Complexities of shrub encroachment: are shrubs important for the maintenance of diversity in Themeda-dominated assemblages on coastal headlands in eastern Australia?

Abstract

Shrub encroachment is an issue worldwide with consequences which may have both positive and negative outcomes for landscape and community health. Themeda-dominated communities on headlands are listed as endangered within New South Wales, Australia with shrub encroachment listed as threat. Coastal headlands are considered harsh environments and positive effects on species diversity patterns may occur due to variables that ameliorate these conditions or through indirect mechanisms. Shrubs are a natural component of coastal headlands in eastern Australian and tall shrub encroachment may have both positive and negative effects on non-shrub communities. I test whether tall shrubs may have a functional role that positively affects species diversity within and between plots and the distribution of species within Themeda-dominated communities on headlands. 352 2 × 2 m plots were placed on 46 headlands along a 530 km stretch of coastline on the North Coast Bioregion, New South Wales, Australia. Within plots vascular plant species were scored on cover and frequency. Species density, diversity, evenness, turnover and gamma diversity were calculated. Variables tested include cumulative tall shrub height from circular transects at 2, 4 and 8 m from plots, slope, aspect, altitude, ground layer height, distance to closest seaward edge and macropod grazing intensity. Rarefaction was performed on a subset of sites that had no shrubs and sites within shrubs within 8 m of plots. The relative interaction intensity (RII) was calculated for all taxa with more than one observation against plots without shrubs and plots with shrubs within 8 m radius. Analyses were performed using Canonical Correspondence Analysis (CCA) and Generalised Additive Modelling (GAM) on species frequency data. A total of 303 native taxa were recorded within plots. The most significant positive effects on species density, evenness and species accumulation included increasing proximity to, and density of tall shrubs. Tall shrubs may allow direct facilitation of less stress tolerant plants to colonise but may have an indirect interaction by reduction in the dominance of Themeda triandra. Other minor significant variables included distance from headland seaward edge, southern and western facing slopes, higher altitudes and a decrease in ground layer height. 178 (80%) of species were found to have a positive RII score associated with the presence of nearby taller shrubs. Tall shrubs may directly facilitate greater species diversity by ameliorating local conditions and indirectly facilitate by reducing the over-dominance of some taxa. In spite of the positive effect of shrubs on diversity eventually tall shrub encroachment may cause replacement of the Themeda-dominated communities with shrubland. Encroaching taller shrubs are therefore shown to be both of benefit to floristic species diversity at all scales within these Themeda-dominated communities on headlands in eastern Australia but also a potential threat. These results highlight the complex nature of community interactions requiring nuanced and potentially novel approaches to management.