Defining the mechanisms driving grass community (composition and functional trait) shifts in African Savannas
Date
2018
Authors
Donaldson, Jason Ernest
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Abstract
Traditionally, landscape structure and variability are thought to be largely determined by climatic
and edaphic conditions (‘green world’ view). However, it is now accepted that herbivore
densities and fire regimes play an important role in shaping vegetation structure due to their role
as consumers. In savannas the roles of fire and herbivory have been recognised for a long time
but their interactions and feedbacks to vegetation are still being elucidated. Savanna trees in the
same environment show evidence for specialisation to either frequently burnt or heavily grazed
communities suggesting that alternative system states, representing either a ‘brown’ or ‘black’
world (heavily grazed vs. frequently burnt), can occur in savanna landscapes. When it comes to
the grass layer we know that grazers can create and maintain patches of high-quality grazing
habitat and exclude fires locally by removing grass biomass and limiting fuel for fires (brown
world). Comparatively, large fires disperse grazing animals over broad areas of the landscape as
they respond to high quality graze available on burn scars in what has been termed pyric
herbivory. The “thinning” of large mammal herds over wide ranges after a fire decreases
concentrated grazing and allows grasses to accumulate biomass and ultimately encourages repeat
burning (black world). Thus, both consumers have self-enhancing feedback loops and we would
expect grass communities to respond to these: different species with divergent functional traits,
either associated with frequent fire or heavy grazing, should be associated with each consumer
state. Abiotic conditions, soils and rainfall, influence the strength of these feedbacks and alter the
dominance of fire vs. herbivory across environmental gradients. This implies that rainfall
variability, and the occurrence of drought, should influence these feedbacks at a particular
location. How drought affects the competition between these consumers by shifting the
conditions governing feedbacks is not well understood, but is becoming increasingly important
in southern Africa where drought events are expected to become more frequent and severe with
climate change.
The main aim of this study was to investigate the processes involved in the establishment and
break-down of grass-consumer feedbacks and understand how these dictate the balance of fire
and grazer driven grass communities in an African savanna. Specifically, I used a landscape
scale experiment setup in the Kruger National Park to test how easily a system switch from fire-
to grazer-dominated can be initiated in a landscape, and how the functional traits of the grasses
drive these switches. I then tracked the impacts of a severe two season drought on the established
consumer driven feedbacks within the study system and analysed the knock-on implications of
these changes in grass community structure and function at broader scales.
The experimental results indicate that altering fire management practices and burning repeated
small (<25 ha) fires concentrated grazers. After three seasons (2013-2015) this resulted in a
structural shift in grass communities that maintained local grass height in a short-cropped state
and excluded fire. Once the dominant consumer pressure shifted there were notable changes in
the local grass species composition and novel grass communities had functional traits that
favoured repeat grazing (low C : N ratios and high leaf moisture content). Thus, feedbacks
between grazers and grass communities established within a relatively short period. This work
adds to a growing number of studies that highlight the importance of feedbacks in the
establishment of dominant consumer processes within savannas.
Drought altered the forage use patterns of grazers as biomass on grazing-lawns became limited
and animals were forced to feed in low-quality tall-grass areas or migrate away from the study
site completely. This more intensive grazing in the broader landscape where fire is usually the
dominant consumer lead to a convergence in the grass community trait space between lawns and
tall-grass areas. This suggests that drought has a homogenising effect on grass community trait
space across fire-driven and grazer-driven grass community patches, at least in the short-term. I
found that grass-grazer feedbacks did remain at broad scales with lawns re-establishing after the
drought, but that these areas were severely restricted in the study system (<4% cover) and only
occurred where strong drivers of grazer feedbacks (water availability) repeatedly concentrated
grazing. Moreover, drought reduced the basal area of tussock grass, and created large areas of
seemingly bare ground in the grazed patches. But grass communities recovered quickly – and
within 3 months after the drought broke productivity was at its maximum.
My results highlight the importance of grass-consumer feedbacks in dictating the structure and
function of African savannas. Clearly, fire and herbivory need to be considered in conjunction
and understanding the state of a given grass community can only be done by assessing the
governing grass-consumer relationship. Drought represents a good example of this; changes in
the short-term abiotic conditions within a savanna appear to have little impact on savanna
function and structure unless it drives changes in the positive feedback loops that govern the
distribution of grazer-driven and fire-driven grass communities within the landscape. Fires
remain one of the only major management tools in large protected savannas and my thesis
underlines the need to manage them by considering how fire influences grass-grazer dynamics
and not simply use fire as a ubiquitous method for improving graze quality in the short-term
Description
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg in fulfillment of the requirements for the degree of Doctor of Philosophy
Johannesburg
2018