Succession after fire in selected fynbos communities of the south-western Cape

[摘要] Successional changes in the vegetation after fire were studied inseveral fynbos communities of the south-western Cape Province of SouthAfrica. The study sites were located in the mountains, at altitudesbetween 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 arehighly leached, derived principally from quartzites. The two mainsites were Zachariashoek near Paarl, where summers tend to be ratherdry, and Jakkalsrivier east of Grabouw, where summer drought isameliorated by fog precipitation and cloudiness. Successional changeswere followed for intervals of up to 10 yr between fires, as well asfor similar periods in vegetation that had been unburnt for 25 yr.Vegetational changes were analysed by means of repeated floristicassessments on permanent quadrats and point-quadrat sampling of canopycover composition on these and on larger plots. At Jakkalsrivier, recently burnt and long unburnt vegetation were also compared bypaired samples. Demographic trends in populations of prominent shrubspecies were followed by repeated censuses of tagged samples inunburnt and recently burnt vegetation. Also at Jakkalsrivier, theeffects of fire on resources available to plants were examined bysampling soil moisture and soil mineral nutrients, as well as byfollowing trends in xylem pressure potentials in selected species ofplants and analysing their foliar nutrient concentrations. Effects offire on microclimate were tested by comparative studies on burnt andunburnt sites.All fynbos communities sampled proved to be highly stable in the faceof fire. Essentially, the pre-fire species composition was regained in2-3 yr in every case. Species were added after fire, partly because ofthe appearance of ephemerals with life histories tied to fire, but also because of the reappearance of longer-lived plants as well asthrough the readier detection of species in vigorous vegetative form.The species richness of the regenerating corrununities tended to bequadratically related to pre-fire biomass, as predicted from currentsuccession theory. Most species in any corrununity (about 70% onaverage) regenerated vegetatively by sprouting after fire. Therelative numbers of species that regenerated germinatively, i.e. theseeders, did not vary in a manner predictively related to corrununitybiomass. There were relatively few species with specialised lifehistories based on reseeding, such as those with canopy-stored seedand ephemerals with presumably specialised requirements forgermination. Virtually no recruitment could be found among plants inthe older (about 25 yr) vegetation, in contrast with lowland fynbossites, where recruitment of herbaceous species occurs, and somemountain fynbos sites on more fertile soils, where forest precursorsmay sometimes colonise.Canopy redevelopment after fire indicated similar resilience among thedifferent corrununities, despite variation in regrowth rates. Pre-firegrowth-form composition was restored within around 10 yr. Maximumleaf-area indices ranged from about 1,5 to 2,5, although corrununi tieson phreatic sites had leaf-area indices exceeding 3,0. There was noevidence for a suppression of the understoreys by overstorey layers,mainly because the latter were sparse despite the abundance of tallbroad-sclerophyllous shrubs in certain habitats. This was because thetaller shrubs had sparse or slender crowns, or both, and becausemortality tended to thin the populations before dense canopiesdeveloped. Trends in the composition of the canopy varied amongcorrununities. corrununities in productive habitats, i.e. in this case onphreatic sites, were dominated in the early stages by a relativelyluxurious growth of ephemeral herbs and soft shrubs which declined within around 3-4 yr. Other sites had very sparse ephemeral cover, theearly stages being dominated mainly by Restionaceae, Cyperaceae, andother sprouting herbs, and sprouting and seeding shrubs, which wereconstituents of the pre-fire canopies. In this respect, the fynbos isclearly 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 relationsof regenerating vegetation, although stream discharge is known to beincreased by fire in these environments. There was tentative evidence,in enhanced foliar concentrations of some mineral nutrients, thatregenerating species of climax plants exploited nutrients released infire. However, any such responses were small, especially in comparsionwith responses observed in chaparral, for example. Ephemeral shrubshad much higher concentrations of foliar nutrients overall than climaxspecies, tending to confirm the correlations found in Australianheathlands between plant life-history and nutrient economy. Theeffects of fire on microclimate were pronounced, especially on thethermal and water vapour regimes experienced by seedlings and sprouts.These extremes did not, however, appear as water stress inregenerating plants. Despite relatively sparse canopies, maturevegetation did reduce light at the ground to levels likely to affectseedling recruitment and survival. Preliminary experiments with alocal dominant shrub, Leucadendron xanthoconus, showed a pronouncedintolerance of shading and hence that light attenuation by canopiesmust be implicated in successional processes.The demographic studies indicated that density-dependent effects werenot important in survival of plants. Two species of fire ephemeralshrubs effectively died out within four years, being characterised bymarkedly higher growth rates than climax species and brief and early fecundities. Climax shrubs had more or less constant rates ofmortality over time, though populations in unburnt vegetation tendedto have slightly higher rates of mortality than young populations.Densities of seedling populations were very high, but mortality rateswere extremely low.In summary, it may be said that the fynbos communi ties studied hereare very stable under a given fire regime. Recovery is rather rapid,being apparently achieved within 10 yr. Not much change occurs inolder vegetation, but there was a gradual attrition of populations ofdominant shrubs, without recruitment, with rare exceptions. Summerdroughts in these montane environments are evidently not sufficientlymarked for water deficits to play a primary role in succession, sothat fire has no effect on plant water relations. Nutrient responsesare relatively weak, and masked in the plants by the low rates ofmetabolism in climax species. Succession after fire is distinguishedby the recovery of pre-fire communities, and subsequent inhibition ofrecruitment. This inhibition is probably through the effects ofcanopies on microclimate, although the interactions between especiallyplants and animals have been implicated in succession in other studies.

[发布机构] University of the Witwatersrand