Interactions between Callitris above-ground biomass, species density and plant form in north-eastern New South Wales John T. Hunter School of Behavioural, Cognitive and Social Sciences, University of New England, Armidale, NSW 2351, Australia. Email: jhunter8@bigpond.com Abstract. 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. Received 28 November 2012, accepted 16 January 2013, published online 21 February 2013 Introduction Of the 13 Callitris species (Cupressaceae) that occur within New South Wales, C. glaucophylla and C. endlicheri are the most common. The former is known from all mainland Australian states and the latter is known from the eastern mainland states. On the basis of anecdotal observations, it has been suggested that where these two species, individually or together, increase in cover to the point of dominance, the diversity of understorey species decreases significantly (Walker et al. 1972; Lacey 1973; Boland et al. 1984; Clayton-Greene and Aston 1990). Because of this perception, thinning is encouraged and frequently carried out as a legitimate land-management practice (Noland 1997; Lunt et al. 2006), even within conservation areas. There is both public and institutional pressure to continue such thinning practices on lands currently incorporated into the state reserve system for biodiversity purposes. Evidence from recent specific studies has contradicted this underlying assumption of loss of species richness with increasing Callitris density. Grazing was shown to have a far greater impact on understorey cover than was increasing C. glaucophylla density (Andrews 2003). Similarly, Thompson and Eldridge (2005a) were unable to find any clear correlation between C. glaucophylla cover, stem or wood density, and the density, cover or diversity of understorey species. Hunter (2011), in regression analyses of increasing C. glaucophylla cover, found a positive effect of increasing cover on species density. Here, I investigate more specifically the effect of AGB of C. endlicheri and C. glaucophylla on species density (richness per site) and on broad components of species composition that include life-form categories, and native and introduced species density is analysed. Such information is important in terms of understanding the dynamics of natural and disturbed systems and is also needed to properly inform management decisions on public and private lands where the maintenance of biodiversity is considered to be a primary goal. Materials and methods Study area The study was undertaken within the western half of the New England Tablelands, Nandewar and Brigalow Belt South Bioregions within New South Wales (Fig. 1). In the region of investigation, 2230 sites were placed within the national reserve CSIRO PUBLISHING Australian Journal of Botany, 2013, 61, 73–79 http://dx.doi.org/10.1071/BT12317 Journal compilation Ó CSIRO 2013 www.publish.csiro.au/journals/ajb