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Determining optimal clearing treatments for the alien invasive shrub Acacia saligna in the southwestern Cape, South Africa
Abstract
The results of a replicated field-scale trial of a range of control treatments for Acacia saligna were used to create a probabilistic model of the effects of repeated treatment applications. Using estimates of costs per plant treated for each treatment and for each of four size classes of plants, it was possible to compute the costs for every treatment combination. A Markov-chain model was created integrating all the transition probabilities and costing factors. A computerised system allows for the input of different initial densities and size-class distributions, different target population specifications, the variable use of fire during the control programme and different programme durations. The system allows for the a priori determination of optimal control treatment combinations and sequences and provides managers with forward estimates of budgetary requirements for control programmes. The approach developed can be applied to the control of other invader plants by modifying the transition probabilities and costing factors.
... Herbicide spraying is sprayed on recent sprouts or in areas with high germination rates [35,36] Stem injection Herbicide is applied directly to the vascular system of the adult plants [39] Biological control Natural enemies (e.g., insects and fungi) from the regions of origin of the acacia or herbivores are employed to reduce its viability or reproductive potential [40,41] Prescribed fire Prescribed fire is used to reduce the seed supply in the soil and eliminate the germinated plants [42] Mechanical and chemical control Cut stump method Acacia trunk is cut followed by herbicide application [39,43] Mechanical and biological Plants are cut and herbivores control their regrowth [44,45] Mechanical and prescribed fire Acacia is cut and followed by prescribed fire use [42] Despite the available variety of control methods and some knowledge of their advantages and disadvantages, in Portugal, few projects are devoted to controlling these IASs, as seen in the low number of applications to the Rural Development Program (RDP2020) [46]. In addition, only two of the mechanical treatments to control acacia (cut and cut followed by herbicide) are supported by the Portuguese Recovery and Resilience Plan (RRP) [47]. ...
... Herbicide spraying is sprayed on recent sprouts or in areas with high germination rates [35,36] Stem injection Herbicide is applied directly to the vascular system of the adult plants [39] Biological control Natural enemies (e.g., insects and fungi) from the regions of origin of the acacia or herbivores are employed to reduce its viability or reproductive potential [40,41] Prescribed fire Prescribed fire is used to reduce the seed supply in the soil and eliminate the germinated plants [42] Mechanical and chemical control Cut stump method Acacia trunk is cut followed by herbicide application [39,43] Mechanical and biological Plants are cut and herbivores control their regrowth [44,45] Mechanical and prescribed fire Acacia is cut and followed by prescribed fire use [42] Despite the available variety of control methods and some knowledge of their advantages and disadvantages, in Portugal, few projects are devoted to controlling these IASs, as seen in the low number of applications to the Rural Development Program (RDP2020) [46]. In addition, only two of the mechanical treatments to control acacia (cut and cut followed by herbicide) are supported by the Portuguese Recovery and Resilience Plan (RRP) [47]. ...
... Due to the abundant acacia seeds in the soil, the emergence of new shoots requires repeated control treatments for these plants and those that survived the previous control [43]. Respondents also recognize this challenge when assessing the sustainability of the different methods [35][36][37][38][39][40][41][42][43][44][45] and when referring to the need to increase the timespan for the funded projects. ...
The global rise in issues related to the invasion of alien tree species is becoming increasingly prominent. Genus Acacia is one of the most pervasive invaders among the non-native flora introduced to Portugal over the centuries. This research aimed to pinpoint the key players engaged in its management, their expertise, and the methodologies applied to control acacias or decrease affected regions. Data were gathered via an online survey and subjected to diverse statistical analyses. Numerous stakeholders have emerged in recent decades, employing various methods to fight acacia invasion. Mechanical, chemical, and combined methods are the most frequently applied. The findings highlight the need for continued funding for long-term acacia control in the same invaded areas, increased training actions and best practices demonstration in the field and increased public awareness and engagement in local communities and the general public.
... After fire, seeds can successfully germinate from soil depths up to 10 cm, quickly outcompeting indigenous re-growth, and increasing the density of the acacia stand (Holmes and Cowling, 1997;Strydom et al., 2012). A. saligna also has the ability to re-sprout if cut, from auxiliary buds left on the stem or from a meristematic zone where the stem transitions to the tap root, the 'coppicing zone' (MacDonald and Wissel, 1992;Love et al., 2009). The combination of impacts from a dense A. saligna invasion can cause major shifts in both ecosystem structure and function, which can be challenging to reverse (Yelenik et al., 2004;Hobbs et al., 2009). ...
... Efforts have been made to improve our understanding of the invasion process of A. saligna in fynbos and to determine effective removal methods while considering natural vegetation recovery in relation to diversity and structure (Arim et al., 2006;Le Maître et al., 2011). Removal of mature stands is usually accomplished by cutting and applying herbicide to cut wounds, or by foliar application of herbicide (MacDonald and Wissel, 1992). Mere removal of the invasive trees, however, does not ensure that indigenous vegetation will recover and does not address the long-lived alien seed banks remaining in the soil (Holmes et al., 1987;Zavaleta et al., 2001). ...
... This may account for large variation in the results. This is not listed as an issue in a previous study, which was limited to the removal of mature, large stumped, A. saligna only at a much lower density (MacDonald and Wissel, 1992). The hand-held sprayers used to apply herbicide to the cut stumps did not indicate calibrated quantity; therefore, the amount applied to each stump was inconsistent. ...
The Greater Cape Floristic Region (GCFR) of South Africa is a biodiversity hotspot threatened by the impacts of habitat transformation and invasive alien species. Cape Flats Sand Fynbos (CFSF) is a critically endangered vegetation type occurring within the GCFR, and its largest remaining fragment is the focus of a large-scale invasive plant control and biodiversity restoration project. Acacia saligna is a highly problematic invasive in CFSF and the main target of the control. To mitigate damage caused by this species, stands are removed and burned, which stimulates both the large invasive seed-bank and the indigenous seed-bank in the soil. Although there are no clear methods on how to manage the re-invasion at this stage without damaging indigenous plant recovery, three post-burn removal methods have the potential to be effective: (1) cutting the Acacia saplings below the coppicing point, (2) cutting the saplings and applying herbicide to the stumps, and (3) foliar herbicide spray. The aims of this study were to (i) find the most effective post-burn A. saligna control treatment, (ii) find the treatment that causes the least harm to indigenous plant recovery, (iii) determine the most cost-effective treatment, and (iv) establish which treatment is the most suitable for large-scale use. Cutting below the coppicing point of the A. saligna provided the most effective removal and was also the least damaging to indigenous vegetation recovery. The foliar spray treatment, however, saved the most time and costs. The best method is therefore dependent on the project goals, scale, and density of the A. saligna invasion. These results may be applicable to other types of fynbos and to other fire-stimulated invasive Acacia species.
... For the control of A. saligna in the South African fynbos, fire, mechanical clearing, chemical control, biological control and combinations of these methods have been employed (Mehta 2001). Nevertheless, owing to the persistent seed banks and the ability of treated plants to coppice, its control is complicated, and the efforts are taxing the authorities involved in the management of fynbos nature reserves (Macdonald and Wissel 1992). Yet, the cost of control programmes can be significantly reduced by utilising available research from other countries (Fowler et al. 2000), especially studies of similar cases of invasion. ...
The impact of the alien Acacia saligna invasion, in terms of species composition and structure, on the autochthonous halophytic and reed-bed communities in the Akrotiri salt marshes (Lemesos, Cyprus) was investigated. Densely invaded (canopy cover > 50%), lightly invaded (canopy cover < 50%) and uninvaded vegetation were sampled and results analysed.
Species composition and structure of the autochthonous communities are altered by A. saligna invasion. In lightly invaded vegetation the deterioration begins with displacement of the most sensitive salt marsh and salt meadow species and reduction of the abundance of the remaining species. In densely invaded vegetation the typical halophytes are eliminated and few wetland species survive, albeit with impoverished abundance and vigour. Thus, regarding composition and structure, the uninvaded communities are similar to the lightly invaded communities but entirely different from the densely invaded vegetation. Species with specialised habitat requirements are more vulnerable to the invasion than broad habitat range species, and annuals are more susceptible than perennial herbs and shrubs. Species richness declines significantly both at habitat-community and at plot level, and is negatively correlated to the A. saligna canopy cover. A. saligna with a combination of competitive and stress-tolerant characters has a high invasion potential and causes habitat loss and local extinction of wetland species, which have limited distribution in Cyprus. Fire and human disturbance are well correlated to A. saligna invasion and spread and the retreat of native wetland communities.
The high ecological value of the Akrotiri salt marshes as well as the scarcity of the coastal halophytic wetlands in Cyprus and the occurrence of threatened wetland species in the area necessitate immediate action to control the spread of A. saligna.
... Semi-arid lands occupy 12.2% of the terrestrial world and many have been significantly altered as a result of land use demands, herbivory, altered fire regimes, invasive species, environmental changes, and woodland expansion (FAO, 1989;UNSO/UNDP, 1997). Woodland expansion and infilling in grasslands, shrublands and savannahs are problematic due to reductions in herbage for livestock and wildlife habitat modification, which may be detrimental to wildlife populations and diversity (Brown and Archer, 1989;Burrows et al., 1990;MacDonald and Wissel, 1992;Van Auken, 2000;Davies et al., 2011). In North American, reduced fire frequency has been recognized as a main cause of woodland development (Burkhardt and Tisdale, 1976;Archer et al., 1988;Miller and Wigand, 1994;Miller and Rose, 1995). ...
... Semi-arid lands occupy 12.2% of the terrestrial world and many have been significantly altered as a result of land use demands, herbivory, altered fire regimes, invasive species, environmental changes, and woodland expansion (FAO, 1989;UNSO/UNDP, 1997). Woodland expansion and infilling in grasslands, shrublands and savannahs are problematic due to reductions in herbage for livestock and wildlife habitat modification, which may be detrimental to wildlife populations and diversity (Brown and Archer, 1989;Burrows et al., 1990;MacDonald and Wissel, 1992;Van Auken, 2000;Davies et al., 2011). In North American, reduced fire frequency has been recognized as a main cause of woodland development (Burkhardt and Tisdale, 1976;Archer et al., 1988;Miller and Wigand, 1994;Miller and Rose, 1995). ...
... Woodland ecosystems have undergone substantial change in most regions of the world as a result of land use demands, herbivore effects, altered fire regimes, invasive species and climate change. Woodlands are categorised as those being reduced or degraded (Zerihun and Backleus 1991;Yates and Hobbs 1997;Bucher and Huszar 1999;Angassa and Baars 2000;Breshears et al. 2005) and those that have expanded in range and in-filled (Brown and Archer 1989;Macdonald and Wissel 1992;Miller and Rose 1995;Holmes and Cowling 1997;Van Auken 2000;Ansley et al. 2001). Reduced fire frequency is recognised as a main causative agent for woodland expansion in North American woodlands (Brown and Archer 1989;Archer 1994;Miller and Wigand 1994). ...
Woodland ecosystems of the world have been changed by land use demands, altered fire regimes, invasive species and climate change. Reduced fire frequency is recognised as a main causative agent for Pinus-Juniperus L. (pinon-juniper) expansion in North American woodlands. Pinon-juniper control measures, including prescribed fire, are increasingly employed to restore sagebrush steppe communities. We compared vegetation recovery following prescribed fire on Phase 2 (mid-succession) and Phase 3 (late-succession) Juniperus occidentalis Hook. (western juniper) woodlands in Oregon. The herbaceous layer on Phase 2 sites was comprised of native perennial and annual vegetation before and after fire. On Phase 3 sites the herbaceous layer shifted from native species to dominance by invasive Bromus tectorum L. (cheatgrass). After fire, shrubs on Phase 2 sites were comprised of sprouting species and Ceanothus velutinus Dougl. (snowbrush). On Phase 3 woodland sites the shrub layer was dominated by C. velutinus. The results suggest that Phase 2 sites have a greater likelihood of recovery to native vegetation after fire and indicate that sites transitioning from Phase 2 to Phase 3 woodlands cross a recovery threshold where there is a greater potential for invasive weeds, rather than native vegetation, to dominate after fire.
... Woodland ecosystems have undergone substantial change in most regions of the world as a result of land use demands, herbivore effects, altered fire regimes, invasive species and climate change. Woodlands are categorised as those being reduced or degraded (Zerihun and Backleus 1991;Yates and Hobbs 1997;Bucher and Huszar 1999;Angassa and Baars 2000;Breshears et al. 2005) and those that have expanded in range and in-filled (Brown and Archer 1989;Macdonald and Wissel 1992;Miller and Rose 1995;Holmes and Cowling 1997;Van Auken 2000;Ansley et al. 2001). Reduced fire frequency is recognised as a main causative agent for woodland expansion in North American woodlands (Brown and Archer 1989;Archer 1994;Miller and Wigand 1994). ...
Background/Question/Methods
Piñon-juniper (Pinus-Juniperus L.) woodlands have expanded into big sagebrush (Artemisia tridentata Beetle) steppe of the western United States primarily as a result of reduced fire disturbances. Prescribed fire in post-settlement piñon-juniper woodlands has been increasingly employed to restore big sagebrush steppe plant communities. We compared vegetation recovery following cutting-prescribed fire treatments on Phase 2 (mid expansion) and Phase 3 (late expansion) western juniper (Juniperus occidentalis Hook.) woodlands on Steens Mountain, Oregon. In Phase II woodlands, trees co-dominate with shrub and herbaceous plants, and in Phase III woodlands trees are dominant and shrubs and herbaceous layers are much reduced. Twelve Phase 2 and nine Phase 3, 0.63 ha plots were established in May 2003 and all study sites were located in Mountain big sagebrush/Idaho fescue (A.t spp. vaseyana /Festuca idahoensis Elmer) associations. Pre and post-fire tree, shrub and herbaceous cover and density were collected over a seven year period. Repeated measures analysis of variance for a completely randomized design was used to test for year, woodland phase, and year by phase interaction for vegetation response variables.
Results/Conclusions
Herbaceous vegetation on Phase II woodland sites was comprised of native perennial and annual vegetation before and after fire. Herbaceous vegetation in Phase 3 woodlands shifted from native species to dominance by cheatgrass (Bromus tectorum L.) and other invasive weeds after fire. Shrubs on Phase 2 sites were comprised of sprouting species and snowbrush (Ceanothus velutinus Dougl.) after fire. The shrub component on Phase 3 sites was dominated by snowbrush following fire. The results indicate that woodland phase influences post-fire vegetation recovery and composition. This suggests that sagebrush steppe in Phase 1 or 2 woodland expansion stages are more likely to recover with their native vegetation components intact than Phase 3 woodland sites. The results also indicate that sites transitioning from Phase 2 to Phase 3 woodlands have crossed a recovery threshold where there is a greater potential for invasive weeds to dominate rather than native vegetation after fire. Phase 3 woodlands that are burned by wildfire or prescribed fire are likely to require additional inputs, primarily seeding and weed control, for vegetation recovery goals to be accomplished. Managers should consider applying alternative treatments in Phase 3 woodlands that would potentially improve sagebrush steppe recovery such as cutting or cutting followed by winter/spring burning.
... The encroachment of woody plants is a major threat to range resources at a global scale, reducing grassland area and carrying capacity, and inhibiting livestock movement (Burkhardt and Tisdale 1976, Strang and Parminter 1980, Gruell 1983, MacDonald and Wissel 1991, Richardson and Bond 1991. Fire suppression, disturbance, climatic variation, and interactions of these factors in the last 100 to 150 years are thought to be responsible for tree encroachment (Tisdale 1950, Parminter 1978, Strang and Parminter 1980, Arno and Gruell 1986, Mast et al. 1998). ...
Grasslands of interior British Columbia are being encroached upon by Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) France) and ponderosa pine (Pinus ponderosa Dougl.). A pot experiment placed in the field was conducted to determine the effect of forest and grassland seedbeds on seedling emergence and early establishment of the 2 species with 2 seed collections each. For these seedbeds, structural characteristics were evaluated and the effect of seedbeds water extracts on seed germination was determined. Seedling emergence of both species was significantly reduced by Douglas-fir needles and enhanced by fescue litter and cattle manure compared to mineral soil, The rate of emergence was reduced by Douglas-fir needles and sagebrush litter, and for some collections, by ponderosa pine needles, but was always enhanced by manure compared to mineral soil. Seedling survival was generally not affected by seedbeds. Douglas-fir seedlings emerging earlier in the season survived better, and both Douglas-fir and ponderosa pine seedlings emerging earlier lived longer than these emerging later. Seed germination of ponderosa pine was not affected by the water extract while that of Douglas-fir was reduced by the water extract from sagebrush litter. Therefore, differences in seedling emergence of the 2 species among seedbeds were related more to structural than to chemical characteristics of seedbeds. Successful establishment of the 2 species in grasslands within this region likely relies on the ability of seeds to germinate early in the growing season on seedbeds in which soil moisture is conserved, as summer droughts are severe.
... Evidence indicates that terrestrial ecosystems invaded by Acacia spp. are not easily restored to their natural state, resulting in high maintenance costs (Macdonald and Wissel 1992; Le Maitre et al. 2011; Marchante et al. 2011). The highly invasive characteristics of A. mangium in coastal savannas and forest boundary regions in French Guiana have resulted in recommendations that this species be restricted, or even phased out, throughout the country (Delnatte and Meyer 2012). ...
... At a site north of Pella, which was largely uninvaded in 1960, A. saligna had by 1984 invaded 155 of 256 ha, of which 13 ha were at densities of over 1000 trees ha ¡1 (Beeston, 1985). A density of 20,769 trees ha ¡1 in the worst invaded areas at this site was recorded in 1984 (Macdonald and Wissel, 1992). Milton and Siegfried (1981) recorded the density of four stands as 3200, 3600, 8530 and 45,200 trees ha ¡1 . ...
Acacia saligna was regarded as the most troublesome invasive alien weed in the Cape Floristic region. The gall-forming rust fungus Uromycladium tepperianum was introduced into South Africa in 1987, and it established throughout the range of the weed. Five populations of the weed were monitored annually from 1991 to 2005. Tree density and stem diameter were recorded in each of four permanent transects at each site. Tree density declined by between 87% and 98% for the period 1991–2005 at these five sites. Average annual mortality rate (±SE) of infected trees dying was 18% (±2%) during the same period. Canopy mass and age were determined by destructively sampling trees at 19 sites. Canopy mass at all sites was lower than data published before introduction of U. tepperianum, and the longer since the last major disturbance (fires or clearing operations), the greater the reduction. Average age of trees in these sites was between 2.2 and 6.1 years, despite the last major disturbance being at least 10–20 years previous at 11 of the sites. Pod and seed production were determined, and seed fall calculated, at each of four sites in 2004. These data were compared to the same variables gathered in 1989. Seed fall in 2004 was reduced compared to that recorded earlier. Results show that U. tepperianum remains a highly effective biological control agent against A. saligna in South Africa.
... 'lindleyi'. Identification of the majority of the sampled A. saligna in SA as the Eastern 'saligna' entity suggests that eradication will require application of arborcide to seedling and coppice regeneration after felling or burning of mature individuals, a control method currently being employed in South Africa (Macdonald and Wissel 1992). ...
Acacia saligna (Labill.) H.L.Wendl. is a species complex with an extensive history of anthropogenic utilisation and distribution. The taxon is naturalised and invasive in many countries. Extensive morphological variation makes accurate taxonomic identification of populations difficult. We used population genetic analysis to determine the biogeographic origins of 12 naturalised populations sampled from throughout south-eastern South Australia and assess the mode of reproduction (seedling or root suckering) at sites with active recruitment. Ten naturalised populations were assigned to Eastern ‘saligna’, although some also showed a lesser degree of affinity with other entities. A single population was assigned to Western ‘saligna’, but showed some affinity with Eastern ‘saligna’, and one population assigned to subsp. ‘lindleyi’ showed some affinity with Northern ‘lindleyi’. These assignments suggest that although several genetic entities of A. saligna are represented in South Australia, the majority of germplasm has originated from native populations of Eastern ‘saligna’ located around Esperance on Western Australia’s southern coast. Genetic diversity is limited in naturalised A. saligna compared with that present in the native range, suggesting a restricted number of historical introductions. Reproduction is predominantly by seedling recruitment, as opposed to clonal reproduction.
... Los mayores cambios en la cobertura de tipos de vegetación ocurrió en elevaciones medias entre 700 y 1,000 m. Los planes de manejo enfocados al control de la expansión de los arboles y la contracción de los bosques debe tomar en consideración estos factores topográficos rying capacity, and inhibiting livestock movement (Burkhardt and Tisdale 1976, Strang and Parminter 1980, Gruell 1983, MacDonald and Wissel 1991, Richardson and Bond 1991. Similar impact on wildlife habitats has also been reported (Kazmaier et al. 2001). ...
Los pastizales en el interior de Columbia Británica en Canadá están adyacentes a los bosques y son susceptibles a la invasión por árboles. El apacentamiento, la supresión del fuego y la variabilidad climática están entre los factores que afectan la dinámica de la vegetación en el ecotono entre el pastizal y el bosque, pero los factores topográficos, tales como la exposición de la pendiente, el grado de pendiente y la elevación pueden interactuar con estos factores y resultar en cambios no uniformes de vegetación entre los elementos del paisaje. Se seleccionaron nueve sitios en la región boscosa de Cariboo/Chilcotin de Columbia Británica, con un total aproximado de 50,000 ha de pastizal y bosque, para estudiar los efectos de la exposición de la pendiente, grado de pendiente y elevación en la distribución de la vegetación, dinámica y expansión del bosque de la década de 1960 a la de 1990. Mapas de las décadas de 1960 y 1990 fueron generados usando fotografías aéreas y sobreponiendolas con capas de GIS que incluían la exposición, la pendiente y la elevación. La clasificación del pastizal abierto, pastizal arbolado, bosque abierto y bosque cerrado se basó en el porcentaje de cobertura de especies de coníferas variando de 0–5%, 5–15%, 15–35% y $35%, respectivamente. Se desarrolló un índice de probabilidad (valor-P) para evaluar los efectos de la exposición, pendiente y elevación en la distribución de la vegetación, dinámica y expansión del bosque basado en la distribución y áreas cambiadas. Los resultados muestran que el pastizal abierto ocurrió en la exposición sur y el cambio hacia pastizal arbolado ocurrió principalmente en estas exposiciones, la probabilidad de un cambio de vegetación de pastizal abierto a pastizal arbolado disminuye con el aumento del grado de pendiente, probablemente debido al régimen de humedad menos favorable en las pendientes pronunciadas. Los pastizales arbolados también cambiaron a bosque abierto en las pendientes con exposición sur y sitios más planos. En contraste, el bosque cerrado ocurrió más frecuentemente en las pendiente con exposición norte y el cambio de bosque abierto a bosque cerrado es más probable a que ocurra aquí. Los mayores cambios en la cobertura de tipos de vegetación ocurrió en elevaciones medias entre 700 y 1,000 m. Los planes de manejo enfocados al control de la expansión de los arboles y la contracción de los bosques debe tomar en consideración estos factores topográficos
... Elements hindering effective management include funding insecurity, the shortage of adequate management capacity (uncertain mandates and uncoordinated management), and weak operational management and monitoring (especially due to the lack of detailed and up-to-date data on the distribution of IAPs). Despite past research efforts to determine strategies for control operations in particular localities (e.g. Macdonald and Wissel, 1992), no objective framework is in place for identifying priorities, and for scheduling and monitoring of IAP control operations. Innovative ways of improving the efficiency of control operations are needed. ...
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.
... However, their use in restoration must be carefully evaluated considering their effect on microhabitat conditions and other aspects of their ecology, particularly the risk of using alien species (Ewel and Putz, 2004). Outside their original distribution area, both A. salicina and E. occidentalis may trigger unwanted changes in community composition and ecosystem function (Macdonald and Wissel, 1992;Nel et al., 2004). But the introduction of A. salicina can be of interest for various reasons, including the production of fodder, fuelwood, furniture and tannins, and the positive effects on soil and vegetation found in our study. ...
Despite low growth rates, plants in arid areas have a strong ability to modify soil surface properties affecting ecosystem processes and community dynamics. But our knowledge on species effects on soil properties in these areas comes largely from observational studies, increasing the risk of confounding factors and precluding estimations of rates of change. We evaluated changes in soil surface properties underneath Acacia salicina, Pinus halepensis and Eucalyptus occidentalis in a 10-year-old common garden experiment established on a degraded Stipa tenacissima steppe in southern Tunisia. The three species tested improved soil properties compared to those of open areas. Acacia salicina ranked first as soil modifier as the soil underneath this species showed higher total organic carbon, total nitrogen, available phosphorus, soil CO2 efflux and infiltration rate, and lower soil hydrophobicity than soil in open areas. The richness of vascular plants was higher under A. salicina than under the other types of cover. This species showed higher ability to improve microsite conditions and foster succession. Short rotations of A. salicina could thus be employed for the restoration of degraded S. tenacissima steppes provided that other aspects of its ecology are controlled. Pinus halepensis represents a good alternative when native species are a priority, albeit facilitative ability is lower.
The decrease in fire activity has been recognized as a main cause of expansion of North American woodlands. Piñon-juniper habitat in the western United States has expanded in area nearly 10-fold since the late 1800s. Woodland control measures using chainsaws, heavy equipment, and prescribed fire are used to restore sagebrush steppe plant communities. We compared vegetation recovery following cutting and prescribed fire on three sites in late Phase 2 (mid succession) and Phase 3 (late succession) western juniper (Juniperus occidentalis Hook.) woodlands in southeast Oregon. Treatments were partial cutting followed by fall broadcast burning (SEP); clear-cut and leave (CUT); and clear-cut and burn in early winter (JAN), late winter (MAR), and spring (APR); and untreated controls. Cover and density of herbaceous, shrub, and tree layers were measured. Five years after treatment, perennial bunchgrasses dominated two sites and co-dominated, with invasive annual grasses, at one site. Except for Sandberg blue-grass (Poa secunda J. Presl), cover and density of bunchgrasses, perennial and annual forbs, and annual grasses increased following treatments at all three sites and were greater than in controls. At each site, shrub, herbaceous, and ground cover response variables equalized or had begun to converge among treatments during the fourth or fifth year following application. SEP and APR treatments were mostly effective at reducing fuel sizes up to and including 1000-hr fuels while JAN and MAR treatments only consumed 1-hr and 10-hr fuels. Winter burning treatments (JAN, MAR) and the CUT treatments did not kill small junipers and seedlings and require additional tree control for sites to fully recover to functional sagebrush-herbaceous plant communities. The results demonstrate that juniper treatments are needed to recover sagebrush steppe plant communities.
Rehwasion of mechanically disturbed juniper communities is possible through contributions from the soil seedbank, seed rain, and the juvenile seedling bank. We compared spatial distribution of the seedbank and seed rain of undisturbed communities to sites where trees were deliberately left as single trees, small mottes of less than 5 trees per group, or large mottes of 5-10 trees per group. Seed density in the litter layer ranged from 1,197 to 1,436 seeds m-' and in the soil layer from 318 to 617 seeds m-*. Seed rain ranged from 275 to 366 seeds m-* over all tree arrangements. The treatment associated with single trees caused the litter layer to be removed resulting in the removal of that portion of the seedbank, consequently most seeds @SO%) were found under the canopy of mature, seed-producing trees. Soil disturbance was less severe in small and large motte arrangements, so only 65% of the soil seed bank was under mature trees. In undisturbed communities, the seed population was distributed evenly under tree canopies and in interspaces. Viability and germinability within the seedbank were low (4% and 0%, respectively). Viability of new seed was 47% and ger-minabiity was approximately 5%. The juvenile seedling bank contained a sufhcient number of seedlings (408 seedlings ha-') for ashe juniper to regain dominance on the site through growth. There was no advantage to any spatial pattern of tree distribu-tion in terms of invasive potential when fewer than 10 trees ha-' were left on a site. However, when 20-50 trees ha-' are left on a site, tree spatial arrangement has a significant effect on rehrva-sion rates.
Acacia mearnsii is an important invasive plant in most of the higher rainfall areas in South Africa. The production of long-lived water-impermeable seeds results in the establishment of large soil-stored seed banks. One way of controlling these seed banks is by felling the plants and burning them to destroy or to stimulate seeds to germinate, so that the resulting seedlings may be controlled. In order to save costs, the hypothesis that the plant population can be burned without felling beforehand, was investigated. A wildfire that swept through a standing population of A. mearnsii plants provided an opportunity to investigate the effects on the population. The mortality of the pre-fire population, the subsequent establishment of seedlings, mortality of seedlings and the spread of the population after the fire were investigated. It was found that the above-ground population increased by a factor of ca. 21,8 due to coppicing of young plants and the mean distance of spread from the edge of an infestation was ca. 20 m. About 90% of seedlings that established, died within two years. Due to the growth of the population, and the dificulty of controlling coppicing plants, a standing burn is not recommended as a control measure for A. mearnsii.
Background: No studies have examined the invasion of exotic species used for forestry purposes in the savannas of the Brazilian Amazonia.
Aims: We investigated the invasion process of Acacia mangium in savanna areas adjacent to large-scale forestry plantations in north-eastern Roraima State, Brazilian Amazonia.
Methods: A tree inventory to record the presence of all A. mangium and native tree individuals was carried out in of 14 plots (each 50 m in width and 1500 m in length) established at five plantation sites. Biometric measurements were taken for all individuals to identify their structure and maturity. Distance categories were created for determining frequency of occurrence in 100 m sections along the plots for all individuals. Correlations and goodness-of-fit tests for discrete data ordered in categories were applied to verify the
occurrence of A. mangium plants in relation to distance from the plantation.
Results: Individuals of A. mangium were dispersed up to 900 m from the plantation edge 8 to 9 years after the plantation was established. Although most recorded individuals were in the juvenile stage, reproductive adults were found in two establishment patterns: non-nucleated and nucleated under native tree species. Crown cover of the savanna’s most abundant native tree species facilitated the regeneration of A. mangium.
Conclusions: Planting of A. mangium in Amazonian savannas provides a source of continuous dispersal and invasion by the species is facilitated by environmental conditions.
The costs of the initial clearing of mixed stands of the alien Acacia cyclops and A. saligna invading fynbos vegetation were compared in a replicated trial on the Pella Fynbos Research Site. Costs were estimated using a regression-based model of the individual cost components. Felling was the cheapest clearing method, with felling plus the stripping of bark from the cut stump, second. Applying arboricides to the cut stump generally doubled the cost relative to felling. Mattocking was the most expensive method with ringbarking second. The estimated cost of clearing a moderate infestation on the Pella site ranged from R26 to R286 ha−1 for the different treatments. Similarly, estimations for an ‘average’ western Cape Acacia thicket ranged from R526 to R2 408 ha−1 while those for the densest infestations encountered on Pella ranged from R299 to R5 170 ha−1. The precision of the model is thought to be low at higher densities. Variations in the density and height of the fynbos and the spatial distribution, size class distribution and growth form of the Acacia plants are all thought to affect clearing costs.
Acacia cyclops is an invasive shrub of natural vegetation in the fynbos biome of South Africa. Its large soil-stored seed banks make eradication difficult. In this study, a dense, mature stand of A. cyclops was cleared using four commonly applied treatments. Few seeds above 30-mm depth in the soil profile survived burning treatments, and percentage viability was reduced at depths up to 40 mm. Seed germinability was not enhanced by fire. Seedling emergence and mortality, monitored over 13 months post-clearing, were much higher following non-burning and ‘fell and burn’ treatments than ‘burn standing’ and ‘fell, pile and burn’ treatments. Nevertheless, the pattern of seedling emergence and survival was similar in all treatments, with survival low until October, 9 months post-clearing. Although all treatments resulted in significant reductions in seed density 1 year post-clearing, burning treatments were more effective than the non-burning treatment. The ‘burn standing’ treatment killed more seeds than the ‘fell and burn’ treatment, and may be recommended where labour costs are prohibitive. Since all treatments induced a rapid seed-bank decline in A. cyclops, it is recommended that the choice of treatment be dictated by other factors, such as possible effects on the recovery of indigenous plant species.
There is no summary to this chapter - only a two-page section entitled 'Summary and Conclusions' which is too long to reproduce here.
Concern about the invasion of this reserve by alien trees and shrubs, principally Acacia cyclops, A. saligna, A. longifolia, Eucalyptus lehmannii and Pinus pinaster, was first expressed in 1941, 2years after its proclamation. Control operations were started by 1943 and were almost totally ineffective for at least the first 35years; no systematic control strategy was implemented, follow-up control work was inadequate to prevent reestablishment of felled thickets and the supervision of control teams was deficient. Linkage of control operations to firewood production was a significant factor in this failure. In 1974 a 10-year control strategy was drawn up and in the late 1970’s began to be effectively implemented. Surveys of 40 plots in the centre of the reserve in 1966, 1976–80 and 1986 showed increasing densities of species other than the easily controlled P. pinaster up to 1976–80. Since then almost all individuals>1,8m in height have been eliminated and indications from smaller height classes are that seed banks are being depleted. Explanations are advanced for the successes and failures experienced during the 47years of control. Some of the side-effects of these invasions and their control are discussed. One conclusion is that this reserve requires a qualified ecologist on its staff.
Percentage viability of A. saligna and A. cyclops seed banks was determined for stands cleared of mature plants for different time intervals. Seedling emergence following recent clearing was monitored for a year at one site of each species. Percentage viability of A. saligna seed banks was uniformly high and was not affected by age since clearing. Percentage dormancy in A. saligna was high since few seedlings emerged in the field (1% of viable seed bank), but after fire, 70% of the remaining viable seed bank germinated. Percentage viability of A. cyclops seed banks differed significantly among sites, with the proportion of viable seeds in the soil increasing significantly with age since clearing at one site. Percentage germination and viability of fresh A. cyclops seeds also differed significantly among sites. Although seedling emergence in A. cyclops (9% of viable seed bank) exceeded that in A. saligna, pre-emergence mortality was implicated as the more important process in A. cyclops seed bank decline. In order to markedly reduce seed banks, it is recommended that A. saligna stands should be burnt and A. cyclops stands cleared in any way which removes the vegetation cover.
(1) Populations of buried viable seeds of Acacia cyclops and A. saligna, invasive aliens of fynbos, were estimated to assess the effects on them of different shrub-clearing treatments. (2) Seed populations of both species declined significantly after 1 year with burning, but in general, not thereafter. (3) Seed banks of A. cyclops, but not of A. saligna, were reduced by shrub felling alone after 1 year, apparently because a high proportion of seeds do not have seed-coat induced dormancy, and thus germinate immediately.
The gall wasp, Trichilogaster acaciaelongifoliae Froggatt, was introduced from Australia to South Africa for the biological control of Acacia longifolia (Andr.) Willd. Comparison of branches from heavily infested trees (those with > 75% of their branches galled) and unifested trees, shows that the gall wasp reduced seed production of A. longifolia by 99% and 95% at two sites in the south western Cape Province, South Africa. Seed production of trees was inversely related to the proportion of infested branches and was reduced by an average of 89% when > 50% of the branches on a tree were infested, irrespective of tree size. The seed reduction was achieved directly by galling of reproductive buds and indirectly by increased abscision of inflorescences on galled branches. Galls caused leaf abscision and reduced vegetative growth markedly. The wasps had been present at these sites for only two generations since release.
The reduction of species richness of indigenous plants is one of the major problems associated with the presence of dense stands of invasive alien trees and shrubs in the Fynbos Biome of the Cape Province, South Africa. A synthesis was made of published and unpublished data on plant species richness in fynbos with different levels of invasion and different histories of control. Linear regressions of species richness on the log of quadrat size were significant for both uninvaded fynbos and fynbos under dense stands of alien trees and shrubs. The slopes of the regression equations did not differ significantly between invaded and uninvaded sites, but elevations were significantly different, indicating a marked reduction in richness of indigenous plant species in invaded areas. The linear regression of species richness on quadrat size for cleared areas was not significant, but quadrats at most cleared sites showed species richness values intermediate to those of uninvaded fynbos and dense stands of aliens. Reductions in species richness at the scale of the sample quadrats used in this study (4–256 m) occur once the canopy cover of aliens exceeds about 50% and there is evidence of reduced species richness with increased time of suppression. For this reason, stands should be cleared before canopy closure is achieved.
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