Figure - uploaded by Frederik Van Daele
Content may be subject to copyright.
Germination percentage (resolution = 2%) (a), and germination lag time (days to germinate after the stratification period) (b). Groups with distinct letters are significantly different at P = 0.05. Capital letters depict statistical comparison of means within each soil phase (i.e. eutrophic, mesotrophic and oligotrophic) and across plant community groups (species). Lowercase letters depict statistical comparison of means within each species group across the soil phases. Error bars are SE.
Similar publications
Cadmium (Cd) pollution is becoming increasingly prevalent, posing a global environmental hazard due to its negative effects on plants growth and human health. Phytoremediation is a green technology that involves uptake of Cd from the soil by a combination of plants and associated microbes. The objective of this study was to investigate the role of...
Ash from power plants that incinerate poultry litter has fertilizer value, but research is lacking on optimal land application methodologies. Experiments were conducted to evaluate calcitic lime and flue gas desulfurization gypsum (FGDG) as potential fillers for poultry litter ash land applications. The ash had phosphorus (P) and potassium (K) cont...
Citations
... The wood chips may also have limited the direct contact of the seeds with the mineral soil (Schulte, 2017). Nevertheless, according to our results, these sites were suitable for colonization by the majority of target and threatened species adapted to oligotrophic and acidic soils (van Daele et al., 2017;Kurtogullari et al., 2020). Seedling establishment of other grassland species, however, was partially hampered by site characteristics (Roem and Berendse, 2000;Schulte, 2017). ...
... In the case of target species, this relationship was negative. The majority of target species are characteristic of Nardus grasslands and are therefore adapted to very oligotrophic conditions (Ellenberg and Leuschner, 2010;van Daele et al., 2017). Only some character species of mountain hay meadows, such as Crepis mollis, grow preferentially at more mesotrophic soils. ...
Introduction
Industrialization has ushered in massive changes in agriculture. Particularly in low mountain ranges, large-scale afforestation with Norway spruce on traditionally managed, semi-natural grasslands has caused a severe decline in biodiversity. Tree removal, hay transfer and resumption of grazing or mowing are typical measures to re-create species-rich grasslands. The aim of this study was to use vascular plants and three insect taxa (leafhoppers, true bugs, and grasshoppers) as bioindicators to evaluate the success of montane grassland restoration on former spruce forests in Central Europe. In addition, we intended to identify the drivers of species richness within the studied grasslands in order to derive suitable recommendations for habitat management.
Methods
We analyzed two different treatments: (i) grazed restoration sites where trees had been cut and species-rich green hay had been applied (N = 9) and (ii) target sites with a long continuity of low-intensity grazing (N = 9).
Results and Discussion
Our study revealed that all studied taxa responded rapidly to the restoration measures. After a development period of 3 to 5 years, we found no differences in species richness and diversity of leafhoppers, true bugs and grasshoppers (all, target and threatened species). In addition, non-metric multidimensional scaling showed a large overlap in species composition between restoration and target grasslands. Among target and threatened species, vascular plants displayed the same pattern as insects and reached similar values when comparing the two treatments. However, total species richness and diversity of vascular plants were still higher on the target sites and species composition overlapped only partially. Grazing intensity was the predictor with the highest explanatory power in multivariable (Generalized) Linear Mixed-effects Models, being negatively related to species richness of leafhoppers and true bugs. We conclude that the measures implemented were effective in re-establishing target communities of different taxa. The transfer of seed-containing hay enabled or accelerated the development of the vegetation. Insects, on the other hand, were able to recolonize the restored grasslands on their own, given that these sites were embedded in a network of species-rich grasslands. With regard to insects (e.g., leafhoppers and true bugs), it should be ensured that grazing is applied at low intensity.
... Therefore, due to their preference for oligotrophic soils, restoration of extensive Nardus grasslands where fertilisers were applied in the past is dicult (Dähler 1992, Hejcman et al. 2007). Van Daele et al. (2017) claim that to restore speciesrich Nardus grasslands, it is crucial to reduce bioavailable P below 10 mg/kg or to select sites with bioavailable P contents below this threshold. Similarly, Korzeniak (2016) found that, especially for mesic Nardus grasslands in the lower montane zone, nutrient levels should be kept low to successfully control the expansion of nitrophilous species. ...
... Grasslands in Europe are semi-natural ecosystems that greatly depend on how they are managed (Butaye et al. 2005). This fact confirms the importance of disturbance in grasslands (Edwards et al. 2007 Grazing mesic and semi-dry semi-natural grasslands, e.g., habitat types 6210(*) and 6230*; not optimal for hay meadows and xeric grasslands selective (might cause an overgrowth of woody and thorny species, not palatable for certain types of livestock -primarily cows, so grazing by goats is recommended in this case); favours grasses over forbs due to their biology; dependent on species phenology; not suitable for xeric grasslands Halada et al. 2001, Muller 2002, Hayes and Holl 2003, Pykälä 2004, Edwards et al. 2007, Galvánek and Lepš 2008, Török et al. 2011, Turtureanu et al. 2014, Török et al. 2016, Bonari et al. 2017, Tóth et al. 2018, Silva et al. 2019, Köhler et al. 2020, Kurtogullari et al. 2020, Mrázková-Štýbnarová et al. 2020, Török et al. 2020, Tölgyesi et al. 2022, Zarzycki et al. 2022 Mowing various semi-natural grasslands, including habitat types 6210(*), 6230*, 6410, 6510, 6520 generally more accessible than grazing; early mowing facilitates the development of early-flowering species; enhancement of target species colonisation; improvement of the development of sown forbs by increasing competitiveness of higher plants; elimination of accumulated biomass and litter beneficial only if carried out at an appropriate time in the season (when most diagnostic grassland forb species are ripe) and not too frequently (regime dependent on grassland type); when carried out with machinery, it is a non-selective and highintensity disturbance single event (compared to grazing) -hand mowing has more advantages that are similar to grazing, but it is costly and timeconsuming; might cause vegetation homogenisation; heavy machinery threatens invertebrate diversity Halada et al. 2001, Muller 2002, Lawson et al. 2004, Edwards et al. 2007, Enyedi et al. 2008, Galvánek and Lepš 2008, Královec et al. 2009, Halada et al. 2011, Török et al. 2011, Török et al. 2016, Bonari et al. 2017, Milberg et al. 2017, Van Daele et al. 2017, Tälle et al. 2018, Török et al. 2020, Zarzycki et al. 2022 Prescribed burning (with long fire-return periods) Table 3. A list of post-restoration grassland management techniques used in the studies reviewed. ...
Currently, the majority of high nature value Slovenian grasslands have an unfavourable conservation status. Based on the available data from habitat type mappings, the surface of high nature value grasslands (6210(*) - semi-natural dry grasslands and scrubland facies on calcareous substrates, 6410 - Molinia meadows, and 6510 - lowland hay meadows) at Natura 2000 sites is decreasing. The existing agri-environment measures have been only partly effective in promoting grassland biodiversity. The main threats to biodiversity are driven by various anthropogenic activities, which result in a continuous change in landscape identity, habitat fragmentation, and ecosystem degradation. Therefore, biodiversity restoration became an urgent step in the conservation of high nature value grasslands. Multiple pathways may lead to the ecological restoration of grasslands with an altered, uncharacteristic floristic composition, or grasslands with an unfavourable conservation status. In this paper, we present an overview of the methods used in the restoration of grasslands from different parts of Europe. In an adapted form, these methods could also be used for the restoration of Slovenian grasslands. Grasslands may be left to spontaneous succession, which is mainly suitable for small-scale areas located in the proximity of grasslands with well-preserved biodiversity. However, to re-establish plant communities with specialist species, a more proactive approach is usually required, such as sowing of regional or commercial seed preservation mixtures, transfer of mature plant material, or topsoil transfer from donor sites with appropriate botanical composition. Grassland restoration methods should be carefully thought-out and carried out before the habitat or species is endangered. We conclude that optimally chosen post-restoration management may have an impact that is comparable to or even greater than the impact of a suitable restoration method. Nevertheless, the maintenance of well-preserved grasslands is still much more cost-effective than the restoration of degraded grasslands.
... However, this is essential to understand how landuse legacies affect contemporary and potential natural vegetation (Perring et al., 2016). This is especially the case for species communities with a slow-growth strategy which are sensitive to land-use intensification due to competitive pressures, such as species-rich Nardus grassland (syntaxonomic class Nardetea strictae; Van Daele et al., 2017). ...
... The habitat quality of acidic grasslands can be explained by chemical soil characteristics to a high degree (Janssens et al., 1998;Kleijn et al., 2008;Ceulemans et al., 2013). Increased availability of N, P and K, often as a result of fertilization, stimulates biomass production, alters the feedback between above-ground and belowground communities and reduces the species richness of grasslands through competitive exclusion (Wardle, 2004;Crawley et al., 2005;Leff et al., 2015;Van Daele et al., 2017). Furthermore, nitrogen deposition and the resulting acidification have been identified as important drivers of degradation (Bobbink et al., 2010;Duprè et al., 2010). ...
... nd specificity and ranges between −1 and 1. The accuracy quantifies the ratio of correct predictions relative to the total amount of predictions. These performance metrics were determined post-hoc based on a threshold of minimally three and four predicted Nardus indicator species respectively, out of a total of 12 listed by Oosterlynck et al., 2020. Van DAELE et al. et al., 2017, factors explaining the plant community niche have to be predominantly integrated in the model (Elith et al., 2006;Öster et al., 2009). To predict the potential suitability for plant communities in time, for example as a tool for ecological restoration, it could be a viable strategy to focus on past land-use and omit presentday landscap ...
Aims:
Historical land use legacies and chemical soil characteristics both explain either directly or indirectly the habitat quality of Nardus grassland, which is protected under the European habitat directive. Yet the relative importance and complementarity of both sets of variables are generally unknown. This knowledge is also relevant for practical reasons, as historical land use variables can be used in desktop spatial analyses, whereas soil characteristics require field surveys to collect samples for laboratory analyses. To this end, we aim to disentangle the relative importance of historical land use legacies and soil chemistry for the Nardus grassland quality, and determine the potential of habitat suitability mapping for predicting potential restoration areas.
Location:
Natura 2000 grasslands in Flanders (northern Belgium)
Methods:
We compared the model performance of three generalized additive models (GAM), using either land use history metrics, soil chemistry, or both as explanatory variables, with the Nardus grassland indicator species count as response.
Results: All three models were able to predict areas suitable for at least 3 Nardus grassland indicator species with high sensitivity and specificity. However, minimum 4 indicator species are required for a favorable conservation status of Natura 2000 Nardus grasslands in Flanders. Using this threshold to detect high priority zones, the model based on historical land use variables resulted in a lower sensitivity than models which included soil chemistry.
Conclusions:
We suggest a two‐step approach, with an a priori desktop spatial analysis based on historical land use variables subdivided in a high priority zone and lower priority zone. If the targeted area for restoration or conservation can be found within the high priority zone, additional soil analysis are only required to help guide conservation and restoration measures. If additional sites are considered within the lower priority zone, a field survey to collect additional soil data is recommended.
... Other species from more productive grasslands were not able to develop dominance after their short-term increase in the following years. The competitive ability and regeneration of these species proved to be strongly constrained by consecutive droughts in oligotrophic systems (van Daele et al., 2017). Therefore, the observed changes in abundance of low-productive and character species, together with the relative stability of species richness, indicate that community filters, which generate an altered set of alpha niches, may play an increased role in the recovery of the community after droughts. ...
Questions:
Mountain grasslands can be strongly affected by extreme droughts such as those related to climate change. What are the impacts of extreme droughts on community composition, diversity, Ellenberg indicator scores and species groups in oligotrophic montane Nardus grasslands, and what are the associated mechanisms of vegetation change?
Location:
Rhön Mountains, Germany
Methods:
In three consecutive years, we investigated the effects of yearly droughts (April-August) in an experimental setup with rainout shelters. Due to the coincidence of ambient extreme dry conditions in those years and our artificial rainfall reduction, we evaluated the contribution to community change of ambient drought conditions and the treatments. We analysed community composition changes by applying redundancy analysis to species differences in comparison with the pre-treatment year, and used mixed-effects models to test for changes in community-weighted means of Ellenberg indicator scores, sociological, and functional groups.
Results:
We found significant changes in species abundances and community structures in response to drought. Evenness increased, but species richness remained rather stable over time. Ellenberg indicator scores for temperature and nitrogen increased, while the score for moisture decreased. Simultaneously, dominant species declined and subdominants increased. Changes occurred with a time lag and were largely driven by the high ambient drought level and less by the artificial treatments.
Conclusions:
Our results show that drought-related community composition changes in Nardus grasslands occur across community structures, characteristic species, and species groups. The post-drought recovery of the community is shaped by community filters, which particularly allow subdominants to take advantage of newly available niches in the matrix, even if they lack strong drought tolerance. Our findings indicate a certain resilience of the community to climate-change-related droughts, which suggests that the observed changes should not lead to an accelerated short-term decline of these grasslands, but that this cannot be excluded in the long term.
... Phosphorus-limited soil conditions in particular allow rare plant species to thrive and thus enable high species richness (Ceulemans et al., 2014;e.g. Nardus stricta, Potentilla erecta and Luzula multiflora (De Saeger and Wouters, 2017), are tolerant to growing in nutrient-poor soils (Diekmann et al., 2019) and slow-growing (Van Daele et al., 2017). When such nutrient-limited communities with numerous slow-growing species are fertilized, they are replaced by communities dominated by a few fast-growing species, i.e. the plant-plant competition is affected (Harpole and Tilman, 2007;Hautier et al., 2009). ...
... Lekberg et al., 2018). In contrast to fast-growing species, slow-growing typical Nardus grassland species are known to be unable to switch from a growth strategy aimed at acquiring enough nutrients to a growth strategy of producing more biomass (Van Daele et al., 2017). Small increases in nutrient availability in nutrient-poor soils can, hence, lead to the loss of slow-growing species specialized at surviving under severe phosphorus limitation (Roeling et al., 2018). ...
The restoration of Nardus grasslands is often hampered by high bioavailability of soil phosphorus and disturbed soil communities. In order to better understand these bottlenecks, we studied Nardus grassland species grown together in communities with fast-growing species in 50-liter pots along a gradient of bioavailable phosphorus with or without inoculated soil biota. These mesocosms allowed the plants to freely interact, including competition for light and nutrients.
We investigated changes in the plant community composition along the phosphorus gradient using Threshold Indicator Taxa Analysis (TITAN). We found a negative threshold of 11.5 mg POlsen kg⁻¹ with six significant indicator plant species. Above the threshold, a small increase in phosphorus resulted in a disproportionally large drop in biomass for the indicator species, including four typical Nardus grassland species. The decline in these ‘oligotrophic indicator species’ was also linked to increasing plant community biomass, so we suggest the oligotrophic indicator species to be outcompeted for light by fast-growing plant species. We did not find an effect of the soil biota treatment on the biomass of the oligotrophic indicator species, but did observe a positive effect of inoculation with soil biota on the total biomass of the plant community.
Interestingly, the threshold for the plant communities in the mesocosm experiment was comparable to the upper bioavailable phosphorus concentrations in remnant Nardus grasslands in northern Belgium. For the restoration of Nardus grasslands, such phosphorus-poor soil conditions appear to be essential, because the plant species that typically occur in these grasslands are able to handle nutrient limitation, but not light limitation.
Stipa grandis steppe as one of the main community types of temperate grasslands in Northern China plays important roles in livestock production and ecological security. Current grassland degradation research links climate change and human activities to soil properties, community traits, and ecosystem functions, but the relationships of plant traits, diversity and ecosystem function remain unclear. To achieve sustainable development of temperate grassland ecosystem, there is a fundamental need to explore the current situations of Stipa grandis steppe and the trade-offs of ecological functions during restoration. We conducted experiments on 120 Stipa grandis community plots (5 m × 5 m) at different restoration stages based on field investigation in Inner Mongolia, China. The relationships of plant traits, diversity, plant organic carbon storage and soil organic carbon storage are analyzed, and the trade-offs of related ecological functions during grassland restoration are explored. The results show that (1) Enclosure effectively promotes plant organic carbon storage, specific leaf area, specific root length, and soil organic carbon concentration of Stipa grandis. (2) Compared with A1 stage (heavy grazing with 1.8–2.7 sheep ha-1 half year-1), enclosure effectively promotes species diversity and functional diversity of Stipa grandis community. (3) Soil carbon storage shows a significant positively correlation with functional diversity in the early stage of grassland restoration, and a significant positively correlation with species diversity in the late stage of grassland restoration. (4) B2 stage (enclosure lasting 8–14 years) has higher function trade-offs and lower functional benefits, which is the critical stage for plant diversity and soil organic carbon storage. Our findings highlight plant traits and community function of Stipa grandis steppe during restoration, which provide scientific basis necessary to promote grassland sustainability and restoration in similar areas.
Species-rich montane Nardus grasslands and their characteristic and highly valued plant species arnica (Arnica montana L.) are two elements of low-intensively-used, species-rich mountain grassland in Central German Uplands. In times of present and future global change, new stressors, like those related to climate change, will come into action, and are predicted to lead to a further decline of Arnica montana and Nardus grasslands. This is of specific concern in lower mountain ranges, where species and habitats are naturally restricted in their altitudinal distribution and where possible refugee areas at higher elevations are limited. As one significant impact of climate change, drought events of different intensity within the growing period of grasslands in spring and summer are projected to be a major threat for semi-natural mountain grasslands and their already endangered plant species, like Arnica montana, that have a pronounced montane distribution. Species and, subsequently, the grassland community need to compensate negative impacts of climate change by adapting to the new environment in their current occurrences and by having a high resilience against these impacts, respectively. However, little is known about the functional trait responses and possible adaptations of the threatened mountain grassland plant species Arnica montana with regard to their intraspecific variability and about the community responses of Nardus grasslands to climate-change-related droughts. In this cumulative PhD thesis present, responses and the adaptability (adaptive plasticity) of Arnica montana via plant functional traits and the community level responses and resilience of Nardus grasslands to drought stress related to climate change were investigated.
Chapter 1 investigates changes in the functional trait performance and variability of Arnica montana in its adult life stage, along a climate gradient from lower elevations with higher- to higher elevations with lower summer aridity and in relation to the grassland management. Results showed a strong positive relationship of most functional traits with reduced summer aridity at higher elevations, which indicates a higher trait performance level at montane sites with less summer aridity. The variability of traits decreased steadily with decreasing summer aridity, suggesting less environmental stress for Arnica montana at higher elevations. Management factors, however, had only a small influence on both performance and variability of Arnica’s functional traits. In Chapter 2 I evaluated in a greenhouse experiment the impact of drought-induced stress on Arnica montana seedlings in their early life stage establishment phase. Together with survival, as a closely linked measure for fitness of an individual, above- and below-ground functional traits were identified that, on the one hand, are related to the species’ fitness decline and, on the other hand, may act as early-warning indicators for reduced fitness. Seedlings showed a high resistance to moderate drought, and senescence was higher and consecutive survival declined only under strong and extreme drought conditions. Based on the trait-survival relationships identified, declines in leaf length, leaf width, and leaf number were identified as sensitive response-indicator traits that indicated a fitness decline prior to a substantial increase in mortality. Chapter 3 studies the effects of extreme droughts on the community composition, diversity, Ellenberg indicator scores, and diagnostic species groups in species-rich Nardus grasslands, and investigates the associated mechanisms of vegetation change. Drought effects were investigated in three consecutive years by a rainout shelter experiment with different levels of rainfall reduction. Nardus grasslands responded with significant changes in species abundance and community structures, which also triggered changes in Ellenberg indicator scores and several species groups. Changes were largely driven by the high ambient drought level but occurred with a considerable time lag.
Taken together, the findings of this thesis enhance our knowledge about the responses of Arnica montana and montane Nardus grasslands, as a representative mountain grassland type, to climate change, and about their relationship to management strategies. Specifically, Arnica montana has the potential to adapt to altered environmental condition in the short term via changes in plant functional traits, but these changes constitute, at the same time, a lower performance level of fitness-related traits, which can lead to a long-term decline in fitness at the individual and population level. The findings at the community level of Nardus grasslands, moreover, indicate a certain resilience of the community to climate-change-related droughts. Overall, the insights gained from the studies can contribute to the development of climate-change-adapted conservation concepts, and can inform habitat- and species-management measures.
Aims
The restoration of degraded ecosystems typically focuses on establishing assemblages of target species, but successful recovery should also be evaluated by the ecosystem’s functioning to guarantee long-term persistence. We investigated how the processes underlying community assembly (i.e. species loss, species gain and changes in abundance of resident species) influenced ecosystem functioning in experimental grassland communities in different restoration states.
Location
A greenhouse experiment in Northern Flanders, Belgium.
Methods
We set up a mesocosm experiment with communities of nineteen planted species, ranging from slow-growing species from poorly productive Nardus grasslands to fast-growing species from highly productive Lolium perenne grasslands. We categorized the mesocosms into different grassland restoration states based on known abiotic and biotic restoration barriers for semi-natural grassland restoration: soil phosphorus levels and soil biota communities. After two growing seasons, we used the CAFE approach, an ecological application of the Price equation, to partition the effects of plant community assembly on ecosystem functioning (here community productivity) for the different restoration states.
Results
Adding soil biota communities sampled from reference Nardus grasslands versus more intensively managed grasslands did not have a significant effect on either plant species richness or biomass productivity. Lower soil phosphorus concentrations (i.e. abiotic restoration) resulted in a higher plant species richness. However, the net effect on productivity was close to zero. The increase in productivity caused by species gains was compensated through decreases in productivity caused by species loss and by decreases in the abundance or functioning of species that are present in both abiotically degraded and abiotically restored states.
Conclusions
Not only species richness but also species identity resulted in changes in ecosystem functioning (i.e. productivity), even though the net functional effects were close to zero. More specifically, we found that species richness-driven increases in productivity were counterbalanced by resource-driven and species identity-driven reductions in productivity.
Aim: Species‐rich Nardus grasslands are high nature‐value habitats. In Switzerland, many of these grasslands are degraded even though they have been under protection since the 1980s. Degradation shows two divergent trends: Nardus grasslands are either dominated by Nardus stricta or by eutrophic plants, both trends leading to the disappearance of typical Nardus grassland species. With this study, we aim to identify the factors that could be adjusted to conserve the integrity of this habitat.
Location: Bernese Alps, Switzerland.
Methods: In 2016, we investigated the underlying causes of this degradation process by assessing vegetation composition in 48 Nardus grasslands located in the Swiss northern Alps of canton Bern and linking it to soil, management and environmental variables. To explore the effect of the degradation on higher trophic levels, orthopteran species richness and densities were assessed.
Results: Results show that Nardus meadows (mown) are rarely degraded compared to Nardus pastures (grazed). Within pastures, eutrophic plants are most abundant on small pastures with low soil carbon/nitrogen ratio, indicating high nutrient availability. Nardus stricta dominance is most problematic on north‐exposed slopes and in summer pastures. A plausible driver of both degradation trends is the grazing management regime: within small pastures at low elevation where the grazing periods are short but intense, soil carbon/nitrogen ratio is low because of high dung deposition, thus the eutrophic species become dominant. Contrastingly, on large summer pastures with low‐intensity and long‐term grazing, N. stricta becomes dominant due to selective grazing. Both degradation trends show a negative impact on the orthopteran density.
Conclusion: Species‐rich Nardus grasslands are a precious alpine habitat for specialised plant species and orthopterans. With an extensive mowing regime or a more controlled grazing regime that homogenises intensity in time and space, species‐rich Nardus grasslands can be conserved in Switzerland.
Grassland ecosystems can be extremely species-rich and harbor a large proportion of European wildlife. Due to agricultural intensification in the 20th century, species-rich Nardus grasslands (a European priority habitat type) are nowadays largely confined to nature reserves, where they are further threatened by e.g. atmospheric nitrogen deposition. Typical plant species of Nardus grasslands are adapted to nutrient-poor soil conditions through their conservative resource use strategy. Land-use intensification causes the plant community to shift from the slow-growth strategy species in Nardus grasslands to fast-growth strategy species in eutrophic grasslands where nutrient cycling is fast and plant species richness is low. The soil biota community also changes, i.e. from fungi-dominated with omnivorous nematodes to bacteria-dominated microbial with herbivorous nematodes. Biodiversity conservation will benefit from better protection of (remnant, i.e. degraded) Nardus grasslands, reducing pressures on degraded grasslands and restoration of degraded grasslands.
The restoration of Nardus grasslands on former agricultural land is impeded by the phosphorus that has accumulated in the soil due to repeated fertilization. Bioavailable concentrations of phosphorus may be tenfold higher in former agricultural fields than in remnant Nardus grasslands. Nitrogen and potassium levels, in contrast, generally decrease again through mowing management when fertilization ceases. Hence, phosphorus is the key nutrient for evaluating the restoration potential of former agricultural grasslands.
In remnant Nardus grasslands and in a mesocosm experiment, we found a soil phosphorus threshold of 12 mg POlsen kg-1. At higher bioavailable soil phosphorus concentrations, typical Nardus grassland species disappeared, probably due to being shaded out by fast-growing plant species (and inoculation with soil biota did not increase their performance). For ecological restoration, phosphorus can be extracted with plant biomass, i.e. through phytomining (by mowing or P-mining). We compared the potential and optimization opportunities of traditional mowing management, i.e. unfertilized cutting and removing hay two or three times a year, and P-mining, i.e. yield maximization by adding growth-limiting nutrients other than phosphorus (i.e. nitrogen and potassium). Mowing for 16-25 years in post-fertilization grasslands did not yield the floristic and phosphorus-poor targets of Nardus grassland. The biomass production was relatively low due to nitrogen and potassium limitation. Another 40 to 114 years of phosphorus removal would be needed to reach the soil phosphorus threshold of Nardus grasslands. With P-mining, the restoration time would be considerately shorter. The potential to phytomine phosphorus with P-mining depended on the soil phosphorus concentration: high phosphorus removal on phosphorus-rich soils, decreasing removal potential with decreasing soil phosphorus concentration. Adding biostimulants, i.e. phosphorus-solubilizing bacteria, humic substances or arbuscular mycorrhizal fungi, did not result in higher phosphorus removal with P-mining. Crop species such as buckwheat, sunflower and maize were considerably more effective at removing phosphorus through P-mining than triticale or flax, at phosphorus-rich soils (all species performed similarly in soils with lower phosphorus concentrations).
Optimization opportunities for phytomining of phosphorus appear to be limited. Restoring phosphorus-poor soil conditions on former agricultural land will remain a challenge. Phytomining phosphorus will always be a long-term commitment, which can be made more efficient by lifting the limitation of nutrients other than phosphorus (P-mining) or selecting crop species that take up more phosphorus. Oligotrophic habitat types deserve high-priority conservation because it is nearly impossible to restore oligotrophic fields on eutrophic fields, unless a drastic technique is used such as topsoil removal. Ecological restoration of semi-natural grasslands on former agricultural land involves a large investment of time and/or money (i.e. decades of mowing or P-mining management or ten thousands of euros per hectare for topsoil removal). Therefore, it is necessary for practitioners in ecological restoration to focus their efforts and carefully select their goals, fields and techniques.
To aid practitioners, we developed a decision tree. We advise that the abiotic and biotic conditions be examined carefully and compared to the targeted habitat type, i.e. measuring the distance to target. We highlight the importance of restoring the abiotic conditions before starting biotic restoration and initiating suitable management. The abiotic distance to target for Nardus grassland restoration is the amount of phosphorus to be removed, which will determine the time needed for phytomining (mowing or P-mining). Evaluating the cost of the restoration technique, the time needed to reach the target and the potential value of the site in the landscape, can help in prioritizing restoration efforts. Restoring Nardus grassland on eutrophic soils requires topsoil removal, a costly measure, and is hence only likely with financial support of e.g. European Life funding or large national nature development projects. Low-budget projects can probably better focus on restoration on fields with a history of less intensive fertilization where phytomining by mowing or P-mining is attainable. It might be most efficient to invest some money (a small fraction when compared to the purchasing price) in the abiotic screening of parcels before purchasing them or before selecting a target habitat type, i.e. a habitat type with less stringent abiotic requirements on eutrophic fields (with high phosphorus levels). For instance, grasslands rich in common native herb species are easier to create, can be beneficial for e.g. pollinators and can be used strategically as corridors between oligotrophic habitats or as buffer zones to prevent inflow of fertilization from nearby agricultural fields. These novel ecosystems, however, can probably never fully replace highly specialized ecosystems such as Nardus grasslands.
