The coastal grasslands of Maputaland, South Africa : effects of fire and grazing on vegetation structure, diversity, and composition.
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Date
2007
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Abstract
A series of trials and investigations were implemented to address concerns
surrounding the dynamics of the fire-climax wooded/edaphic grasslands within the
iSimangaliso Wetland Park, northern KwaZulu-Natal, South Africa. The research
problem surrounded inadequate historical evaluations of changes in vegetation structure,
grasslands progressing to a woody dominated composition, and increases in Helichrysum
kraussii (Curry bush).
These were addressed as follows: Firstly, the recovery of vegetation in response to
different periods of fire exclusion in different communities along a topographical
gradient of a coastal dune area, was assessed over a two year period. Secondly, the
regeneration after wildfire of the persistent, stress tolerant shrub H. kraussii, was studied
on different catenal positions with differing fire exclusion periods and with and without
defoliation of surrounding plant biomass in the coastal edaphic grasslands north of
Manzengwenya, South Africa. Thirdly, aerial photography from 1937, 1975, and 2000
was georectified, digitised and analysed using a Geographic Information System to
examine broad vegetation changes in response to different management regimes for a site
on the Eastern Shores of Lake St Lucia and a site within the Tewate Wilderness Area.
In the absence of fire, the coastal edaphic grasslands progressed to a closed canopied
scrub forest within six years. An increase in fire exclusion period resulted in a decrease in
species abundance, an increase in woody height, and a decrease in plant density. Richness
increased initially but declined marginally with increased fire exclusion period. Higher
lying east and west facing sites had a better veld condition index compared with bottom
sites and had an increased response (vigour) to defoliation but were far more likely to
succeed through to woody scrub forest. Woody plant biomass vigour was greater for west
facing sites.
Ordination of species composition across sites in response to fire exclusion and
catenal position revealed greater similarities within exclusion periods than between.
Bottom sites were more similar with similarity decreasing for east and west facing sites.
Fire exclusion resulted in an initial increase in woody species and a subsequent increase
in herbaceous species.
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Growth response of H. kraussii was unaffected by catenal position and fire exclusion
period, whereas defoliation of surrounding grass tended to increase in size (P<0.05).
Density and height for this species however increased with increasing fire exclusion. An
increase in soil moisture negatively affected H. kraussii growth indicating susceptibility
to high water tables. The number of other woody species establishing beneath H. kraussii
may be due to changes in the transmission of light through the canopy where an increase
in canopy diameter resulted in an increase of photosynthetically active radiation at the
soil surface.
The effects of fire on landscape change were investigated for the Eastern Shores and
Tewate Wilderness Area, iSimangaliso Wetland Park, South Africa using aerial
photography. Changes to historical disturbance regimes largely through active exclusion
of fire resulted in the majority of the higher lying coastal grasslands changing to savanna
scrub or closed canopied forest within 63 years on the Eastern Shores. The degree of
fragmentation of these grasslands was greatly reduced within the Tewate Wilderness
Area where disturbance regimes included greater frequencies of fire. Hygrophilous
grasslands remained largely unaffected by woody encroachment but did not preclude
woody species establishment indicating possible susceptibility during long drier periods.
Frequent fires result in the maintained distribution of the higher grasslands. This
vegetation type is a system which becomes resilient in response to fire, whereas in the
absence of fire readily progresses to Dune Forest. The coastal grasslands above the high
water table are therefore highly unstable and transformed easily in the absence of regular
disturbance. It would appear that a threshold of approximately six years exists, after
which substantial management intervention may be required to reverse the succession
back to grassland.
The growth of H. kraussii was unaffected by fire and remained persistent irrespective
of fire exclusion period. An ability to attain size (height and canopy diameter) was
limited with increased soil moisture but density was reduced through regular burning.
Frequent fires are necessary to reduce density of H. kraussii and reduce the competitive
advantage gained with age.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2007.
Keywords
Rangelands--Fire management--KwaZulu-Natal--Maputaland., Woody plants--KwaZulu-Natal., Helichrysum--Effect of fires on--KwaZulu-Natal--Maputaland., Plant succession--KwaZulu-Natal., Range management--KwaZulu-Natal--Maputaland., Grassland ecology--KwaZulu-Natal., Theses--Grassland science.