The functional biogeography of African acacias (Vachellia and Senegalia) at adult and seedling life stages

Abstract

Savannas constitute the largest biome on the African continent. Savanna systems are characterised by the coexistence of C4 grasses in the understorey and trees or shrubs in the overstorey. This balance is maintained by gradients of rainfall, fire and herbivory, which influence woody cover by limiting tree recruitment and persistence. Therefore, in order to persist in savannas, trees must possess adaptations to withstand these limiting factors. The aim of this thesis was to investigate the trait biogeography of African savanna trees of the Mimosoid clade, which is one of three taxa dominating the woody component of African savannas. More specifically, I assessed whether trait trade-offs exist, if fire, herbivory and precipitation drive species trait variation, and whether trait syndromes associated with fire, herbivory and precipitation exist, for a) southern African mimosoid seedlings and for b) African acacias (Vachellia and Senegalia). To measure seedling traits, twelve mimosoid species were grown in an experimental setup and several above- and belowground traits were recorded at an age of 60 days. To measure adult traits, a trait database was compiled using species descriptions of acacias from literature. Estimates of water stress, fire regime and browser biomass experienced across the distribution range of each species were extracted from macroecological maps. Linear models detected trade-offs between traits of adult Senegalia species only and revealed trait responses to aridity, fire and browser predictors for both seedlings and adults. Hierarchical clustering, t-tests and an NMDS ordination analysis were used to show drought severity was the only predictor that differed between the two trait clusters and formed a syndrome at the seedling stage. Adult syndromes were identified using hierarchical clustering, principal component analysis and canonical discriminant analysis; both Vachellia and Senegalia had four species clusters linked to different combinations of aridity, fire and browser gradients. The provenance of savanna species influenced the traits of woody species at both the seedling and adult stages. At the seedling stage, traits reflected strategies to secure a water supply and develop large resilient seedlings to avoid desiccation for species from arid areas. Certain adaptations seem to remain from seedling to adult stages, such as reduced leaf area in arid-adapted species or shorter architectures in species from highly browsed areas. In contrast, some strategies show the opposite trend in seedlings versus adults. Recovery strategies were prioritised in seedling species that originated from areas subjected to intense fire or browsing. In contrast, resistance strategies were prioritised in the adult stage and some strategies indicate a broad adaptation to more than one savanna driver. These findings have important ecological implications as they indicate how traits at both the seedling and the adult stage may offer resistance or tolerance to environmental pressures that would control woody densification and bush encroachment.