Esler K
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- ItemA dynamic modelling tool to anticipate the effectiveness of invasive plant control and restoration recovery trajectories in South African fynbos(2021) Hall, S.A.; Bastos, R.; Vicente, J.; Vaz, A.S.; Honrado, J.P.; Holmes, P.M.; Gaertner, M.; Esler, K.J.; Cabral, J.A.Invasive alien plants negatively impact ecosystems, necessitating intricate management actions. In a critically endangered vegetation type within the fynbos biome of South Africa, a study was performed comparing different management interventions over plots invaded by Acacia saligna. A dynamic modelling approach was designed to analyze field data and simulate effectiveness of several restoration methods. Field data for vegetation recovery rates over the course of two years were fed into the model, which allowed the extrapolation of multiple recovery trajectories over a long time‐span, not possible to obtain from traditional short‐term field surveys. Our model simulations show that different treatments in similarly degraded states at the time of clearing can result in vastly different recovery trajectories. Active seed sowing was initially most expensive but resulted in most successful native shrub recovery, decreasing costs of longer‐term follow‐up acacia clearing. Clearing without burning was cheapest but resulted in limited establishment of both native and acacia cover, providing an opportunity for secondary invasion by alien forbs. In this case, biotic thresholds may have been crossed which prevented recovery of certain vegetation components. Active sowing can partially reverse thresholds by restoring shrub cover but not structural diversity. Therefore even applying this treatment did not resemble vegetation structure of the reference condition after an extended period of 30 years, but does show how restoration can be improved by native seed sowing compared to passive restoration alone. Our model simulations provide a useful tool to support decision‐making by providing management recommendations for optimizing alien plant clearing protocols.
- ItemABSTRACT: The influence of fire on germination and plant species diversity in Swartland Shale Renosterveld in the Cape Floristic Region, South Africa(Elsevier B.V., 2017) Cousins, S.R.; Esler, K.J.; Witkowski, E.T.F.
- ItemAcacia saligna seed banks: Sampling methods and dynamics, Western Cape, South Africa(Elsevier, 2012) Strydom, M.; Esler, K.J.; Wood, A.R.Acacia saligna is potentially the most damaging invasive species in the coastal lowlands of the south-western Cape. The gall rust fungus, Uromycladium tepperianum, has been highly successful as a biological control agent for A. saligna populations in South Africa and has effectively reduced density, canopy cover and seed production of the tree. However, concerns still remain about the soil-stored seed bank and knowledge of seed bank status and dynamics is crucial for effective management. This study evaluated the effectiveness of two different sampling methods in assessing the status of the seed bank, how the seed bank of A. saligna at two different sites varies over time and how these findings compare to findings of other seed bank studies of A. saligna across southern Africa. Even with the reduction in seed production caused by biological control, numbers of seed in the soil seed bank are high enough to maintain high levels of recruitment after management or natural disturbances. Both sampling methods (grid and random sampling)attempted were effective in assessing the vertical distribution of the seed bank and estimated the size of the seed bank to be within the same order of magnitude. However, random sampling is more effective in assessing seed bank size as it was found that the seed has a clumped horizontal distribution. The vertical distribution of seed in the seed bank was found to be influenced by soil properties. The largest portion of the seed bank is situated in the upper 10 cm of the soil profile and declines in size with depth.
- ItemActive seed sowing can overcome constraints to passive restoration of a critically endangered vegetation type(2021) Hall, S.A.; Holmes, P.M.; Gaertner, M.; Esler, K.J.Invasive alien plants negatively impact ecosystems, but recovery of native vegetation may fail following standard methods of alien species removal alone. Alternative management actions may thus be required. Cape Flats Sand Fynbos is a critically endangered vegetation type in the Cape Floristic Region of South Africa which is threatened by Acacia saligna invasion, but standard clearing methods have failed to restore native vegetation structure. A restoration study was performed comparing passive treatments i.e. clearing without burning (stack-block) versus clearing and burning (burn-block), as well as active intervention by sowing seeds of native species, either initially after burning or a year later, in which seeds were either not pre-treated or pre-treated with smoke and heat exposure before sowing. After two years all treatments resulted in different recovery trajectories, although none resembled the reference condition. Clearing without burning facilitated recovery in less degraded areas with higher initial native shrub cover, but otherwise resulted in limited vegetation recovery. Limited recovery facilitated secondary invasion by herbaceous weeds. Active seed sowing resulted in the highest recovery of native shrub cover and diversity. These findings suggest that passive restoration is constrained by seed limitation, due to the lack of recovery of vegetation components under passive clearing treatment. Active sowing was able to partially overcome this constraint through improved recovery of total shrub cover. However, non-sprouting shrub cover was higher while resprouting shrubs and species of Restionaceae were lower compared to the reference condition. Pre-treatment of seeds before sowing improved establishment of some species. Active treatment involving sowing pre-treated seeds after clearing and burning therefore resulted in best fynbos recovery compared to either of the passive treatments tested. A decision tree has been developed based on these findings in order to guide best protocol for managers.
- ItemAlien tree invasion into a South African montane grassland ecosystem: impact of Acacia species on rangeland condition and livestock carrying capacity(Taylor & Francis, 2018) Yapi, T.S.; O'Farrell, P.J.; Dziba, L.E.; Esler, K.J.The degree to which invasive Acacia species affect South Africa’s livestock production has received little attention. We investigated the ecological impacts of Acacia mearnsii invasion on forage quality and quantity and on soil resources, along A. mearnsii invasion gradients, on South African rangelands and the subsequent conditions following clearing. Grazing capacity was reduced by 72% in densely invaded sites, whereas clearing improved grazing capacity by 66% relative to densely invaded sites within 5 years. In densely invaded sites total grass species basal cover was reduced by up to 42%. As a result, A. mearnsii reduced grazing capacity, from 2 to 8 ha required to support one large stock unit (ha/LSU) in uninvaded and densely invaded sites, respectively. Soil moisture content was lower in densely invaded sites compared with lightly invaded and cleared sites. Plant litter increased from 1.3% to 4.2%, carbon content of the soil increased from 2.0% to 4.0% and nitrogen concentrations increased from 0.1% to 0.2% in response to invasion by A. mearnsii. Clearing resulted in improved grazing capacity within 5 years. These results also showed that, if left uncontrolled, wattle species can reduce livestock carrying capacity within montane grasslands in South Africa.
- ItemBest practice for the use of scenarios for restoration planning(Elsevier, 2017) Metzger, J.P.; Esler, K.; Krug, C.; Arias, M.; Tambosi, L.; Crouzeilles, R.; Acosta, A.L.; Brancalion, P.H.S.; D’Albertas, F.; Teixeira Duarte, G.T.; Garcia, L.C.; Grytnes, J-A.; Hagen, D.; Jardim, A.V.F.; Kamiyama, C.; Latawiec, A.E.; Rodrigues, R.R.; Ruggiero, P.G.C.; Sparovek, G.; Strassburg, B.; Saraiva, A.M.; Joly, C.Scenarios are important tools to facilitate the communication among scientists, practitioners, and decision-makers, and, thus to support policy and management decisions. The use of scenarios has an enormous potential to reduce ecosystem restoration costs and to optimize benefits, but this potential remains poorly explored. Here, we recommend and illustrate six best practices to guide the use of scenarios for planning native ecosystem restoration. We argue, first, for a participatory process to consider aspirations of multiple stakeholders along the whole scenario building process, from planning to implementation and review phases. Second, targeted restoration outcomes should be defined by key-actors (those who have direct interests in restoration) and directly involved stakeholders, within a clear socio-environmental context and under a well-defined problem statement, considering a broad range of nature and human benefits that can be derived from ecosystem restoration. Third, methodological choices, such as scenario types, spatial and temporal scales, drivers, restoration related variables, and indicators, should be defined according to the multiple desired outcomes. Fourth, we encourage the consideration of the interactions among variables, within a spatially explicit, and temporally dynamic multi-criteria approach. Fifth, analysis and dissemination of scenario results should highlight the trade-offs and synergies among different restoration outcomes, identifying the scenarios that maximize benefits and minimize costs and resistance (i.e. the cost effective and most feasible scenario) for multiple targets. Finally, promoting capacity building, through a wider consultation process including interaction with a broader group of stakeholders, is critical for the successful implementation and review of restoration interventions. Scenarios that support ecosystem restoration should follow an adaptive and iterative process, aiming to continuously improve restoration interventions and outcomes.
- ItemCan wetland plant functional groups be spectrally discriminated?(Elsevier Inc., 2018) Rebelo, A.J.; Somers, B.; Esler, K.J.; Meire, P.Plant functional traits (PFTs) underpin ecosystem processes and therefore ecosystem service provision. If PFTs are possible to detect and discriminate spectrally, then it may be possible to use remote sensing applications to map ecosystem processes or services within and across landscapes. As a first step towards this application, we explored whether functional groups of 22 dominant South African wetland species were spectrally separable based on their PFTs. We measured 23 biochemical and morphological PFTs in combination with spectra from 350 to 2349 nm using a handheld radiometer. First, we evaluated the possibility of accurately predicting morphological and biochemical PFTs from reflectance spectra using three approaches: spectrum averaging, redundancy analysis (RDA), and partial least squares regression (PLSR). Second, we established whether functional groups and species were spectrally distinguishable. We found seven PFTs to be important in at least two of the three approaches: four morphological and three biochemicals. Morphological traits that were important were leaf area (PLSR: r2=0.40, regression: r2=0.41), specific leaf area (r2=0.67), leaf mass (r2=0.43, r2=0.38), and leaf length/width ratio (r2=0.62). Biochemical traits that play a role in the structural composition of vegetation, like lignin content (r2=0.98, r2=0.54), concentration (r2=0.45) and cellulose content (r2=0.57, r2=0.49), were found to be important by at least two of the analyses. Three other traits were important in at least one of the analyses: total biomass (r2=0.56), leaf C/N ratio (r2=0.99), and cellulose concentration (r2=0.76). Redundancy analysis suggests that there is a large percentage (52%) of the spectrum not explained by the PFTs measured in this study. However, spectral discrimination of functional groups, and even species, appears promising, mostly in the ultraviolet A part of the spectrum. This has interesting applications for mapping PFTs using remote sensing techniques, and therefore for estimating related ecosystem processes and services.
- ItemCompetitive interactions between the alien invasive annual grass Avena fatua and indigenous herbaceous plants in South African Renosterveld: the role of nitrogen enrichment(Springer Science+Business Media B.V., 2010) Sharma, G.P.; Muhl, S.A.; Esler, K.J.; Milton, S.J.Nitrogen enrichment may play a role in successful invasion of indigenous South African mediterranean shrublands by alien invasive annual grasses. To test the hypothesis that an increase in nitrogen would result in a greater increase in biomass for an alien annual grass than for various indigenous plant functional groups, we conducted a field study in Western Cape Renosterveld shrubland fragments, surrounded by wheat or vinyards, to assess alien grass abundance in relation to soil nitrogen availability. Significant decreases in invasive annual grass Avena fatua cover and soil nitrogen were observed from the edges to the interior of Renosterveld habitat fragments and there was a significant positive relationship between Avena fatua cover and soil nitrogen. In addition, Avena fatua was grown in competition with three indigenous species of different functional types, an annual forb (Dimorphotheca pluvialis), a geophyte (Oxalis purpurea) and an indigenous perennial grass (Tribolium uniolae) at three concentrations of soil nitrogen in a pot experiment. Results revealed that the alien grass Avena fatua had significant increases in biomass when nitrogen was added, whereas the indigenous species did not. Alien grass competition significantly influenced performance of the annual forb and the indigenous grass but did not affect the geophyte. Results suggest the prioritization of hierarchical management options for the different functional groups in Renosterveld in response to invasive grasses. Nutrient enrichment through run off must be restricted to conserve remnant Renosterveld fragments.
- ItemThe database of the PREDICTS (Projecting Responses of Ecological Diversity In Changing Terrestrial Systems) project(John Wiley & Sons Ltd, 2017) Hudson, L.N.; Newbold, T.; Contu, S.; Hill, S.L.L.; Lysenko, I.; De Palma, A.; Phillips, H.R.P.; Alhusseini, T.I.; Bedford, F.E.; Bennett, D.J.; Booth, H.; Burton, V.J.; Chng, C.W.T.; Choimes, A.; Correia, D.L.P.; Day, J.; Echeverria-Londono, S.; Emerson, S.R.; Gao, D.; Garon, M.; Harrison, M.L.K.; Ingram, D.J.; Jung, M.; Kemp, V.; Kirkpatrik, L.; Martin, C.D.; Pan, Y.; Pask-Hale, G.D.; Pynegar, E.L.; Robinson, A.N.; Sanchez-Ortiz, K.; Senior, R.A.; Simmons, B.I.; White, H.J.; Zhang, H.; Aben, J.; Abrahamczyk, S.; Adum, G.B.; Aguilar-Barquero, V.; Aizen, M.A.; Albertos, B.; Alcala, E.L.; del Mar Alguacil, M.; Alignier, A.; Ancrenaz, M.; Andersen, A.N.; Arbelaez-Cortes, E.; Armbrecht, I.; Arroyo-Rodríguez, V.; Aumann, T.; Axmacher, J.C.; Azhar, B.; Azpiroz, A.B.; Baeten, L.; Bakayoko, A.; Báldi, A.; Banks, J.E.; Baral, S.K.; Barlow, J.; Barratt, B.I.P.; Barrico, L.; Bartolommei, P.; Barton, D.M.; Basset, Y.; Batáry, P.; Bates, A.J.; Baur, B.; Bayne, E.M.; Beja, P.; Benedick, S.; Berg, A.; Bernard, H.; Berry, N.J.; Bhatt, D.; Bicknell, J.E.; Bihn, J.H.; Blake, R.J.; Bobo, K.S.; Bóçon, R.; Boekhout, T.; Böhning-Gaese, K.; Bonham, K.J.; Borges, P.A.V.; Borges, S.H.; Boutin, C.; Bouyer, J.; Bragagnolo, C.; Brandt, J.S.; Brearley, F.Q.; Brito, I.; Bros, V.; Brunet, J.; Buczkowski, G.; Buddle, C.M.; et al.The PREDICTS project—Projecting Responses of Ecological Diversity In Changing Terrestrial Systems (www.predicts.org.uk)—has collated from published studies a large, reasonably representative database of comparable samples of biodiversity from multiple sites that differ in the nature or intensity of human impacts relating to land use. We have used this evidence base to develop global and regional statistical models of how local biodiversity responds to these measures. We describe and make freely available this 2016 release of the database, containing more than 3.2 million records sampled at over 26,000 locations and representing over 47,000 species. We outline how the database can help in answering a range of questions in ecology and conservation biology. To our knowledge, this is the largest and most geographically and taxonomically representative database of spatial comparisons of biodiversity that has been collated to date; it will be useful to researchers and international efforts wishing to model and understand the global status of biodiversity.
- ItemDetermining the relationship between invasive alien species density and a country’s socio-economic status(Open Journals Publishing, 2010) Sharma, G.P.; Esler, K.J.; Blignaut, J.We explored the relationships between various socio-economic variables and the prevalence and density of invasive alien species (IAS) on a global scale using country-level data sets. We did this by testing the hypothesis that the abundance and distribution of populations of IAS are correlated with various socio-economic indicators, with the direction of causality being that the state of IAS is determined by socio-economic conditions. We found a positive and statistically significant relationship between the prevalence and density of IAS and the human development index (HDI), the satisfaction with life index and the gross domestic product (GDP) among all the countries tested. Additionally, the density of IAS increased significantly with human-population density, total geographic area, GDP and HDI. We also found a positive relationship between the density of IAS and the top 10 road networks of the world. This provides some insight into the development of renewed policies and management strategies for invasive species across both continents and countries. We do caution, however, that the results are likely to be influenced by the sampling factor, whereby affluent countries have more resources to measure and monitor IAS than poorer countries and hence have better records of such, which then indicates a stronger relationship with the level of development.
- ItemEffect of soil type and climatic conditions on the growth and flowering phenology of three Oxalis species in the Western Cape, South Africa(2013) Haukka, A.K.; Dreyer, L.L.; Esler, K.J.Generally, invasive plant species havewide environmental tolerance that enables them to exploit various climatic conditions and soil types, allowing them to invade new habitats easily. In the Cape Floristic Region, South Africa, the diversity of flowering plants can in part be ascribed to variation in these habitat characteristics that limit their distribution. Climate change has been predicted to bring warmer and drier conditions to the region, with possible influences on the climatic barriers that influence species distributions. We tested the effects of soil type and climate on the growth and flowering of the non-weedy Oxalis tomentosa, and the two weedy species Oxalis purpurea and Oxalis pes-caprae on an altitudinal gradient. The three species, all native to the region, exhibit a range of tolerances to environmental conditions: the first is habitat specific whereas the others are both well known indigenous weeds that have wider tolerance. The results showed that O. purpurea tolerates a variety of conditions well, whereas O. pes-caprae is more restricted by soil type, but would potentially profit from future climatic changes. O. tomentosa, when removed from its native habitat, was stressed under all conditions. These results suggest that habitat-restricted species will be threatened if the predicted level of climate change occurs, while invasive weeds will profit. Studying species responses to different environmental conditions is essential in determining future distributions.
- ItemEffects of Prosopis(mesquite) invasion and clearing on vegetation cover in semi-arid Nama Karoo rangeland(NISC (Pty) Ltd and Taylor & Francis, 2016) Ndhlovu, T.; Milton, S.J.; Esler, K.J.The Nama-Karoo is the largest of the three biomes that comprise the semi-arid Karoo-Namib ecoregion of southern Africa. Large tracts of Nama-Karoo rangeland have been invaded by alien leguminous trees of the genus Prosopis. We assessed the impact of Prosopis invasion and clearing on vegetation cover in heavily grazed Nama-Karoo rangeland on two sheep farms in the Beaufort West district of the Western Cape province of South Africa. Our results suggest that, below critical threshold cover levels, Prosopis trees do not affect indigenous vegetation and add to overall rangeland vegetation cover. However, once Prosopis cover exceeds critical thresholds, the trees begin to negatively impact on indigenous vegetation (especially grasses) and overall vegetation cover. In addition, our data suggest that vegetation cover recovers after Prosopis clearing, being temporarily higher than pre-invasion levels possibly as a result of enhanced post-invasion soil fertility. Lastly, our results suggest that Prosopis trees reduce rangeland erosion potential at low cover and increase it after they exceed critical threshold cover. Information about thresholds of Prosopis impact can be used to direct clearing efforts for maximum benefit when resources are limited.
- ItemFoliar nitrogen dynamics of an invasive legume compared to native non-legumes in fynbos riparian zones varying in water availability(Creative Commons Attribution License, 2019) Crous, C.J.; Drake, D.C.; Jacobsen, A.L.; Pratt, R.B.; Jacobs, S.M.; Esler, K.J.The legume Acacia mearnsii invades South Africa’s fynbos riparian zones and may alter the nitrogen (N) dynamics and supply in these areas that typically support few native N fixers. Nitrogen uptake by A. mearnsii may also be influenced by water availability, potentially affecting riparian-specific performance and impact estimations. We expected to find functional differences between the invasive legume and the two co-occurring but non-leguminous native species Brabejum stellatifolium and Metrosideros angustifolia. We also wanted to examine whether in-situ water availability affected N source or uptake in the invasive species. We found A. mearnsii was indeed functioning differently from non-N-fixing native species, and had considerably higher foliar %N. Interestingly, 15N abundance and uptake were associated with site hydrology, meaning water availability should be scrutinised when assuming N-fixing in A. mearnsii using δ15N. Nonetheless, higher water availability to A. mearnsii in fynbos riparian ecosystems did increase foliar N uptake. This has implications for prioritizing clearing of sites with increased nutrient deposition, such as dense stands in relatively moist riparian zones.
- ItemFunctional traits explain the Hutchinsonian niches of plant species(2020) Treurnicht, M.; Pagel, J.; Tonnabel, J.; Esler, K.J.; Slingsby, J.A.; Schurr, F.M.Aim: The Hutchinsonian niche is a foundational concept in ecology and evolutionary biology that describes fundamental characteristics of any species: the global maximum population growth rate (rmax); the niche optimum (the environment for which rmax is reached); and the niche width (the environmental range for which intrinsic population growth rates are positive). We examine whether these characteristics are related to inter- and intraspecific variation in functional traits. Location: Cape Floristic Region, South Africa. Time period: Present day. Major taxa studied: Twenty-six plant species (Proteaceae). Methods: We measured leaf, plant-architectural and seed traits across species geographical ranges. We then examined how species-mean traits are related to demographically derived niche characteristics of rmax, in addition to niche optima and widths in five environmental dimensions, and how intraspecific trait variation is related to niche widths. Results: Interspecific trait variation generally exceeded range-wide intraspecific trait variation. Species-mean trait values were associated with variation in rmax (R-2 = 0.27) but were more strongly related to niche optima (mean R-2 = 0.56). These relationships generally matched trait-environment associations described in the literature. Both species-mean traits and intraspecific trait variability were strongly related to niche widths (R-2 = 0.66 and 0.59, respectively). Moreover, niche widths increased with intraspecific trait variability. Overall, the different niche characteristics were associated with few, largely non-overlapping sets of traits. Main conclusions: Our study relating functional traits to Hutchinsonian niches demonstrates that key demographic properties of species relate to few traits with relatively strong effects. Our results further support the hypothesis that intraspecific trait variation increases species niche widths. Given that niche characteristics were related to distinct sets of traits, different aspects of environmental change might affect axes of trait variation independently. Trait-based studies of Hutchinsonian niches thus yield important insights into the mechanisms shaping functional biodiversity, which should reinforce the role of traits in functional biogeography.
- ItemIdentifying research questions for the conservation of the Cape Floristic Region(2019) Allsopp, N.; Slingsby, J.A.; Esler, K.J.We conducted a survey among people working in the nature conservation community in an implementation, research or policy capacity to identify research questions that they felt were important for ensuring the conservation of the Cape Floristic Region. Following an inductive process, 361 submitted questions were narrowed to 34 questions in seven themes: (1) effective conservation management; (2) detecting and understanding change: monitoring, indicators and thresholds; (3) improving governance and action for effective conservation; (4) making the case that biodiversity supports critical ecosystem services; (5) making biodiversity a shared concern; (6) securing sustainable funding for biodiversity conservation; and (7) prioritising research. The final questions were evaluated against the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services Conceptual Framework to test whether the questions addressed elements identified by this Framework as those essential to ensure that conservation contributes to a positive future for the Cape Floristic Region. We found that all elements in this Framework received attention from the collective group of questions. This finding suggests that the conservation community we approached recognises implicitly that research in multiple disciplines as well as interdisciplinary approaches are required to address societal, governance and biological issues in a changing environment in order to secure the conservation of the Cape Floristic Region. Because the majority of people responding to this survey had a background in the natural sciences, a challenge to tackling some of the questions lies in developing integrative approaches that will accommodate different disciplines and their epistemologies.
- ItemImpact of alien pines on local arbuscular mycorrhizal fungal communities – evidence from two continents(Federation of European Microbiological Societies, 2016) Gazol, A.; Zobel, M.; Cantero, J.J.; Davison, J.; Esler, K.J.; Jairus, T.; Opik, M.; Vasar, M.; Moora, M.The introduction of alien plants can influence biodiversity and ecosystems. However, its consequences for soil microbial communities remain poorly understood. We addressed the impact of alien ectomycorrhizal (EcM) pines on local arbuscular mycorrhizal (AM) fungal communities in two regions with contrasting biogeographic histories: in South Africa, where no native EcM plant species are present; and in Argentina, where EcM trees occur naturally. The effect of alien pines on AM fungal communities differed between these regions. In South Africa, plantations of alien EcM pines exhibited lower AM fungal richness and significantly altered community composition, compared with native fynbos. In Argentina, the richness and composition of local AM fungal communities were similar in plantations of alien EcM pines and native forest. However, the presence of alien pines resulted in slight changes to the phylogenetic structure of root AM fungal communities in both regions. In pine clearcut areas in South Africa, the richness and composition of AM fungal communities were intermediate between the native fynbos and the alien pine plantation, which is consistent with natural regeneration of former AM fungal communities following pine removal. We conclude that the response of local AM fungal communities to alien EcM pines differs between biogeographic regions with different histories of species coexistence.
- ItemThe impact of anthropogenically induced degradation on the vegetation and biochemistry of South African palmiet wetlands(Springer, 2018) Rebelo, A.J.; Emsens, W.-J.; Meire, P.; Esler, K.J.There are many different anthropogenic causes of wetland degradation, such as disturbances which affect the physical structure of wetlands, resulting in erosion (altered fire regimes, road and railway building through wetlands, channelization of wetlands), pollution, land-cover change, and climate change. These different types of degradation have various impacts, depending on the type of wetland, soils, biochemistry and other factors. We researched a poorly-studied South African valley-bottom peatland that is dominated by the ecosystem engineer Palmiet: Prionium serratum. We ask the question: what is the impact of degradation by gully erosion, pollution and alien tree invasion on biochemistry and plant community composition of palmiet wetlands? In 39 plots from three palmiet wetlands situated approximately 200 km apart we found that channel erosion, through a loss of alluvium, has probably resulted in leached soils with lower soil organic matter and water content, less able to retain nutrients and cations. Soil leaching is a possible explanation for the groundwater of degraded wetlands having higher electrical conductivity and pH than that of pristine wetlands and a lower soil cation exchange capacity (21.3 ± 5.80–7.7 ± 4.91 meq/100 g). The loss of alluvium typically resulted in a completely new plant community, composed mostly of pioneer species and several alien species. The increase in base saturation (17.5 ± 8.46–30.2 ± 17.85%) and soil pH (4.8 ± 0.51–5.1 ± 0.50) with degradation was hypothesized to be the result of liming practices. Once extremely degraded, i.e. all the alluvium is lost, it is unlikely that these sensitive palmiet wetlands will recover original vegetation communities and lost functions, except on long timescales. We recommend conservation of the few pristine wetlands that remain, and rehabilitation of those that still retain some of their original function.
- ItemInfluence of fire on critically endangered Swartland Shale Renosterveld in the Cape Floristic Region(International Association for Vegetation Science, 2018) Cousins, S.R; Witkowski, E.T.F.; Esler, K.J.Questions: The degree to which renosterveld shrublands are fire-dependent is currently unclear. To address this issue, the following questions were asked: (1) does smoke stimulate germination of soil-stored seeds in renosterveld; (2) does recently-burned renosterveld display changed composition and higher diversity than unburned vegetation; and (3) how do the species compositions of renosterveld soil seed banks and standing vegetation compare? Location: Swartland, Cape Floristic Region, South Africa. Methods: Soil seed bank samples from a north-and south-facing slope were smoke-treated and germinated to test for smoke-stimulated germination. Burned standing vegetation was surveyed 16 months post-fire, as was unburned vegetation on the same slopes. Seed bank species richness and density were compared between smoke-treated and untreated samples within and between slopes. Burned and unburned standing vegetation were compared within and between slopes in terms of species richness, abundance and aerial cover. Compositional similarity of the seed banks and standing vegetation was assessed. Results: Seed banks were dominated by annuals and graminoids. Smoke treatment had no effect, except for driving significantly higher species richness and seedling density in south-facing slope perennial shrubs. Species richness and seedling density were significantly higher in seed banks on the south-facing slope compared to the north-facing slope. Burned standing vegetation exhibited significantly higher diversity than unburned vegetation. Annuals and graminoids displayed significantly higher species richness and aerial cover in burned renosterveld. The north-facing slope contained less than half the number of species/m2 compared to the south-facing slope. The seed banks and standing vegetation showed low to intermediate similarity (Sørensen = 31%– 53%), but grouped close together on an NMDS plot, suggesting intermediate similarity overall. Conclusions: Elevated germination of perennial shrubs in smoke-treated seed bank samples and increased diversity of post-fire standing vegetation suggest the renosterveld in this study shows elements of a fire-driven system. Certain species only recruited in burned sites, suggesting fire-stimulated germination. Aspect had a major influence on plant community composition, with the mesic south-facing slope being more diverse than the xeric north-facing slope. The similarity between the seed banks and standing vegetation was higher than previously shown for renosterveld, and appears to be higher than for fynbos.
- ItemNitrogen and phosphorus influence Acacia saligna invasiveness in the fynbos biome(2020) Esterhuizen, N.; Forrester, J.; Esler, K.J.; Wigley-Coetzee, C.; Morcillo, R.J.; Kleinert, A.; Perez-Fernandez, M.; Valentine, A.J.This study attempts to understand how invasive legumes such as Acacia saligna may compete with indigenous legumes such as Virgilia divaricata. The two species are trees with similar growth forms. We studied the competitive ability of invasive and indigenous seedlings under variations in soil phosphorus availability. South African fynbos vegetation is threatened by invasive Acacia. The indigenous tree legume, Virgilia, grows in similar phosphorus soil conditions as Acacia although there is a gap in the knowledge of their physiology. We investigated the utilization of different inorganic P sources by the invasive A. saligna and the native V. divaricata in the presence and absence of root nodules in each species. Plant performance in terms of photosynthesis and biomass production was also analysed. Plants were cultivated in silica sand supplied with Long Ashton nutrient solution, modified to contain either 50 mu M P or 500 mu M P applied as NaH2PO4 center dot 2H(2)O. Rate of growth was estimated as the increase in mass in plants harvested after 4 and 8 weeks of growth. After 4 weeks of growth, the seedlings of Virgilia grew quicker and produced more biomass than Acacia, under both phosphorus conditions. However, this was reversed after 8 weeks of growth, with Acacia out-competing Virgilia. Increased growth of the invasive legumes was achieved by relying on soil nitrogen under high phosphorus conditions and shifting to atmospheric sources under lower phosphorus levels. The strategies of altering photosynthetic carbon balance and nitrogen acquisition under varying soil phosphorus conditions potentially underpin the invasive potential of Acacia in fynbos soils.
- ItemSoil seed banks of remnant and degraded Swartland Shale Renosterveld(International Association for Vegetation Science, 2013) Heelemann, S.; Krug, C.B.; Esler, K.J.; Reisch, C.; Poschlod, P.Questions: What are the characteristics of soil seed banks in highly endangered renosterveld vegetation and adjacent degraded sites? What is the contribution of the soil seed bank to restoring renosterveld vegetation after degradation through agriculture or afforestation? Location: Tygerberg Nature Reserve – one of the largest remaining fragments of Swartland Shale Renosterveld, Cape Floristic Region, South Africa. Methods: Assessment of vegetation, soil parameters and soil seed banks in three renosterveld sites, two adjacent abandoned fields, one pine plantation and one pine plantation clear-cut site. Smoke primer (i.e. fire surrogate) was applied to soil seed bank samples to evaluate fire as a possible management and restoration tool. Results: Abandoned agricultural fields adjacent to renosterveld remnants are characterized by alien grass cover, nutrient enrichment of the soil and depletion of the indigenous soil seed bank. In contrast, pine plantations show less alien species infestation, soil nutrient alteration and have a viable soil seed bank as well as re-development of indigenous renosterveld vegetation after clearance. Seedling recruitment was not significantly influenced by application of a smoke primer compared to the magnitude of fire response in fynbos. Conclusion: Abandoned agricultural fields (previously renosterveld) at Tygerberg have a very low restoration potential. In contrast, pine plantation sites should be given priority in restoration, because soil chemistry is less significantly altered and a viable indigenous soil seed bank is still present, which can be successfully activated through clearance management.