Biomass and biodiversity of African savanna woodlands: spatial patterns, environmental correlates and responses to land-use change
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Date
30/06/2015Item status
Restricted AccessEmbargo end date
31/12/2100Author
McNicol, Iain Morton
Metadata
Abstract
Tropical savannas and woodlands are the dominant vegetation cover in
Southern Africa covering 4 million km2. Their large spatial extent means they
are potentially a globally important store of biomass carbon with implications
for global climate, and an area of high biodiversity value. They provide natural
resources such as food, fuel and timber that help sustain the livelihoods of over
100 million people. The ability of these savanna woodlands to maintain these
important ecological functions is under question due to increases in land use and
land cover change. This thesis addresses a set of science questions aimed at (i)
improving our knowledge of the amount of carbon and biodiversity stored in
these ecosystems and how they co-vary, (ii) how these variables are spatially
distributed at landscape scales and the factors which underlie these patterns,
and (iii) how they respond over time to human disturbance.
In Chapter 2 I examine how patterns in aboveground woody carbon storage
(AGC) are linked to differences in forest structure, tree species diversity and
floristic composition across a recently established network of 25 permanent
sample plots in south-east Tanzania. Large stems were a significant
contributor to plot-level AGC stocks with the top 3% of individuals (>40cm) in
terms of size containing 35% of the total measured C. This data can potentially
be used to simplify future measurements of biomass in these systems. Tree
species diversity was positively related to AGC indicating the potential to align
forest conservation efforts. The linear relationship suggests a functional
relationship between the variables and is consistent with ecological theory on
niche complementarity and selection effects, however based on the available
data the mechanisms underlying this relationship can only be theorised.
Changes in tree species composition were also noted across plots with
differences in vegetation structure between plots explaining 16% of the variation
in composition, with environmental differences related to climate and soils
explaining only 3%.
In Chapter 3, the focus shifts to understanding larger-scale spatial patterns
in AGC. Field plots are spatially limited in this regard, therefore radar remote
sensing data was used to generate a map of AGC in order to improve our
knowledge on what principally controls its spatial variability at landscape
scales. Results showed that factors related topography, climate and soils
explained very little of the variation in C stocks across the landscape (r2 = 15 –
20%). Differences in slope angle and topographic position were important in
discriminating between low biomass savannas and moderate biomass
woodlands, while differences in annual precipitation were more important in
separating woodlands and denser forests. A large proportion of the variation in
C stocks (~80%) was unexplained highlighting the role of unmeasured variables.
It is suggested that fire may play a key role in shaping patterns in tree species
composition and C stocks across these landscapes. This data has important
implications for a local REDD+ project which is aiming to generate carbon
credits through improved fire management.
In the second part of the thesis the attention shifts to understanding the
long-term ecological impacts of shifting cultivation and the sensitivity and
resilience of these woodlands to anthropogenic change. In Chapter 4 I examined
how carbon stored in trees and soils recover across a 40-year chronosequence of
abandoned agricultural land, and how this patchy disturbance impacts spatial
pattern in tree species composition and diversity. I show that re-growing
woodlands can act as carbon sinks through the accumulation of woody biomass
(0.83 tC ha-1 yr-1), with soil texture having no clear impact on accumulation
rates. Re-growing woodlands were also found to contain considerable
biodiversity value by promoting novel species assemblages. Bulk soil carbon
stocks appeared to be largely unaffected by the full cycle of shifting cultivation.
However in Chapter 5 I show evidence of a previously unquantified legacy effect
of land clearance on soil CO2 production with more recently abandoned fields (c.
6 years) exhibiting significantly higher efflux rates than the older
abandonments (15 -25 years) and mature woodlands. Total soil nitrogen was the
most important predictor of soil respiration across the plots (r2 = 0.3) followed by
fine root density (r2 = 0.12). Soils in the younger sites were found to be more
nitrogen rich which was used to explain the greater CO2 fluxes in these areas,
however, it is still unclear why this pattern exists.
The thesis concludes by discussing the wider implications of the results, as
well as outlining further work needed to solidify some of the conclusions drawn
in this thesis.