Interactions between 'Callitris' above-ground biomass, species density and plant form in north-eastern New South Wales

Dense 'Callitris endlicheri' (Parl.) F.M.Bailey (black cypress pine) and 'C. glaucophylla' Joy Thomps. & L.A.S.Johnson (white cypress pine) stands are often viewed as problematic and thinning is often encouraged from a biodiversity perspective. In the present investigation, canonical correspondence analyses (CCAs) of 997 survey sites were undertaken within the public and private reserve network that contains a variety of above-ground biomass (AGB) of 'C. endlicheri' and 'C. glaucophylla' (as measured by diameter at breast height (DBH) and dispersion) and the evenness of the species distribution was undertaken. This was done to further dissect the effect of 'Callitris' AGB on species density (richness per quadrat) of native and introduced species and on broad life-form groupings. Other landscape features such as altitude, physiography, drainage and soil depth were also included in analyses. 'C. endlicheri' and 'C. glaucophylla' grow in different biophysical locations in most instances and this was reflected in the results of the study. No level of AGB or clumping of 'C. endlicheri' was found to affect species density of native or introduced taxa or the distribution of life forms. Increasing AGB of 'C. glaucophylla' had a positive effect on native species density. The species density of introduced taxa was also increased with an increase in 'C. glaucophylla' AGB. The distribution of life-forms was significantly affected by an increase in 'Callitris' AGB with a decrease in trees, shrubby taxa and hemi parasites, although herbaceous species had a concomitant increase in number. There is no reason to thin dense Callitris stands to increase local species richness. However, because the distribution of life-form types is significantly affected by 'C. glaucophylla', there is a need to understand what is occurring in species replacements and what landscape mosaic of structural types is required for this species. It is likely that dense stands of 'Callitris' are important, along with a variety of stand densities so as to maintain the highest regional diversity.