Disturbance affects spatial patterning and stand structure of a tropical rainforest tree

The distribution and spatial patterns of plant populations in natural ecosystems have recently received much attention; yet the impacts of human-induced disturbances on these patterns and underlying processes remain poorly understood. We used the sub-canopy tree, Ryparosa kurrangii (Achariaceae), to explore the possible effects of such disturbances on stand structure and spatial patterning in an Australian tropical rainforest. We studied three populations that differed in their extent of habitat modification: anthropogenic disturbance (proximate settlement and roads) and internal damage by an invasive alien species, the feral pig (Sus scrofa). Populations were mapped, characterized, and three size cohorts (seedlings, saplings, trees) were analysed using a suite of spatial point pattern analyses (univariate: Diggle's G and F and Ripley's K; bivariate: Diggle's G and Ripley's K). Ryparosa kurrangii has a typical stand structure for a sub-canopy tree species, but occurs at high densities locally (> 400 stems ha-1). At all sites, the tree cohort were randomly distributed and saplings were spatially aggregated at distances of up to 2-3 m. Between sites there were distinct differences in the size structure and spatial pattern of seedlings, the cohort most affected by recent habitat modification. That is, the least disturbed site had no aggregation among seedlings, the site with the greatest anthropogenic disturbance had many small, clustered seedlings that were spatially associated with trees, and the site with pig damage had clustered seedlings that had no spatial relationship with trees. We propose that habitat modification by anthropogenic and pig disturbance disrupts seed dispersal and establishment regimes, which leads to altered seedling spatial patterns. These disturbances could have long-term implications for the population structure and health of R. Kurrangii.