Tiffany Pillay

Spatial pattern analysis and competition between Acacia karroo trees in humid savannas

Plant Ecology, 2012

Abstract It is unclear whether inter-tree competition plays a role in determining the woody plant... more Abstract It is unclear whether inter-tree competition plays a role in determining the woody plant cover of humid savannas. Spatial point-pattern analysis can give insights to the underlying processes affecting the individuals in a population. We assessed the spatial patterns of Acacia karroo trees from savannas in KwaZulu-Natal, using conventional nearest neighbour analysis and second-order spatial statistics such as Ripley&amp;#x27;s K-and L-functions, and the univariate and bivariate O-ring statistics. We predicted that juvenile trees would be ...

Grass competition is more important than fire for suppressing encroachment of Acacia sieberiana seedlings

Plant Ecology, Nov 3, 2020

We describe a simple computing technique for the tournament choice problem. It rests upon relatio... more We describe a simple computing technique for the tournament choice problem. It rests upon relational modeling and uses the BDD-based computer system RelView for the evaluation of the relation-algebraic expressions that specify the solutions and for the visualization of the computed results. The Copeland set can immediately be identified using RelView's labeling feature. Relation-algebraic specifications of the Condorcet non-losers, the Schwartz set, the top cycle, the uncovered set, the minimal covering set, the Banks set, and the tournament equilibrium set are delivered. We present an example of a tournament on a small set of alternatives, for which the above choice sets are computed and visualized via RelView. The technique described in this paper is very flexible and especially appropriate for prototyping and experimentation, and as such very instructive for educational purposes. It can easily be applied to other problems of social choice and game theory.

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Nutrient addition, fire and grass competition affects biological nitrogen fixation in Vachellia sieberiana, and associated soil respiration

Pedobiologia, Mar 1, 2023

Nutrient addition, fire and grass competition affects biological nitrogen fixation in Vachellia sieberiana, and associated soil respiration

Pedobiologia

Spatial analysis reveals facilitation in young clonal trees and competition in older trees during re-invasion of encroaching trees in an African savanna

Plant Ecology

Reinvasion of Native Invasive Trees after a Tree-Thinning Experiment in an African Savanna

Rangeland Ecology and Management, 2022

Woody plant encroachment has serious negative consequences for ranchers because of the reduction ... more Woody plant encroachment has serious negative consequences for ranchers because of the reduction in herbaceous fodder for domestic livestock. A long-term monitoring of a tree-thinning experiment was established near Magudu (northern KwaZulu-Natal, South Africa) in 1990. The objective of this experiment was to determine the optimal tree density that would maximize grass production while benefiting grasses from nitrogen fixation, hydraulic lift, and/or shade from the leguminous trees. The initial dominant species was the nitrogen-fixing tree, Dichrostachys cinerea. There were 12 plots (6 pairs) with trees removed to approximate 0, 179, 277, 428, and 625 trees ha–1, with 1 pair left as a control (unmanipulated). One plot of each replicate (except the controls) was treated with short-acting herbicide (picloram) once in 1990. We collected data on herbaceous cover and species composition, tree species composition, sizes, and mortality. We also used remote sensing to examine the patterns o...

Differential effects of nutrient addition and woody plant encroachment on grassland soil, litter and plant dynamics across a precipitation gradient

Pedobiologia, 2021

Grass competition is more important than fire for suppressing encroachment of Acacia sieberiana seedlings

Plant Ecology, 2020

We describe a simple computing technique for the tournament choice problem. It rests upon relatio... more We describe a simple computing technique for the tournament choice problem. It rests upon relational modeling and uses the BDD-based computer system RelView for the evaluation of the relation-algebraic expressions that specify the solutions and for the visualization of the computed results. The Copeland set can immediately be identified using RelView's labeling feature. Relation-algebraic specifications of the Condorcet non-losers, the Schwartz set, the top cycle, the uncovered set, the minimal covering set, the Banks set, and the tournament equilibrium set are delivered. We present an example of a tournament on a small set of alternatives, for which the above choice sets are computed and visualized via RelView. The technique described in this paper is very flexible and especially appropriate for prototyping and experimentation, and as such very instructive for educational purposes. It can easily be applied to other problems of social choice and game theory.

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Competitive effect and response of savanna tree seedlings: comparison of survival, growth and associated functional traits

Journal of Vegetation Science, 2013

Questions: What is the effect of neighbour competition on the survival, growth and biomass of mes... more

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Competitive interactions between savanna trees

despite increasing neighbourhood competition while the RGR of humid species decreased as neighbou... more despite increasing neighbourhood competition while the RGR of humid species decreased as neighbour density increased. The total biomass of both humid and mesic species also declined as the neighbourhood competition increased. In terms of competitive effect and response, we found that these two aspects of competitive ability were not concordant (i.e. good effect competitors were not necessarily good response competitors). Lastly, we found that plant traits such as specific leaf area and above-ground features (e.g. shoot biomass and leaf number) were significantly related to the competitive response or effect of savanna tree seedlings. Spatial distribution patterns of a dominant humid savanna species, Acacia karroo, revealed that juvenile plants are aggregated, as expected due to facilitation, seed dispersal and vegetative reproduction. However, the regular spacing of larger individuals due to competition and densitydependant mortality were not detected. We found, using nearest neighbour analysis, that trees with closer neighbours had smaller canopy diameters. This suggests that while competitive interactions are present, they may be weak and insufficient to cause mortality, rather resulting in decreased plant performance. Overall, I found that, at the seedling stage, neighbourhood competition was particularly important for both humid and mesic savanna trees. Competitive interactions between mesic seedlings resulted in significantly higher mortality rates, greatly reducing the recruitment of these species. Humid species, although able to successfully recruit, experienced reduced growth rates under dense neighbourhood competition. In the field, patterns of competitive interactions were difficult to detect using spatial statistics alone. However, we did find evidence of weak competitive interactions among humid savanna trees. In summary, competitive interactions were important for all savanna species at the crucial seedling stage. However, field comparisons showed that competitive interactions were relatively weak in A. karroo and resulted in reduced performance rather than differential mortality.

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Experimental drought suppresses grass productivity and passive warming promotes tree sapling performance: Insights from African savanna species

Acta Oecologica, 2022

Spatial pattern analysis and competition between Acacia karroo trees in humid savannas

Plant Ecology

It is unclear whether inter-tree competition plays a role in determining the woody plant cover of... more It is unclear whether inter-tree competition plays a role in determining the woody plant cover of humid savannas. Spatial point-pattern analysis can give insights to the underlying processes affecting the individuals in a population. We assessed the spatial patterns of Acacia karroo trees from savannas in KwaZulu-Natal, using conventional nearest neighbour analysis and second-order spatial statistics such as Ripley's K-and L-functions, and the univariate and bivariate O-ring statistics. We predicted that juvenile trees would be ...

Soil Organic Carbon Increases in Semi-Arid Regions while it Decreases in Humid Regions Due to Woody-Plant Encroachment of Grasslands in South Africa

Scientific reports, Jan 19, 2018

Grasslands and savannas are experiencing intensive land-cover change due to woody plant encroachm... more Grasslands and savannas are experiencing intensive land-cover change due to woody plant encroachment. This change in land cover is thought to alter soil carbon (C) and nitrogen (N) storage in these ecosystems. Some studies have reported a negative correlation between soil C and N and mean annual precipitation while others have indicated that there is no relationship with mean annual precipitation. We quantified the changes in C and N pools and δC and δN values to a depth of 1 m in pairs of encroached and adjacent open grassland sites along a precipitation gradient from 300 mm to 1500 mm per annum in South Africa. Our study showed a negative correlation between changes in soil organic C stocks in the 0-100 cm soil layer and mean annual precipitation (MAP). The most humid site (1500 mm MAP) had less C in shrub-encroached sites while the drier sites (300-350 mm MAP) had more C than their paired open grasslands. This study generally showed soil organic C gains in low precipitation areas...

Soil Organic Carbon Increases in Semi-Arid Regions while it Decreases in Humid Regions Due to Woody-Plant Encroachment of Grasslands in South Africa

Scientific reports, Jan 19, 2018

Grasslands and savannas are experiencing intensive land-cover change due to woody plant encroachm... more Grasslands and savannas are experiencing intensive land-cover change due to woody plant encroachment. This change in land cover is thought to alter soil carbon (C) and nitrogen (N) storage in these ecosystems. Some studies have reported a negative correlation between soil C and N and mean annual precipitation while others have indicated that there is no relationship with mean annual precipitation. We quantified the changes in C and N pools and δC and δN values to a depth of 1 m in pairs of encroached and adjacent open grassland sites along a precipitation gradient from 300 mm to 1500 mm per annum in South Africa. Our study showed a negative correlation between changes in soil organic C stocks in the 0-100 cm soil layer and mean annual precipitation (MAP). The most humid site (1500 mm MAP) had less C in shrub-encroached sites while the drier sites (300-350 mm MAP) had more C than their paired open grasslands. This study generally showed soil organic C gains in low precipitation areas...

Spatial pattern analysis and competition

It is unclear whether inter-tree competition plays a role in determining the woody plant cover of... more It is unclear whether inter-tree competition
plays a role in determining the woody plant cover of
humid savannas. Spatial point-pattern analysis can
give insights to the underlying processes affecting the
individuals in a population. We assessed the spatial
patterns of Acacia karroo trees from savannas in
KwaZulu-Natal, using conventional nearest neighbour
analysis and second-order spatial statistics such as
Ripley’s K- and L-functions, and the univariate and
bivariate O-ring statistics. We predicted that juvenile
trees would be spatially aggregated, because of
facilitation between shrubs when zones of overlap
are relatively small, while adult trees would be
regularly spaced because of the effects of densitydependent
mortality (i.e. consistent with the honeycomb
rippling model).

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Competitive effect and response of savanna tree seedlings: comparison of survival, growth and associated functional traits

by David Ward and Tiffany Pillay

Questions: What is the effect of neighbour competition on the survival, growth and biomass of mes... more Questions: What is the effect of neighbour competition on the survival, growth and biomass of mesic and humid savanna tree species? Can competitive effect and response be linked to plant functional traits?
Location: Neil Tainton Arboretum, University of KwaZulu-Natal, Pietermaritzburg, ZA.
Methods: Using a target–neighbour design, all combinations of four humid and four mesic savanna tree seedlings were tested in a greenhouse experiment, to establish the effects of neighbourhood competition on target plant performance. The competitive response and effect of each species was quantified, and regressed against several functional traits to determine which traits are predictive of competitive ability.
Results: We found that neighbour density negatively affected the survival of mesic tree seedlings but not humid tree seedlings. Mesic plants were able to maintain their relative growth rates (RGRs) despite increasing neighbourhood competition. The RGRs of humid species significantly decreased as the density of neighbours increased, indicating that competition may be a factor affecting plants in savannas receiving more than 650 mm mean annual rainfall. We used the quantile regression method to showthat the biomass of humid tree seedlings decreased with increased neighbour biomass, when considering competitive response only. We also found that resource-acquiring traits (such as maximum RGR, plant height and root biomass) were positively correlated with the competitive effect of mesic savanna seedlings, while specific leaf area captured the competitive response of humid savanna seedlings to neighbour competition.
Conclusions: Competitive interactions between savanna trees negatively affect survival and growth rate, although the competitive effect or response of a particular species depends on its adaptation to either high- or low-resource environments.
Our results show significant relationships between plant traits and
competitive ability, which may be used to predict competitive interactions between tree seedlings fromhumid and mesic savannas.

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