FIRE AND COMPETITION IN AUSTRALIAN HEATH - A CONCEPTUAL-MODEL AND FIELD INVESTIGATIONS

We describe a model of heath vegetation, in which species were classif ied into five functional groups based on characteristics of their prop agule pools, post-fire growth, timing and mode of reproduction and com petitive status. The model assumes no recruitment without fire and a s imple competitive hierarchy based on vertical stature. A critical feat ure of the model is an initial post-fire window of 5-6 yr in which com petition from overstorey species on understorey species is reduced. Un derstorey functional groups differ in their ability to exploit this wi ndow. In the field, we tested five predictions derived from the model: (a) overall species richness of understorey varies inversely with ove rstorey density as a result of a trend in richness of woody species, b ut not in herbaceous species; (b) where an overstorey was present in t he previous fire interval, post-fire population density is reduced in a functional group of understorey serotinous resprouting shrubs, but n ot in a group of understorey obligate-seeding shrubs with soil seed ba nks; (c) in understorey serotinous resprouting shrubs, post-fire regro wth in resprouting individuals is adversely affected by the presence o f an overstorey in the preceding fire interval; (d) in understorey ser otinous resprouting shrubs, levels of pre-fire propagules are lower in the presence of an overstorey, reducing the density of post-fire recr uits; and (e) in understorey serotinous resprouting shrubs, recruitmen t relative to the pre-fire population is unaffected by overstorey spec ies within the window of reduced competition. Of these, three tests (a ,b,d) supported the model, one (e) may support the model, but the resu lts were inconclusive and one (c) did not support the model. Limitatio ns and further applications of the model are discussed. Our results su ggest that maintenance of high densities of overstorey populations is in conflict with conservation of some understorey species. Models of t he type we propose will help identify and resolve such conflicts and p romote the judicious use of fire to maintain full species diversity of plant communities.