall-to-run-off ratios; growth and changes in biomass between fires; and effects of these changes on streamflow. 3. The simulations of water yield were modelled with the Arc/Info GRID module using a 200 x 200-m grid. It was assumed that the interval between fires was 15 years and that proliferation and dispersal of alien plants took place only after fires. 4. Between fires, the model simulated the growth of the vegetation and its effects on streamflow, using relationships between rainfall and run-off, and run-off and above-ground biomass. 5. Results for the Kogelberg area in the Western Cape Province showed that alien plants invaded about 40% of the grid cells within 50 years. Cover of alien plants increased from an initial estimate of 2.4% to 62.4% after 100 years. 6. Invasion of catchment areas would result in an average decrease of 347 m3 of water per hectare per year over 100 years, resulting in average losses of more than 30% of the water supply to the city of Cape Town. In individual years, where large areas would be covered by mature trees, losses would be much greater. 7. In addition, invasion of fynbos by alien plants will cause the extinction of many plant species, increase the intensity of fires, destabilize catchment areas with resultant erosion and diminished water quality, and decrease the aesthetic appeal of mountain areas. 8. Control of alien weed species is necessary to avert the above impacts, and the costs of control operations could be justified by the savings achieved in maintaining adequate water run-off from stable catchments in the long term.

A Field Demonstration of the Effect on Streamflow of Clearing Invasive Pine and Wattle Trees from a Riparian Zone

Article

Jul 1995

Two portable weirs were used to quantify changes in streamflow following clearfelling of a dense stand of self-sown Pinus patula and Acacia mearnsii along a riparian zone on Kalmoesfontein, a SAPPI forest plantation south-east of Lydenburg, Eastern Transvaal. The weirs were set up at positions 500 m apart on the same stream, and used to monitor streamflow levels before and after the clearing of all trees between the weirs to an average distance of 25 m from the stream. Analysis of streamflow data revealed that the clearing operation resulted in a 120% increase in streamflow at the lower weir, equivalent to 30,5 m3 of water per day. Streamflow at the lower weir was less than at the upper weir before clearfelling, but equalled that at the upper weir after clearfelling. This demonstrates that the intervening trees were responsible for the initial difference in streamflow. Two further lines of evidence point to the adjacent trees exerting a strong influence on streamflow. Firstly, a clear daily fluctuation in streamflow was evident at both weirs. This is a consequence of transpiration by trees taking place only during hours of daylight. Secondly, the occurrence of cloudy, rain-free weather led to an increase in streamflow. Such weather reduces the evaporative demand of the air, causing transpiration rates to decline as well.We conclude that invasive exotic trees should be removed from riparian zones to promote significant streamflow increases from afforested catchments. There is a need for a broader body of information to assess the effects of species, density and age distribution, as well as catchment characteristics, on streamflow responses to clearing invasive trees. This study has demonstrated the suitability of the portable weir technique in capturing such information.

THE EFFECT OF RIPARIAN VEGETATION ON WATERLOSS FROM AN IRRIGATION FURROW AT JONKERSHOEK

Article

Jan 1955

H. B. Rycroft M.Sc., B.Sc.(For.), Ph.D

Water-losses from the soil and streams are caused by several factors, the most important of which is probably transpiration, especially during periods of hot, dry weather. This paper describes these losses and, as a result of an experiment carried out at Jonkershoek, suggests that water-loss can be reduced by removing the riparian vegetation during periods of acute water shortage.

Diurnal fluctuations in Jonkershoek streams due to evaporation and transpiration

Article

Jan 1941

C. L. WICHT

Water-use by Riparian Vegetation at Cathedral Peak

Article

Mar 1972

U. W. Nänni

The technique applied in measuring streamflow with small portable weirs is described. In an experiment, two such weirs were placed in series in a stream. After a period of calibration, the riparian vegetation between the two weirs was removed. Vapour losses were reduced after treatment and an increase in streamflow was recorded. These changes were presumably caused by the treatment.The need to repeat this type of experiment before results can be considered reliable, is emphasised.

The dependence of storm flows on rainfall intensity and vegetal cover in South Africa

Article

Dec 1984
J HYDROL

Analysis of 1546 storm flows from eight small basins in South Africa provides new evidence that hourly rainfall intensity, and consequently overland flow, play a minor role in flood production in many source areas. Regressed against gross storm rainfall and maximum hourly rainfall intensities, storm-flow volumes showed: (1) decreasing sensitivity to rain intensity on high response basins; and (2) decreasing sensitivity as rainstorm size increases on the same basin. In other words, small channel-source storm flows respond to hourly intensity, but large storm flows from expanded source areas do not. Afforestation of a 195-ha basin with Pinus patula produced a detectable but unimportant decrease in storm flows and peak flows, while periodic burning of grass veld (Themeda grasses) showed no detectable change in storm flows or peak flows. Results are consistent with previous analyses of actual data from a number of drainage basins large enough to contain perennial streams.

The water yield gains obtained from clear felling riparian zone vegetation

D F Scott

Streamflow responses to the clearing of alien invasive trees from riparian zones at three sites in the Western Cape Province

Abstract

No full-text available

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