The influence of drought and abiotic drivers on a mesic grassland.

Abstract

A significant percentage of southern African grasslands have undergone deteriorative transformation due to climate-related changes. This PhD research study investigated the impacts of drought on species composition dynamics in a South African mesic grassland with data collected over a period of 3 years. Chapter 3 investigated the effects of a short-term drought on plant species composition under different burning management regimes on the long-term veld burning experiment at Ukulinga Research Farm, University of KwaZulu Natal, South Africa. The results showed that annual burning produced higher species diversity and richness while no burning promoted the development of woody plants such as Vachellia nilotica and forbs. The short-term drought did not have a significant impact on the species composition of the grassland. Chapter 4 assessed the influence of drought on grass-on-grass interactions in pot trials at the Neil Tainton Arboretum greenhouse and the study found that the presence of competition reduced phytometer biomass, but at varying levels with different species. Broad-leaved species had a stronger competitive effect over narrow-leaved species as they reduced the number of tillers, biomass production and tuft size of narrow-leaved species. The competitive strength within broad-leaf species was species-specific, with Megathrysus maximus being more competitive than Cymbopogon excavatus. Palatable species had a dominant competitive effect over unpalatable species without exposure to defoliation and Tristachya leucothrix had a stronger competitive effect over Themeda triandra. Drought did not have any significant impact on final dry biomass, but significantly reduced the tiller numbers for palatable species as well as tiller numbers and tuft circumference for broad-leaved species. Chapter 5 focused on how the grass sward composition, biomass production as well as root biomass and length were affected by different moisture levels, including drought at 50% rainfall reduction on the Drought Net Experiment at Ukulinga Research Farm. Species diversity, richness and aboveground biomass were significantly affected by varying weather patterns, while all were not changed by moisture manipulations. Species diversity and richness increased in 2021/2022 while the biomass was greater in 2020/2021. On the contrary, high moisture levels reduced root biomass and this can be attributed to resources being allocated to greater above-ground biomass during moisture availability. Greater root biomass was found in the top 16cm of soil. Chapter 6 assessed the influence of nutrient addition (nitrogen, phosphorus, potassium) on species composition, diversity and biomass production under different moisture levels including drought (50% rainfall reduction) over 3 years on the Drought Net Experiment at Ukulinga Research Farm. Interannual climate variability significantly increased species richness and diversity by 2021/2022. An interaction between fertilization and years shows a significant difference in response to interannual climate variability across fertilizer levels. Annual weather patterns refer to the differences in each 12-month cycle, centred around the growing season The unfertilized plots were unaffected by annual climate fluctuations and the fertilized plots showed significantly greater diversity in 2021/2022, the growing season with the highest rainfall. Species evenness fluctuated annually in the high and ambient rainfall treatments while it remained unchanged in the drought treatment. Tristachya leucothrix, Themeda triandra and Cymbopogon excavatus were abundant in both unfertilized and fertilized sites but most species increased in abundance in the fertilized areas. High moisture levels significantly increased plant biomass production while drought had the opposite effect. The findings from this study have shown that interannual climate variability and fertilization had a significant impact on species composition and productivity. The length of drought is a key factor that determines if a drought causes significant impacts on a plant community and long-term experimental applications such as fire, have more pronounced implications on a grassland community. Lastly, the competitive effect of plants is species-specific, and influenced by plant morphological traits. The results of this research study emphasize the importance of long-term experimental research for varying levels of drought and fire on mesic grasslands.