Fire frequency effects in a grassy woodland: Trees and grasses

Abstract Our study investigated the effects of fire frequency on the tree and grass components of Cumberland Plain Woodland, an endangered grassy eucalypt woodland in eastern Australia. We located three sites within each of three fire frequency classes (high, moderate, low) based on fire history and a similar time since last fire. For trees, we asked whether fire frequency affected density, eucalypt population structure, basal area and spatial patterning. For grasses, we tested for fire frequency effects on total abundance and the abundance of two common species. Density of trees was not significantly affected by fire frequency for juveniles, saplings or adult trees and neither was basal area per hectare. Features of a recruitment bottleneck model were present for eucalypts. There was a large pool of suppressed juveniles making up more than half the population at each fire frequency. Saplings were the smallest group and were susceptible to fire‐induced stem mortality, with particularly low numbers relative to juveniles and small trees where fire was frequent or intense. Despite this difference in sapling mortality, numbers of small trees did not differ with fire frequency. Saplings could transition to small trees in equal numbers if they did so more quickly at high than at lower fire frequencies, and if recruitment of saplings into the tree layer was controlled independently of fire frequency. The size hierarchy of small to medium eucalypt trees was homogeneous over eight of the nine sites, spatial patterning of adult trees was random tending to regular, and mean tree size decreased with density at all sites. These features of eucalypt population structure are indicative of possible resource competition which could regulate tree recruitment. Total grass cover index was high across all fire frequencies, with Themeda triandra dominating at high and moderate fire frequencies and Microlaena stipoides at low fire frequency.