Vegetation in an age of global change : encroachment, succession, and phenology

Abstract

To successfully predict how ecosystems and associated species will respond to global change drivers, an understanding of the underlying drivers of the assembly, diversity and functional responses of biological communities is needed. The objective of this thesis was to provide insights into the ecology of vegetation through means of exploring phenological and successional patterns and drivers in an age of global change. First, using herbarium records, the timing and climatic drivers of fruiting phenology of 58 widespread South African fleshy- and dry-fruited tree species, which possess different physiological requirements, were tested. Second, the patterns of succession through bush clump formation (a form of woody encroachment) in a South African savanna were explored, and the microclimatic determinants of succession determined. Third, a systematic review to assess whether consistent trends exist in plant functional traits across successional gradients, and to assess whether a bias exists in trait-based successional studies (e.g., between geographic distribution, growth forms, and vegetation types) was conducted. Significant differences in fruiting phenology of fleshy- and dry-fruited species were found: fleshy-fruited species fruited earlier than dry-fruited species, and the effects of climatic predictors on fruiting phenology across species depended on fruiting type: only fleshy-fruited species were limited by winter temperatures, while high temperature seasonality typically resulted in later fruiting for both fleshy- and dry-fruited species. Bush clump formation was initiated by the establishment of a founder individual which resulted in microclimatic changes, facilitating the establishment of other species, and resulting in a deterministic successional change (as bush clump size increased) from an open habitat (characterised by savanna species) to a closed habitat (characterised by forest species). Out of 398 recorded traits reported in successional studies, only six were frequently reported across trait-based successional studies; however, all traits showed consistent trends across successional gradients. Clear biases in trait-based successional studies were observed: woody communities have been studied more often than herbaceous communities, with forest communities being disproportionally studied; and most studies have been conducted in Europe. By linking patterns of phenology and succession to environmental drivers, the impacts that global change drivers may have on community assembly patterns could be inferred. The dependence of phenology on climatic factors suggests that climate change will result in shifts in phenology with implications for seed dispersal and food availability of frugivorous animals. Changes in the microclimate conditions brought about by the associated succession of bush clump formation suggests that microclimate plays a role in the replacement of ancient savannas by forest species; such replacement will compromise the diversity and ecosystem services of savannas. This suggests that management interventions should include processes that naturally keep the system open, such as fire. Finally, our ability to generalise how traits change with succession, over broad vegetation types and geographic regions, is mostly limited to forest systems, and consistent trends observed may not hold in other biomes. Therefore, in a changing, disturbance-driven world in which succession may become increasingly important in community assembly, it is pertinent to predict how communities other than forests, and non-woody taxa, assemble post-disturbance.