Savanna woody plant dynamics; The role of bark thickness

J.J. Midgley, G. Hempson, L. Kruger, K. Vickers
2013 South African Journal of Botany  
While tree-grass interactions have been widely documented, the effects of tree-tree competition have been largely overlooked. This study investigated the effects of neighbour competition on the survival, growth and biomass of mesic and humid savanna tree species. Using a target-neighbour design, all combinations of four humid and four mesic savanna tree seedlings were grown in a greenhouse experiment, to establish the effects of neighbourhood competition on target performance. The competitive
more » ... sponse and effect of each species was quantified, and regressed against several functional traits to determine which traits are predictive of competitive ability. We found that neighbour density negatively affected the survival of mesic tree seedlings only. However, mesic plants were able to maintain their relative growth rates despite increasing neighbourhood competition, while the relative growth rates 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 also found that resource-acquiring traits (such as maximum relative growth rate, plant height and root biomass) were related to the competitive effect of mesic savanna seedlings, while specific leaf area captured the competitive response of humid savanna seedlings to neighbour competition. 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, and aid in control of bush encroachment savanna sites. b Organisation for Tropical Studies, Skukuza, Mpumalanga South Africa Fire tests the absolute thickness of the bark of woody plants. However, absolute bark thickness is not a species specific trait because it largely depends on stem size, which in turn depends on plant age and growth rates. Relative bark thickness (ratio of bark thickness to stem diameter) is a species specific trait. However, it is not straight forward to determine because of non-linear relationships between stem size and bark thickness. In this presentation we consider appropriate ways of determining relative bark thickness and whether relative bark thickness is a useful trait for understanding woody plant dynamics in fire-prone savannas. The savanna biomes are unique with regard to the co-existence of trees and grass. Variability in tree demography largely determines the structure of savannas. The physiognomy of savannas ranges from open grasslands with few trees to closed woodlands with an underlying grass layer. Sapling stage is the most vulnerable phase in the life history of trees which is mainly affected by resource limitation (water, light and nutrients), grass competition and disturbances (fire and herbivory). All these factors play a crucial role in tree sapling establishment, critical to long-term ecosystem functioning, either directly or by interacting with one another. We tested the effects of rainfall (frequent watering vs natural rainfall), shade (presence vs absence), nutrients (presence vs absence), grass competition (presence vs absence) and presence vs absence of defoliation (used to simulate herbivory) on savanna tree sapling survival and growth in a controlled field experiment. We conducted our experiment in a humid (N 1000 mm mean annual precipitation) South African savanna at KwaMbonambi, KwaZulu-Natal, South Africa. We used four humid savanna species (Acacia karroo, A. sieberiana, Schotia brachypetala and Strychnos spinosa) and four mesic savanna (~750 mm MAP) species (Acacia tortilis, A. nigrescens, Colophospermum mopane and Combretum apiculatum). Sapling survival was not dependent on any of the treatment factors provided. This may be due to the presence of favourable conditions such as low irradiance, temperature and high rainfall associated with humid savannas. We found that light is the most important limiting factor affecting sapling growth, among all the resources provided. Grass competition had a substantial suppressive effect on tree sapling growth. High stem growth rates after defoliation indicated that saplings recover well after herbivory. It is commonly understood that in humid savannas, frequent fires restrict tree recruitment, due to the presence of high grass fuel loads. However, we show that shade has a high potential to alter tree-grass dynamics in humid savannas. (1939) proposed a two-layer hypothesis, an equilibrium explanation for coexistence of savanna trees and grasses. This hypothesis relies on vertical niche partitioning and assumed that grasses are more water-use efficient than trees and use subsurface water while trees also have access to deeper water sources. Thus, in open savannas, grasses were predicted to predominate because of their water use efficiency and access to subsurface water. This hypothesis has been a prominent part of the savanna literature since first proposed. We review the literature on Walter's hypothesis and reconsider his original intentions. Walter intended this hypothesis to be restricted to dry savannas. In his opinion, mesic and humid savannas were controlled by biotic factors and disturbances. We surveyed the global savanna literature for records of vertical niche partitioning by grasses and trees. We find that, within the scope of Walter's original intentions, this hypothesis works remarkably well, and in some cases is appropriate for deserts as well as for dry temperate systems and even some mesic savannas.
doi:10.1016/j.sajb.2013.02.014 fatcat:wwrvdhogzncwfemnvxs4nwbqxe