Environmental drivers of Drakensberg grass species distributions at broad and fine scales

Abstract

The Poaceae is an ecologically and economically important plant family worldwide, as grasses are diverse and abundant in many regions and key to a range of ecosystem services. Numerous factors may influence the distribution of grass species, but the relative influence of these factors remains mostly unknown, both at coarse- and fine-scales. The Maloti-Drakensberg Mountain Range is a rugged and topographically complex environment that supports high levels of biodiversity, and is dominated by grassland vegetation. These montane grasslands contain a high diversity of grass species, which, in turn, support many human communities that rely on the region for livestock grazing. In Chapter 1, I (a) identify which environmental variables are related to the distribution of these grass species across broad-scales, (b) test if the different taxonomic, morphological and functional groupings of the grass species differ in the variables related to their distributions, and (c) predict how climate change will impact the distribution of these species. I show that mean temperature of the coldest quarter and precipitation of the wettest quarter were the most important predictors on average for 43 common grass species’ distributions. There were significant differences in the importance of the variables that were included in the final models between grasses that differ in photosynthetic pathways (C3 vs C4) and life forms (perennial vs annual). Future climatic conditions are predicted to reduce the area of suitable habitat for most of the species, with many of the species shifting their distribution ranges to higher elevations. In Chapter 2, I (a) determine which factors are related to the fine-scale cover and occurrence patterns of ten dominant grasses, and (b) determine if the factors influencing these grass species cover and occurrence patterns change with elevation in the Maloti-Drakensberg grasslands. I show that mesotopography and soil depth (both of which are non-climatic environmental variables) were consistently important predictors of the grass species distributions. The importance of the predictors changed with elevation, where at the highest elevation site mesotopography and rock cover were the most important predictors, the mid-elevation site indicated that soil depth was important and the lowest elevation site highlighting soil depth and soil moisture as important predictors. This study highlights the intricate relationships between environmental factors, grass species distributions and their various physiological and ecological classifications. The broad-scale study results emphasized the importance of climatic variables (including measures of temperature and precipitation), in contrast to the fine-scale study where the importance of the non-climatic variables (mesotopography and soil depth) was highlighted. Therefore, this study emphasises the imperative for further research into understanding the intricacies and complexities of the relationships between species distributions and the environment, allowing for the facilitation of well-informed conservations efforts.