Effects of climate and land-use change on grass and tree populations and their consequences to the ecosystem multi-functionality, Limpopo, South Africa

Abstract

Changes in climate and land-use, collectively called environmental changes, have been a source of concern globally, particularly in dryland grasslands, where people still heavily rely on services from these ecosystems. Extreme climatic conditions have been projected to increase both in intensity and frequency globally. In semi-arid regions, drought is anticipated to occur more frequently and to last longer as a consequence of climate change. Moreover, as human populations continue to grow, there is an increase in demand for natural resources that are already diminishing. Consequently, the combination of these factors has a negative effect on the functions and services of the dryland grassland ecosystems. Therefore, to counteract the degradation of these socioeconomically significant ecosystems, it is vital to understand how these systems respond to the long-term effects of drought and grazing. Limpopo province is largely dominated by drylands; comprising arid, semi-arid and dry sub-humid ecosystems. In Limpopo province, rangelands and agroforestry systems deliver important ecosystem services. Arable lands, rangeland, agroforestry, and orchards are three major land-use types contributing greatly to local livelihoods within Limpopo’s multi-use landscapes. Motivated by the above mentioned factors this study had the following objectives; (i) to review the impact of climate change on dryland grasslands, (ii) to evaluate ecosystem functioning through the assessment of climate related effects on taxonomic diversity and density demography from the grass layer, (iii) to analyse the effects of drought and grazing on the grass layer and to understand the factors affecting tree populations, particularly tree establishment patterns, (iv) to measure ecosystem service provision from the savanna ecosystem and also, to bridge the knowledge gap on the importance of biodiversity in an ecosystem. To achieve the aforementioned objectives, a comprehensive literature analysis was conducted on the effects of climate change on dryland grasslands to assess the magnitude of this impact and the existing understanding of vegetation dynamics in the face of climate change. The study also took advantage of the large-scale field experiment which evaluated, through precipitation manipulation, the impact of drought on grazed and ungrazed vegetation in the dryland grasslands of Limpopo province, South Africa, labeled drought Act experiment. In the Drought Act experiment, passive rain-out shelters and grazing ex-closure fences were set up to simulate a severe drought in combination with differing resting schemes of the rangeland. This was done in order to assess the effect of previous drought events on herbaceous vegetation. Grazing and drought treatments were implemented across four treatment plots per block, via a full factorial design. The study also took advantage of the steep gradient of climatic aridity in Limpopo province and used a space-for-time substitution to evaluate the effects of climate-induced risks and factors impacting the establishment of encroaching woody species under conditions of climate change. Two climate zones and soil types were selected; semi-arid vs. dry sub-humid zone, and Glenrosa soil vs. Hutton soil. Data analysis was executed using the R statistical software package. The examination of literature revealed that African dryland ecosystems are especially vulnerable to the effects of climate change, resulting in biodiversity loss, structural and functional changes to the ecosystem, and a diminished capacity to deliver ecosystem services. Climate change’s most susceptible species and functions have a great potential to be utilized as early warning signs. Furthermore, precipitation manipulation experiments are a great tool for investigating the impact of climate change as they allow for precipitation reduction below the natural range. There is still a general lack of information regarding the effects that extreme climatic conditions have on ecosystems and the mechanisms that determine how ecosystems respond and recover from stress and disturbances. The Drought Act experiment showed that prolonged drought had a substantial and negative impact on the biomass output of the vast majority of taxonomic groups and plant functional types (PFTs). This reduction in biomass production from the grass layer results in limited grazing for livestock, which is a primary ecosystem service provided by dryland grasslands. However, the study revealed that few species and PFTs were resistant to the effects of prolonged drought and grazing. In general, the study showed that long-term drought and grazing winners were primarily forbs and narrow-leaved perennial grasses with low leaf area (LA) and high leaf dry matter content (LDMC). Furthermore, the negative impact of drought on the taxonomic richness and species per unit area and ultimately diversity, worsened as the drought period increased. Additionally, grazing exclusion (resting) was shown to have a negative influence on species richness, abundance, and diversity, especially over long periods of time. Bottom-up mechanisms such as soil type had a greater impact on the establishment, recruitment, and survival of invading woody species than top-down mechanisms such as precipitation. In addition, the significant correlation that was established in the study between the age of trees and the circumferences of their stems, measured at breast height, provided evidence that non-destructive methods of estimating the age of trees are feasible. Further development of non-invasive approaches in the field of dendrochronology is also made possible by these findings. The findings of this thesis indicate, on the whole, that; to gain a better understanding of dryland vegetation dynamics in the face of drought, researchers need to investigate further the impact of climatic extremes on ecosystem functions and services. Moreover, winners and losers of long-term drought can be distinguished by their unique characteristics; hence, taxonomic groups and functional characteristics could be utilized as early markers of veld degradation, which would permit timely management interventions. The negative impact of long-term drought and grazing on the grass layer limits the ecosystem’s capacity to carry livestock and wildlife for extended periods, thus impacting the livelihoods of the people who rely on these ecosystems. In addition, the tendency of higher tree establishment in lower rainfall years suggests that drought could be a driving factor for woody vegetation propagation. The trait-based approach is very instructive when it comes to researching the dynamics of vegetation in dryland grasslands. This is especially true when considering the effects of changing climate and land-use. This study has contributed to a better knowledge of the ecosystem function under changing climate and land-use, which is the basis of enhancing the resilience of different land-use systems and reducing risks to ecosystem functions and services while optimizing production.