Thesis (Ph.D. (Plant Production)) -- University of Limpopo, 2022 / 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. / NRF, DAAD, UL, SALL net, Universitat Potsdam
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ul/oai:ulspace.ul.ac.za:10386/4147 |
| Date | January 2022 |
| Creators | Mokoka, Malesela Vincent |
| Contributors | Ayisi, K. K., Linstadter, A. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | xxii, 132 leaves |
| Relation |
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