Spatio-temporal dynamics of woody vegetation structure in a human-modified South African savanna
Date
2016
Authors
Mograbi, Penelope Jane
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Abstract
Ecosystem services, nature’s benefit to people, contribute to human well-being. Extensive
reliance on, and unsustainable use of, natural resources is typical of the rural poor in
developing countries and can lead to ecosystem degradation, decreased ecosystem service
provision, and increased vulnerabilities of rural populations. Most ecosystem services are
intangible or difficult to quantify, but fuelwood provisioning can be measured directly and
can serve as a proxy for the status of other ecosystem services (e.g. aesthetic and spiritual
services, nutrient cycling, carbon sequestration). South African rural communities have a
high reliance on fuelwood despite extensive access to electricity. Within this context, live
wood harvesting occurring around rural settlements in increasing amounts has been deemed
unsustainable. However, the ‘fuelwood crisis’ of the 1970s, and subsequent predictions of
woodland collapse through fuelwood supply-demand models, has still not occurred despite
substantial population growth in developing countries. Hypothesised reasons for modelled
supply-demand mismatches are based on underestimation of fuelwood supply and woodland
regeneration, as well as overestimation of fuelwood demand by discounting behavioural
adaptability of users. It is likely that the spatial configuration of fuelwood use allows for the
co-adaptability of both humans and ecosystems. A lack of understanding of the spatial
configuration of these social-ecological dynamics limits our insights into current and future
adaptive responses and thus, the degree of sustainability. This thesis aimed to assess woody
biomass stocks and vertical structure changes, as a proxy for provisioning ecosystem
services, in a spatially and temporally explicit manner, to describe the status and impact of
wood extraction in semi-arid, savanna communal lands. Using repeat, airborne light detection
and ranging (LiDAR) data from 2008 and 2012, we surveyed three-dimensional woodland
structure in Bushbuckridge Municipality communal lands – the grazing and harvesting areas
for densely populated rural settlements in former Apartheid ‘homelands’ in South Africa.
Woody biomass in 2008 ranged from 9 Mg ha-1 on gabbro geology to 27 Mg ha-1 on granitic
geology. Land-use pressure was associated with compensatory regrowth of savanna tree
species through post-harvest coppice in the 1-3m height class. Woody biomass increased at
all sites, contrary to previous fuelwood models of the area. Change detection in the vertical
canopy structure revealed that biomass increases were almost solely attributable to the 1-3m
and 3-5m height classes. These changes were exacerbated by wood extraction intensity in the
communal lands – the communal land with the highest wood extraction pressure experienced the greatest biomass increases, likely a strong regrowth response to high harvesting levels.
Within communal lands, areas closest to roads and settlements experienced substantial
biomass increases as a result of shrub level gains. This relationship was mediated by the
usage gradient – the greater the wood extraction pressure on the communal land, the larger
and more spatially coalesced the ‘hotspots’ of shrub-level increases were in relation to ease of
access to the communal land. However, biomass increases are not necessarily indicative of
woodland recovery, as shrub-level increases were coupled with losses of trees >3m in height.
To explore these tall tree dynamics further, we tracked >450 000 individual tree canopies
over two years over contrasting landscapes – a private reserve containing elephants, two
communal lands under different wood extraction pressures, and a nature reserve fenced off
from both elephants and humans. Humans are considerable drivers of treefall (defined here as
a ≥75% reduction in the maximum height of each tree canopy) in communal lands. Humanmediated
biennial treefall rates were 2-3.5 fold higher than the background treefall rate of
1.5% treefall ha-1 (in the control site – the reserve containing neither elephants nor humans).
Elephant-mediated treefall was five fold higher than the background rate. Rate and spatial
patterns of treefall were mediated by geology and surface water provision in the elephantutilised
site where relative treefall was higher on nutrient-rich geology, and intense treefall
hotspots occurred around permanent water points. Human-mediated rates and spatial patterns
of treefall were influenced by settlement and crop-land expansion, as well as ease of access to
communal lands. Frequent fires facilitated the persistence of trees >3m in height, but was
associated with height loss in trees <3m. The combined loss of large trees and gain in shrubs
could result in a structurally simple landscape with reduced functional capacity. Shrub-level
increases in the communal lands are likely an interactive combination of newly established
woody encroachers and strong coppice regrowth in harvested species. The more intensely
used the communal land, the greater the bush thickening and the stronger the relationship
between biomass gains and structural changes in the lowest height classes. The exacerbation
of bush thickening in natural resource-dependent communities has critical implications for
ecosystem service provision. There is potential for coppice regrowth to provide fuelwood to
communities using ‘tree thinning’ programmes, but there is a lack of data on the quantity and
quality of the regrowth, as well as the sustainability of coppice, the impacts of different
harvesting methods, and the potential feedbacks with changing climate and CO2 fertilisation.
Woody resource spatial distribution in communal lands is centred around settlement-level
wood extraction pressure, as well as natural resource accessibility in the woodlands. In highly
utilised areas, woodland regenerative capacity has been underestimated. Additionally, natural resource extraction is still highly localised, even at the communal land scale, with major
structural changes occurring around the periphery or close to existing infrastructure.
However, it is these underrated coupled adaptive responses in social-ecological systems that
explain the failure of fuelwood supply-demand models’ predictive abilities. Nevertheless,
loss of large trees in the landscape and the persistence of ‘functionally juvenile’ coppice
stands will have implications for seedling production and establishment in the landscape with
repercussions for the future population structure and ecosystem service provision. I discuss
the implications of increased natural resource reliance in an African development context and
the positive feedback between rural poverty and environmental impoverishment. Potential
constraints to the data are unpacked, together with opportunities for further research in this
area.
Description
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. 25 October 2016 in Johannesburg, South Africa.
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Citation
Mograbi, Penelope Jane (2016) Spatio-temporal dynamics of woody vegetation structure in a humanmodified South African savannah, University of Witwatersrand, Johannesburg, <http://wiredspace.wits.ac.za/handle/10539/21753>