Site-to-site variation in the demography of a fire-affected perennial, Acacia suaveolens, at Ku-ring-gai Chase National Park, New South Wales, Australia

Acacia suaveolens (Sm.) Willd is a perennial shrub that forms even-aged stands, recruited from a soil seed-bank following fire. It has previously been subject to demographic studies, which used a space-for-time substitution to investigate temporal patterns following fire. In the present study the potential for spatial variation across sites was investigated by sampling at several similarly aged populations in Ku-ring-gai Chase National Park, northern Sydney, Australia. Significant variation in mean size and fecundity of A. suaveolens individuals was observed among sites, over a 2-4.6-fold range in plant size, and a sevenfold range in mean fecundity. The observed variation at 3 years after fire encapsulated most of the variation previously observed among sites 0-17 years since fire, emphasizing the importance of spatial variation in this species. For each site a two-stage (seed, plant) matrix model was constructed, and projected from 3 to 25 years following fire. Population growth was measured as number of seeds per 3-year-old plant, and found to vary 1.4-fold across models for different sites. This site-to-site variation, as well as that in size, fecundity and survival, was statistically significant. Variation in projected seeds per plant could mostly be attributed to differences in fecundity rather than plant survival. Sensitivity analyses emphasized the biological significance of the variation in fecundity. Whereas previous studies have focused on temporal variation, this work demonstrates the importance of extending our understanding of a species to include the spatial component of population dynamics.