13 results on '"Esler, Karen J."'
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2. Soil seed banks of remnant and degraded Swartland Shale Renosterveld
- Author
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Heelemann, Steffen, Krug, Cornelia B., Esler, Karen J., Reisch, Christoph, and Poschlod, Peter
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- 2013
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3. Xylem Transport Safety and Efficiency Differ among Fynbos Shrub Life History Types and between Two Sites Differing in Mean Rainfall
- Author
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Pratt, Robert B., Jacobsen, Anna L., Jacobs, Shayne M., and Esler, Karen J.
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- 2012
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4. Water Stress Tolerance of Shrubs in Mediterranean-Type Climate Regions: Convergence of Fynbos and Succulent Karoo Communities with California Shrub Communities
- Author
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Jacobsen, Anna L., Esler, Karen J., Pratt, R. Brandon, and Ewers, Frank W.
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- 2009
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5. A landscape-scale assessment of the long-term integrated control of an invasive shrub in South Africa
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Esler, Karen J., van Wilgen, Brian W., te Roller, Kerry S., Wood, Alan R., and van der Merwe, Johannes H.
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- 2010
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6. Fire and life history affect the distribution of plant species in a biodiversity hotspot.
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Magadzire, Nyasha, Klerk, Helen M., Esler, Karen J., Slingsby, Jasper A., and Syphard, Alexandra
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PLANT species ,PLANT diversity ,SPECIES distribution ,PHYTOGEOGRAPHY ,LIFE history theory ,FIRE ecology ,FIRE management - Abstract
Aim: Species distribution models (SDMs) provide valuable insights into species–environment relationships and potential climate change impacts on diversity. Most SDMs do not account for the role of natural disturbance regimes such as fire in determining current and future species distributions, or how species traits mediate their response to these stressors. Here, we investigate the importance of fire in determining the distributions of species in fire‐prone fynbos vegetation, and how this varies in relation to different life history traits (growth form and fire‐response strategy). Location: Cape Floristic Region, South Africa. Methods: We modelled the distribution of 104 plant species with different life history traits, using Maxent. The model included five climatic variables, one edaphic and one fire variable. Post hoc analyses of model output and permutation procedures were conducted to assess variable importance across different life history traits. We accounted for phylogenetic autocorrelation using sister species comparisons. Results: Permutation importance scores identified fire return interval as a major determinant of fynbos species' distributions. Linear mixed effect analyses revealed that seeder species were significantly more sensitive to fire than resprouters. Coefficients from the (linear) response curves of the different predictors indicated that the occurrence of species across all life histories was negatively associated with longer fire return intervals. Main conclusions: Fire and life history traits governing species' response to fire are key factors determining species distributions in our study system. SDMs that ignore the role of fire in driving species distributions, and how this varies across different life history types, compromise our ability to understand species–environment relationships in fire‐prone ecosystems. There is great need for better spatial data describing historical, current and future fire regimes and for models that can incorporate different responses based on species life histories, to improve vulnerability assessments for fire‐prone ecosystems. [ABSTRACT FROM AUTHOR]
- Published
- 2019
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7. Physiological responses to folivory and phytopathogens in a riparian tree, Brabejum stellatifolium, native to the fynbos biome of South Africa.
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Maoela, Malebajoa A., Esler, Karen J., Jacobs, Shayne M., and Roets, Francois
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RIPARIAN plants , *FYNBOS , *ALMOND , *PHYTOPATHOGENIC fungi , *ARTHROPODA - Abstract
Abstract: The canopies of many tree species sustain a large diversity of folivorous arthropods and phytopathogenic fungi. These organisms are thought to influence overall tree and stand productivity. Leaf diseases caused by Phyllosticta owaniana and Periconiella velutina, phytopathogenic fungi commonly found on the native riparian tree Brabejum stellatifolium (wild almond), like any other leaf disease, can potentially reduce a plant's photosynthetic efficiency. In addition to these two phytopathogens, the weevils Setapion provinciale and Setapion quantillum are abundant in wild almond canopies. Despite their pervasive occurrence, the impacts of these phytopathogens and arthropods on host tree leaf physiology have not been examined. The gas exchange response of wild almond leaves to phytopathogens and folivore damage was assessed. Leaf nitrogen, phosphorus and water content were also determined. Declines in photosynthetic rates and other physiological parameters were associated with increasing damage severity by weevils and phytopathogens in leaves of B. stellatifolium. Nitrogen and phosphorus contents were negatively associated with disease severity. Water and phosphorus contents were also negatively correlated with increased weevil damage, while nitrogen content was positively correlated with it. The observed responses of B. stellatifolium metabolic functioning to fungal pathogen and folivory indicate a possibility of suppressed wild populations of wild almond. [ABSTRACT FROM AUTHOR]
- Published
- 2018
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8. Xylem density, biomechanics and anatomical traits correlate with water stress in 17 evergreen shrub species of the Mediterranean-type climate region of South Africa.
- Author
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JACOBSEN, ANNA L., AGENBAG, LIZE, ESLER, KAREN J., PRATT, R. BRANDON, EWERS, FRANK W., and DAVIS, STEPHEN D.
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CLIMATE change ,XYLEM ,BIOMECHANICS ,ANATOMICAL specimens ,EVERGREENS ,SHRUBS ,FYNBOS ,PHYLOGENY - Abstract
1 Climate change in South Africa may threaten the sclerophyllous evergreen shrubs of this region. Available data suggest that they are not as tolerant of water stress as chaparral shrubs occurring in climatically similar California, USA. 2 Seventeen species from nine angiosperm families, including both fynbos and succulent karoo species, were studied at a field site in Western Cape Province, South Africa. Minimum seasonal pressure potential ( P
min ), xylem specific conductivity ( Ks ), stem strength against breakage (modulus of rupture, MOR), xylem density, theoretical vessel implosion resistance ( ) and several fibre and vessel anatomical traits were measured. 3 Species displayed great variability in Pmin , similar to the range reported for chaparral and karoo shrub species, but in contrast to previous reports for fynbos shrubs. 4 More negative Pmin was associated with having greater xylem density, MOR and . There was no relationship between Pmin and traits associated with increased water transport efficiency. 5 Xylem density integrates many xylem traits related to water stress tolerance, including Pmin , MOR and , as well as percentage fibre wall, parenchyma, vessel area and fibre lumen diameter. 6 Xylem density may be an integral trait for predicting the impact of climate change on evergreen shrubs. [ABSTRACT FROM AUTHOR]- Published
- 2007
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9. Ecological research and conservation management in the Cape Floristic Region between 1945 and 2015: History, current understanding and future challenges.
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van Wilgen, Brian W., Carruthers, Jane, Cowling, Richard M., Esler, Karen J., Forsyth, Aurelia T., Gaertner, Mirijam, Hoffman, M. Timm, Kruger, Frederick J., Midgley, Guy F., Palmer, Guy, Pence, Genevieve Q. K., Raimondo, Domitilla C., Richardson, David M., van Wilgen, Nicola J., and Wilson, John R.U.
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ECOLOGICAL research ,CONSERVATION of natural resources ,ENVIRONMENTAL protection ,BIODIVERSITY research - Abstract
In 1945, the Royal Society of South Africa published a wide-ranging report, prepared by a committee led by Dr C.L. Wicht, dealing with the preservation of the globally unique and highly diverse vegetation of the south-western Cape. The publication of the Wicht Committee’s report signalled the initiation of a research programme aimed at understanding, and ultimately protecting, the unique and diverse ecosystems of the Cape Floristic Region. This programme has continued for over 70 years, and it constitutes the longest history of concerted scientific endeavour aimed at the conservation of an entire region and its constituent biota. This monograph has been prepared to mark the 70th anniversary of the Wicht Committee report. It provides a detailed overview of the circumstances that led up to the Wicht Committee’s report, and the historical context within which it was written. It traces the development of new and substantial scientific understanding over the past 70 years, particularly with regard to catchment hydrology, fire ecology, invasive alien plant ecology, the harvesting of plant material and conservation planning. The Wicht Committee’s report also made recommendations about ecosystem management, particularly with regard to the use of fire and the control of invasive alien plants, as well as for the establishment of protected areas. Subsequently, a combination of changing conservation philosophies and scientific conservation planning led to the creation and expansion of a network of protected areas that now covers nearly 19% of the Cape Floristic Region. We also review aspects of climate change, most of which could not have been foreseen by the Wicht Committee. We conclude that those responsible for the conservation of these ecosystems will face many challenges in the 21st century. These will include finding ways for effectively managing invasive alien plants and fires, as foreseen by the Wicht Committee. While the protected area network has expanded beyond the modest targets proposed by the Wicht Committee, funding has not kept pace with this expansion, with consequences for the ability to effectively manage protected areas. The research environment has also shifted away from long-term research conducted by scientists embedded in management agencies, to short-term studies conducted largely by academic institutions. This has removed a significant benefit that was gained from the long-term partnership between research and management that characterised themodis operandusof the Department of Forestry. Growing levels of illegal resource use and a changing global climate also pose new challenges that were not foreseen by the Wicht Committee. [ABSTRACT FROM PUBLISHER]
- Published
- 2016
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10. Invasion, alien control and restoration: Legacy effects linked to folivorous insects and phylopathogenic fungi.
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Maoela, Malebajoa A., Jacobs, Shayne M., Roets, Francois, and Esler, Karen J.
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INTRODUCED species ,BIOLOGICAL invasions ,PLANT introduction ,TREE varieties ,PLANT species - Abstract
Invasive alien trees increase native tree stress and may increase attack by herbivores and pathogenic fungi. Alien tree removal should ameliorate such impacts. Here we compared the levels of damage by phylopathogenic fungi and folivorous insects on Brabejum stellatifolium and Metrosideros angustifolia (native trees) and Acacia mearnsii (invasive tree species) among near-pristine, invaded and restored sites. Generally, foliar damage levels were higher at invaded than at near-pristine sites. Damage levels at restored sites were similar, or even higher than those at invaded sites. Decreased native tree species richness did not explain these patterns, as restored sites had native tree species richness levels similar to those of near-pristine sites. Increased host abundance and leaf nitrogen content did not significantly correlate with increased damage in most cases. Therefore, plant species richness recovers following restoration, but native trees still experience increased pressure from folivores and phylopathogenic fungi, which may even exceed levels experienced at invaded sites, thus impacting recovery trajectories. [ABSTRACT FROM AUTHOR]
- Published
- 2016
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11. Soil nitrogen availability favours the growth but not germination of secondary invaders after clearing invasive Acacia saligna.
- Author
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Nsikani, Mlungele M., Gaertner, Mirijam, Latombe, Guillaume, and Esler, Karen J.
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NITROGEN in soils , *GERMINATION , *WILD oat , *INTRODUCED species , *ACACIA , *NATIVE plants - Abstract
• Secondary invader germination is not influenced by available soil nitrogen. • Elevated available soil nitrogen favours secondary invader growth. • Secondary invader species respond differently to available soil nitrogen. • Secondary invasion can be managed through a reduction in soil nitrogen levels. Invasive alien nitrogen-fixing species, such as Australian acacias, often leave a legacy of elevated available soil nitrogen after their removal. This legacy effect can facilitate secondary invasion by other alien species, thereby preventing natural restoration of areas being managed. To restore viable native plant communities in ecosystems where secondary invasion is a barrier to restoration, it is important to understand the soil legacy effects of invasions. Using Acacia saligna (Labill.) H. L. Wendl. (Fabaceae) invasions in the South African fynbos as case study, we determined (1) the extent to which levels of available soil nitrogen influence the germination and growth of secondary invaders; and (2) how this differs between secondary invader species. We chose five of the most common species that have been identified as secondary invaders after clearing invasive A. saligna in the Cape Flats Sand Fynbos: (i) Avena fatua (L.) (Poaceae), (ii) Briza maxima (L.) (Poaceae), (iii) Bromus diandrus (Roth.) (Poaceae), (iv) Hypochaeris radicata (L.) (Asteraceae), and (v) Raphanus raphanistrum (L.) (Brassicaceae). Using proportional fertigation, we created soil nitrate levels similar to those found in non-invaded areas (1mg/kg), and the lowest (3 mg/kg), median (7.5 mg/kg) and highest (12 mg/kg) levels typically found in areas previously invaded by A. saligna up to ten years after clearing. For each soil nitrate level, we germinated secondary invader seeds (five seeds per petri dish) in an incubator (five species × four soil nitrate levels × five replicates = 100 petri dishes). Furthermore, for each soil nitrate level, we grew secondary invaders (one plant per pot) in a greenhouse tunnel for five months (five species × four soil nitrate levels × five replicates = 100 pots). There was no significant relationship between germination success and soil nitrate level for any species. However, root and shoot dry mass were significantly positively correlated to soil nitrate level for all species. The relationship was nonetheless only linear for Bromus diandrus, Hypochaeris radicata , and Raphanus raphanistrum. These results indicate that the legacy of elevated available soil nitrogen does not have an effect on the germination of secondary invaders, but favours their growth. Therefore, secondary invasion can be managed through a reduction in soil nitrogen levels. However, secondary invader growth does not decrease at the same rate for all species in response to a decrease in available soil nitrogen. Thus, the use of soil nitrogen reduction as a tool for the management of secondary invasions should take into account the species being managed. However, given that multiple secondary invader species often dominate a restoration site, restoration efforts should reduce soil nitrogen to non-invaded levels to account for the different response rates to available soil nitrogen, and simply management efforts. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
- View/download PDF
12. Active seed sowing can overcome constraints to passive restoration of a critically endangered vegetation type.
- Author
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Hall, Stuart A., Holmes, Patricia M., Gaertner, Mirijam, and Esler, Karen J.
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SOWING , *SEEDS , *INTRODUCED plants , *NATIVE plants , *INTRODUCED species , *ENDANGERED plants - Abstract
• Active sowing results in successful establishment of native shrub cover and diversity. • Pre-treatment of seed can improve success of a restoration sowing intervention. • Passive restoration is seed limited, suggesting a biotic threshold has been crossed. • Invasive plant seedbanks are not more effectively depleted by burning after clearing. • A decision tree has been developed to determine best protocols for restoration based on site-specific characteristics. 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. [ABSTRACT FROM AUTHOR]
- Published
- 2021
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13. Foliar nitrogen dynamics of an invasive legume compared to native non-legumes in fynbos riparian zones varying in water availability.
- Author
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Crous, Casparus J., Drake, Deanne C., Jacobsen, Anna L., Pratt, R. Brandon, Jacobs, Shayne M., and Esler, Karen J.
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ACACIA mearnsii , *STABLE isotopes , *FYNBOS , *RIPARIAN areas , *NITROGEN-fixing plants , *NITROGEN fixation - Abstract
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. [ABSTRACT FROM AUTHOR]
- Published
- 2019
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