Thesis presented for the Degree of Doctor of Philosophy at the
University of the Witwatersrand / Successional changes in the vegetation after fire were studied in
several fynbos communities of the south-western Cape Province of South
Africa. The study sites were located in the mountains, at altitudes
between 300 and 1000 m a.s.l., in areas with winter rainfall regimes
-1 and annual precipitation of about 900 to 1000 mm. yr Soils are
highly leached, derived principally from quartzites. The two main
sites were Zachariashoek near Paarl, where summers tend to be rather
dry, and Jakkalsrivier east of Grabouw, where summer drought is
ameliorated by fog precipitation and cloudiness. Successional changes
were followed for intervals of up to 10 yr between fires, as well as
for similar periods in vegetation that had been unburnt for 25 yr.
Vegetational changes were analysed by means of repeated floristic
assessments on permanent quadrats and point-quadrat sampling of canopy
cover composition on these and on larger plots. At Jakkalsrivier, recently burnt and long unburnt vegetation were also compared by
paired samples. Demographic trends in populations of prominent shrub
species were followed by repeated censuses of tagged samples in
unburnt and recently burnt vegetation. Also at Jakkalsrivier, the
effects of fire on resources available to plants were examined by
sampling soil moisture and soil mineral nutrients, as well as by
following trends in xylem pressure potentials in selected species of
plants and analysing their foliar nutrient concentrations. Effects of
fire on microclimate were tested by comparative studies on burnt and
unburnt sites.
All fynbos communities sampled proved to be highly stable in the face
of fire. Essentially, the pre-fire species composition was regained in
2-3 yr in every case. Species were added after fire, partly because of
the appearance of ephemerals with life histories tied to fire, but also because of the reappearance of longer-lived plants as well as
through the readier detection of species in vigorous vegetative form.
The species richness of the regenerating corrununities tended to be
quadratically related to pre-fire biomass, as predicted from current
succession theory. Most species in any corrununity (about 70% on
average) regenerated vegetatively by sprouting after fire. The
relative numbers of species that regenerated germinatively, i.e. the
seeders, did not vary in a manner predictively related to corrununity
biomass. There were relatively few species with specialised life
histories based on reseeding, such as those with canopy-stored seed
and ephemerals with presumably specialised requirements for
germination. Virtually no recruitment could be found among plants in
the older (about 25 yr) vegetation, in contrast with lowland fynbos
sites, where recruitment of herbaceous species occurs, and some
mountain fynbos sites on more fertile soils, where forest precursors
may sometimes colonise.
Canopy redevelopment after fire indicated similar resilience among the
different corrununities, despite variation in regrowth rates. Pre-fire
growth-form composition was restored within around 10 yr. Maximum
leaf-area indices ranged from about 1,5 to 2,5, although corrununi ties
on phreatic sites had leaf-area indices exceeding 3,0. There was no
evidence for a suppression of the understoreys by overstorey layers,
mainly because the latter were sparse despite the abundance of tall
broad-sclerophyllous shrubs in certain habitats. This was because the
taller shrubs had sparse or slender crowns, or both, and because
mortality tended to thin the populations before dense canopies
developed. Trends in the composition of the canopy varied among
corrununities. corrununities in productive habitats, i.e. in this case on
phreatic sites, were dominated in the early stages by a relatively
luxurious growth of ephemeral herbs and soft shrubs which declined within around 3-4 yr. Other sites had very sparse ephemeral cover, the
early stages being dominated mainly by Restionaceae, Cyperaceae, and
other sprouting herbs, and sprouting and seeding shrubs, which were
constituents of the pre-fire canopies. In this respect, the fynbos is
clearly distinguished from the California chaparral, for example,
where ephemerals tend to dominate the post-fire stages on most sites.
There was no evidence that fire had any effect on the water relations
of regenerating vegetation, although stream discharge is known to be
increased by fire in these environments. There was tentative evidence,
in enhanced foliar concentrations of some mineral nutrients, that
regenerating species of climax plants exploited nutrients released in
fire. However, any such responses were small, especially in comparsion
with responses observed in chaparral, for example. Ephemeral shrubs
had much higher concentrations of foliar nutrients overall than climax
species, tending to confirm the correlations found in Australian
heathlands between plant life-history and nutrient economy. The
effects of fire on microclimate were pronounced, especially on the
thermal and water vapour regimes experienced by seedlings and sprouts.
These extremes did not, however, appear as water stress in
regenerating plants. Despite relatively sparse canopies, mature
vegetation did reduce light at the ground to levels likely to affect
seedling recruitment and survival. Preliminary experiments with a
local dominant shrub, Leucadendron xanthoconus, showed a pronounced
intolerance of shading and hence that light attenuation by canopies
must be implicated in successional processes.
The demographic studies indicated that density-dependent effects were
not important in survival of plants. Two species of fire ephemeral
shrubs effectively died out within four years, being characterised by
markedly higher growth rates than climax species and brief and early fecundities. Climax shrubs had more or less constant rates of
mortality over time, though populations in unburnt vegetation tended
to have slightly higher rates of mortality than young populations.
Densities of seedling populations were very high, but mortality rates
were extremely low.
In summary, it may be said that the fynbos communi ties studied here
are very stable under a given fire regime. Recovery is rather rapid,
being apparently achieved within 10 yr. Not much change occurs in
older vegetation, but there was a gradual attrition of populations of
dominant shrubs, without recruitment, with rare exceptions. Summer
droughts in these montane environments are evidently not sufficiently
marked for water deficits to play a primary role in succession, so
that fire has no effect on plant water relations. Nutrient responses
are relatively weak, and masked in the plants by the low rates of
metabolism in climax species. Succession after fire is distinguished
by the recovery of pre-fire communities, and subsequent inhibition of
recruitment. This inhibition is probably through the effects of
canopies on microclimate, although the interactions between especially
plants and animals have been implicated in succession in other studies. / AC2017
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/22342 |
| Date | January 1987 |
| Creators | Kruger, Frederick John |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | Online resource (230 leaves), application/pdf |
Page generated in 0.0025 seconds