Prescribing Fire Frequencies in Cape Fynbos in Areas Causes Marked Changes in Vegetation

Abstract--Regulating fire frequency in Cape fynbos Prescribing Fire Frequencies in Cape Fynbos in areas causes marked changes in vegetation. Very Relation to Plant Demography1 frequent fire eliminates seeding shrubs with long youth periods; infrequent fire reduces shrubs because of senescence. Variation between these ex- tremes will cause more subtle compositional changes. F. J. Kruger2 Research on plant fecundity schedules and on the dynamics of seed pools is necessary to understand these. Problems in the ecology and management of South other of his goals. Therefore, our empirical know- African mediterranean-type ecosystems have been ledge of the effects of fire regime must be supple- outlined in several recent papers (Bands 1977, mented through the study of plant demography and Kruger 1977 and these proceedings, Day and others hence the dynamic responses of vegetation to fire. 1979): fire is generally acknowledged as a useful tool in these ecosystems. In this paper I attempt SPROUTING vs. SEEDING PLANTS to set out a framework for research on the demography of Mountain Fynbos plants, so that a de- In fynbos as in other mediterranean-type vegeta- ductive and a predictive system can be developed tion, fire regime strongly influences the relative for deciding on proper burning frequencies for abundance of plants that regenerate by sprouting any given management goal. and those that rely on seed. Very frequent fire favours sprouting plants. Survival of fire seems Management goals for Mountain Fynbos centre to be high. First, mortality in Watsonia pyrami- mainly on maintenance of certain dynamic equili- data, a geophyte, is zero or nearly so (Kruger and bria in the ecosystem, for sustained yield of Bigalke in press). For Protea nitida, Haynes clean water, and so on. Designing the managed (1976) reported mortalities of 1 to 13 per cent in fire regime is largely dependent on our under- different fires. Second, fires stimulate vegetative standing of how a change in regime will determine reproduction among herbaceous species (Kruger and a new equilibrium in the vegetation. We know, for Bigalke in press) and, possibly, sexual reproduc- example, that a change in fire frequency, from tion among shrubs such as Protea nitida (Haynes about once in 20 yr to about once in 6 yr, can 1976). Although long absence of fire causes a de- change a dense tall shrubland to a low vegetation cline in herbaceous sprouters, most sprouters, dominated by herbaceous plants, eliminating species woody or herbaceous, appear to be very long-lived with slow life cycles (Van Wilgen 1981a, b). On so that infrequent fires may also favour sprouters the other hand, if fire is kept out for 40 yr or over seeders (Bond 1980, Van Wilgen 1981a, b, more, shrub populations decline, seedling recruit- Kruger and Bigalke in press). Plants of species ment after fire is reduced, and there is an appa- such as Watsonia pyramidata and Protea nitida seem rent trend toward a sparse shrubland (Bond 1980, to be very long-lived and populations are there- Van Wilgen 1981a, b); under favourable conditions, fore well able to persist despite long intervals long intervals between fire could result in re- of low recruitment. It is only at very high and placement of fynbos by evergreen rain forest. very low fire frequencies that sprouters are markedly favoured over seeding shrubs in fynbos. Of the elements of fire regime that are regula- Otherwise, sprouters rarely form dense stands, and ted by management, fire frequency has the greatest most usually are subordinate to seeders. This is potential for making immediate and marked changes unusual in mediterranean-climate ecosystems (Kruger to fynbos. The manager must invariably reconcile in press) and it would be intriguing to find the conflicting goals. Fire control, for example, is reasons. For management, however, the present best achieved through frequent burning, which re- need is for data that describe the dynamics of duces the average biomass in the area. Nature sprouter populations. conservation, where species diversity is to be maintained, probably requires intermediate fire DEMOGRAPHY OF SEEDING SHRUBS frequencies. It is not sufficient to select an arbitrary frequency that will safely achieve one or Regeneration A review shows that seeding shrub populations in mediterranean-type ecosystems are mostly even- 1Presented at the Symposium on Dynamics and aged and that effective population recruitment Management of Mediterranean-type Ecosystems, from seed is generally confined to the period June 22-26, 1981, San Diego, California. immediately post-fire, and further that this is from propagule stocks of three kinds (a) soil seed- 2Deputy Director for Conservation Research, banks (b) canopy-stored seed and (c) continually South African Forestry Research Institute, replenished stocks (Kruger in press). In the first Department of Water Affairs, Forestry and case, populations can survive long intervals be- Environmental Conservation, Pretoria, Republic tween fires in the dormant state after death of of South Africa. adult plants, though the relative decay rates of Gen. Tech. Rep. PSW-58. Berkeley, CA: Pacific Southwest Forest and Range 118 Experiment Station, Forest Service, U.S. Department of Agriculture; 1982. the seed-banks must partly determine the composition Primary youth periods and fecundity schedules of the regenerating flora, depending on the length of the fire-free interval. Circumstantial evidence In the even-aged shrub populations of the fyn- (Kruger 1977) indicates considerable longevity bos the reproductive potential of a species in a among such seed in fynbos, and the habit would stand of uniform fire history depends on its age. therefore be adaptive in a fire regime marked by Hence, fire frequencies prescribed for conserva- variability in the fire recurrence interval. It tion of fynbos are based in the first instance on is apparently present in many fynbos species, in- some estimate of the minimum age at which repro- cluding Proteaceae, Aspalathus, and many other ductive potential suffices to replace the parent large taxa. population; latterly, the question of the period of peak fecundity has become important (e.g. Bond Effective reproduction by canopy-stored seed or 1980). continually replenished stocks, where dormancy is not marked and seed life after dispersal is short, The first question of importance relates to the depends on an extant parent population and hence youth period, i.e. time between establishment and is ineffective where intervals between fire exceed age at the beginning of the fecundity period. In plant life-spans; Bond (1980) has produced data fynbos there is interspecific variability in this to support this. Fynbos is often dominated by respect, but relatively little (Kruger and Bigalke shrub species that store seed in closed capitula in press). Thus, from records for nursery plants on the plant, until the parent dies, usually after of 42 fynbos Protea species quoted by Rourke fire. Seed released in unburnt vegetation is sub- (1980) we find the following: ject to very heavy predation (Bond 1980); seed in these species is relatively large, and contains Youth period, yr No. of species high concentrations of proteins (Van Staden 1978); 2-4 22 thus storage in capitula is apparently adaptive in 5-7 16 a low-nutrient environment, in that seed tend to > 7 4 be protected against predators and efficient reproduction ensured because seed tend to be re- These nursery records are not grossly unrepresen- leased by a reliable cue into the post-fire environ- tative of field performance, as indicated by ment where predation pressure is low and germina- Kruger and Bigalke (in press). tion conditions favourable. The effectiveness of canopy-stored seed is in some dispute. Seed ripens Species with differing youth periods sometimes over seven or more months after flowering, and both occur in disparate habitats. Thus, Protea rupicola, Jordaan (1949, 1965) and Van Staden (1978) have one of those exceptionally slow to mature, inhabits claimed or suggested that seed in capitula are cliff faces near mountain crests, where short-lived and that there are insufficient viable fires are infrequent and short primary youth periods seed to ensure regeneration if parents are killed are not necessarily adaptive. In management it during the time of seed ripening (winter and spring). is important to know the distribution of such This does not accord with results reported by Lom- species and the behaviour of fire in such habitats; baard (1971), nor with observations in Williams the compartment management system followed in fyn- (1972) and Rourke (1980). Assuming that canopy- bos does not allow fine-grained burns and relies stored seed is effective in ensuring regeneration, presently on the assumption that if a compartment the habit is adaptive in a predictable fire regime, contains patches of such habitat then prescribed where recurrence intervals are longer than the burns will not traverse these patches at excessive youth period but do not exceed the reproductive frequencies, despite their spreading elsewhere life-span of parent plants. within the compartment. Martin (1966) has implied that certain important The present working hypothesis in prescribing fynbos taxa, including Ericaceae, are dependent fire frequencies is that sympatric species tend on the annual seed rain for reproduction. He main- to have similar youth periods, and that these are tained further that, in his study area, no seed relatively short, i.e. 3-8 yr (Kruger 1979 and un- store was available on site for post-fire regene- published). Prescribed burns at intervals that ration. This habit will not be adaptive in the approach these will begin to eliminate species fire environment of the fynbos, especially where locally (Van Wilgen 1981a). Where precocious many species are local, unless species have highly species occur with others that are markedly slow dispersible seed. In view of the generally to mature (for example, Protea repens with low capacity for long-range dispersal reported P.

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