Effects of fire frequency on plant species composition of sandstone communities in the Sydney region: Combinations of inter‐fire intervals

Fire frequency is the number of fires experienced by a particular community within a given time period. This variable can potentially be resolved into a number of interacting components, including time since the most recent fire, and the length of the inter‐fire intervals. We estimated the effects of inter‐fire intervals and sequences of inter‐fire intervals, independently of time‐since‐fire, on the floristic composition of 26 samples from dry sclerophyll vegetation with different fire histories in Brisbane Water National Park near Sydney. In particular, we examined the effects in the recent fire history of: the length of the shortest inter‐fire interval (1–26 years); repetition of very short (1–3 years) and medium (7–14 years) inter‐fire intervals; and recovery from very short (1–3 years) and short (4–6 years) inter‐fire intervals. Our analyses suggest that inter‐fire interval may account for about 55% of the floristic variation among our samples, which were taken at a uniform 18 months after the most recent fire. There appear to be at least three general and unrelated effects of the recent history of inter‐fire intervals on the floristic composition of this vegetation: (i) shorter inter‐fire intervals were associated with a reduction in the number of species present in the community, along with a more unequal abundance of the remaining species, particularly of those large Proteaceae shrubs that often dominate the community biomass in dry sclerophyll shrublands of southeastern Australia; (ii) the presence of an inter‐fire interval that is 1–6 years long was associated with an additional reversible reduction in the number of fire‐sensitive species present in the community, particularly those common shrub species with a canopy‐stored seed‐bank and non‐leguminous species with a soil‐stored seed‐bank; and (iii) repetition of inter‐fire intervals that are 1–5 years long was associated with an increase in the abundance of herbaceous fire‐tolerant species. This means that variation in these characteristics is associated with changes in abundance of different plant species, and that the floristic composition of any one area can be influenced in three different ways by these variables. Management of fire for plant species conservation must therefore be based on both length of time‐since‐fire and length of inter‐fire intervals if it is to be effective.