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

Abstract

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 show that 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 from humid and mesic savannas.

Competitive response of savanna tree seedlings to C4 grasses is negatively related to photosynthesis rate

“…The relationship with photosynthetic rates suggests a trade-off between the ability of tree seedlings to tolerate high grass biomass (i.e., conditions of resource limitation) and their ability to perform under high-resource conditions. Previously, Pillay and Ward (2014) had found a positive relationship between SLA and tree competitive response across four savanna species (which would be broadly consistent with our hypothesis), but this response was in competition with other tree species rather than with grasses. This suggests that a trade-off exists between the inherent ability to acquire resources by savanna trees and their ability to tolerate grass competition.…”

Dispersal and recruitment limitations in secondary forests

“…The understorey plant community (i.e. seedlings/saplings) comprises the species that have been successfully dispersed to a site and can provide an indication of the future composition of the adult plant community (Dalling et al, 1998;Pillay and Ward, 2014). The adult plant community composition (i.e.…”

Patterns in woody vegetation structure across African savannas

“…In dry savannas, water availability determines the establishment, growth and survival of plants and competitive plant traits are often of a water-saving nature (Chesson et al, 2004;Pillay and Ward, 2014). Abiotic environmental factors, such as the rainfall regime, soil type, and topography, impact ecohydrological processes by controlling infiltration rates, runoff generation, and available water capacity, which in turn impact the growth and survival of woody plants in the landscape .…”