More Resistant Than Resilient: Effect of 15 Years Conservation Management on Ground-Layer Vegetation Composition in Temperate Woodland and Forest

Abstract

We monitored ground-layer vegetation in permanently marked plots in a temperate grassy woodland/sclerophyll forest mosaic, after the removal of sheep, but with continuing macropod grazing. Data collection over 15 years encompassed extremes of drought and high rainfall. Plots were stratified to sample 13 subhabitats reflecting variations in vegetation type and landscape position. Species composition varied with: (i) site productivity (grassy vs sclerophyll forest), (ii) soil moisture (landscape position) and (iii) presence of tree canopies in open grassland. After 15 years, most subhabitats retained their distinctiveness, with net changes suggestive of nutrient decline under tree canopies and increasingly mesic conditions on higher slope positions. Analysis of species richness of 18 origin and life-form groups over time revealed three types of responses: (i) stable, (ii) linear increase and (iii) a quadratic response. The latter was identified for native and exotic species in all habitats, specifically for annuals and short tussocks. There were sustained linear increases in the species richness of three native life forms (geophytes, subshrubs and leafy herbs) suggesting improvements in conservation status, while most native perennial groups were stable. Large perennial native grasses (Themeda triandra, Rytidosperma pallidum, Poa sieberiana and Aristida ramosa) maintained their initial dominance, while that of Lomandra filiformis and Melichrus urceolatus increased. The level of dominance of exotics was largely sustained, although exotic annuals fluctuated in response to seasonal conditions. The frequency of exotic perennial dicots increased slightly, despite intensive control efforts. We attribute the persistence of biomass under severe drought to the high dry matter content of many ground-layer dominants, which served to protect soil and provide refuge to palatable geophytes and native herbs. Our findings are consistent with ecological models that predict that cessation of fertiliser inputs and heavy livestock grazing will promote vegetation with ecosystem attributes resistant to climate extremes.