The effects of burning and mowing on microclimate and soil resources and implications for species change in the southern tall grassveld of KwaZulu-Natal.
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Date
2004
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Abstract
Promotion of a predictive understanding of plant community response to various forms,
frequencies and seasons of disturbance, either through the direct physical effect on biota and
or indirect effect on plants, through modification of microclimate and soil attributes is
currently a major goal in plant ecology. In particular, the effect of disturbance on altering the
ratio between available light and nutrients and their resultant effect on growth, shoot/root
allocation, and thus community composition has gained considerable recognition in
connection with the mechanisms of plant succession under a popular heading "the resource
ratio hypothesis of plant succession". Contemporary and long-term (>50 years) burning and
mowing experiments in KwaZulu-Natal (KZN) provide important sites for investigation that
in the mesic grasslands of KZN, community composition changes in response to the
frequency, time and type of disturbances such as burning, mowing and veld fertilization.
However, the relationship between disturbance-resource-plant traits and their interactive role
in species change is virtually unknown. This study sought to improve understanding of mesic
grassland dynamics in. KZN, using short-term pot and plot experiments. The principal
objectives were: 1) by subjecting plants to different levels of resources viz. light, nutrients,
water and cutting to determine the relative above and below-ground growth performances
(biomass allocation) of species from contrasting habitat preference in KZN, which implies
their relative competitive ability for limiting resources and tolerance to cutting, 2) by using a
short-term (one-season period) burning and mowing experiment to determine the effect of
different veld management practices on microclimate and availability of soil resources and
their subsequent effect on plant growth performances, 3) testing the relative shade tolerance of
representative species from contrasting habitat preferences, 4) by combining the outcomes
from these experiments, to provide a general synthesis concerning species' response to
disturbance/resource which further signifies species change.
The hypothesis that competitive ability as a function of biomass allocation is fertility
dependent was supported by a pot experiment. In low nutrient treatments short grass species
that predominate infertile soils in KZN viz. Aristida funcifarmis and Themeda triandra
attained double the shoot biomass, more than double root biomass, initiated more tillers and re-grew better (after cutting) than those inherently tall species that predominate fertile sites
viz. Eragrostis curvula and Hyparrhenia hirta. In contrast, in high nutrient treatments, tall
species attained far higher shoot biomass and grew taller in height. Interestingly, short species
had a smaller shoot: root ratio than tall species, consistent with the prediction of the resource
ratio hypothesis. However, no evidence was obtained suggesting that tall species were more
shade tolerant than short species. A field-based shade experiment rather showed that, those
species that initiate tillers below-ground viz. Aristida junciformis, Eragrostis curvula and
Tristachya leucothrix were more shade tolerant than those species that initiate tillers above-ground
viz. Hyparrhenia hirta and Themeda triandra.
On the other hand, the effect of disturbance/resource relationship in influencing the
growth (biomass production, growth rate, and basal circumference) of contrasting species was
examined by conducting a short-term (one season) burning and mowing experiment. The
effect of disturbance, its form and frequency through its effect on light and soil moisture was
able to account for a substantial difference in species vigour, which can potentially impact
community composition. Short species (Themeda triandra and Tristachya leucothrix) showed
their highest biomass production and higher basal circumference enlargement in burnt summer
mown sites, whereas medium to tall species (Aristida junciformis, Eragrostis curvula and
Hyparrhenia hirta) were less tolerant to summer mowing. Aristidajunciformis and Eragrostis
curvula appeared to be more vigorous (both in terms of above-ground biomass production and
growth rate) in burnt but not mown and control treatment respectively. High biomass and litter
accumulation on sites protected from disturbance appeared to have a large influence on
species vigour. Species such as Aristida junciformis, Eragrostis curvula and Tristachya
leucothrix had high tolerance to litter accumulation while in contrast Hyparrhenia hirta and
Themeda triandra were more vulnerable. In total this study has revealed that the association of
some short species e.g. Themeda triandra with the reccurrence of disturbance is mainly due to
increases in light availability and lowered dominance from tall species in frequently disturbed
sites rather than nutrient related. However, this study has revealed that there are some
indications whereby the notion that the inverse relationship between available light and
nitrogen are important driving variables in species change is an important working theory in the mesic grassveld of KZN.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2004.
Keywords
Grasslands--KwaZulu-Natal., Rangelands--Fire management., Range management--KwaZulu-Natal., Prescribed burning--Environmental aspects--KwaZulu-Natal., Theses--Grassland science.