Esler K


Recent Submissions

Now showing 1 - 5 of 68
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    A 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.
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    Active 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.
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    Nitrogen 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.
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    Levers and leverage points for pathways to sustainability
    (2020) Chan, K.M.A.; Boyd, D.R.; Gould, R.K.; Jetzkowitz, J.; Liu, J.; Muraca, B.; Naidoo, R.; Olmsted, P.; Satterfield, T.; Selomane, O.; Singh, G.G.; Sumaila, R.; Ngo, H.T.; Boedhihartono, A.K.; Agard, J.; de Aguiar, A.P.D.; Armenteras, D.; Balint, L.; Barrington-Leigh, C.; Cheung, W.W.L.; Díaz, S.; Driscoll, J.; Esler, K.J.; Eyster, H.; Gregr, E.J.; Hashimoto, S.; Hernández Pedraza, G.C.; Hickler, T.; Kok, M.; Lazarova, T.; Mohamed, A.A.A.; Murray-Hudson, M.; O'Farrell, P.; Palomo, I.; Saysel, A.K.; Seppelt, R.; Settele, J.; Strassburg, B.; Xue, D.; Brondízio, E.S.
    Humanity is on a deeply unsustainable trajectory. We are exceeding planetary boundaries and unlikely to meet many international sustainable development goals and global environmental targets. Until recently, there was no broadly accepted framework of interventions that could ignite the transformations needed to achieve these desired targets and goals. As a component of the IPBES Global Assessment, we conducted an iterative expert deliberation process with an extensive review of scenarios and pathways to sustainability, including the broader literature on indirect drivers, social change and sustainability transformation. We asked, what are the most important elements of pathways to sustainability? Applying a social–ecological systems lens, we identified eight priority points for intervention (leverage points) and five overarching strategic actions and priority interventions (levers), which appear to be key to societal transformation. The eight leverage points are: (1) Visions of a good life, (2) Total consumption and waste, (3) Latent values of responsibility, (4) Inequalities, (5) Justice and inclusion in conservation, (6) Externalities from trade and other telecouplings, (7) Responsible technology, innovation and investment, and (8) Education and knowledge generation and sharing. The five intertwined levers can be applied across the eight leverage points and more broadly. These include: (A) Incentives and capacity building, (B) Coordination across sectors and jurisdictions, (C) Pre‐emptive action, (D) Adaptive decision-making and (E) Environmental law and implementation. The levers and leverage points are all non-substitutable, and each enables others, likely leading to synergistic benefits. Transformative change towards sustainable pathways requires more than a simple scaling-up of sustainability initiatives—it entails addressing these levers and leverage points to change the fabric of legal, political, economic and other social systems. These levers and leverage points build upon those approved within the Global Assessment's Summary for Policymakers, with the aim of enabling leaders in government, business, civil society and academia to spark transformative changes towards a more just and sustainable world.
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    Water-use characteristics of Palmiet (Prionium serratum), an endemic South African wetland plant
    (2020) Rebelo, A.J.; Jarmaln, C.; Esler, K.J.; Cowling, R.M.; Le Maitre, D.C.
    Palmiet, Prionium serratum, is an endemic wetland plant which dominates oligotrophic wetlands throughout the Cape Floristic Region, South Africa. Palmiet is often perceived as undesirable by landowners, in part because it is thought to have high water-use, although little is known about the water-use of this important wetland species. We estimated the water-use dynamics of Palmiet at the leaf scale, using stomatal conductance measurements, and at the wetland scale, by modelling evapotranspiration using remote sensing and an energy-balance model. Factors that influenced Palmiet water-use were also considered, and seasonal variations were analysed. The aim was to estimate Palmiet wetland water-use, and to develop a set of crop factors (Kc) for use in hydrological modelling of catchments containing Palmiet wetlands. Results show that Palmiet has a comparatively low stomatal conductance (11–152 mmol∙m-2∙s-1), which was lower in summer than winter, and moderate evapotranspiration for a riparian species (1 220 mm∙a-1 compared to a local reference evapotranspiration of 1 302 mm∙a-1 and A-Pan evaporation of 2 809 mm∙a-1), which was higher in summer (more energy to drive evapotranspiration and higher vapour pressure deficits). Morphological and physiological adaptations to nutrient poverty or periodic drought are suggested theories which may explain the controls on transpiration for Palmiet.