The impact of individual Callitris glaucophylla (white cypress pine) trees on agricultural soils and pastures of the north-western slopes of NSW, Australia

Woody vegetation thickening occurs in agri-ecosystems worldwide, often with negative consequences for production. Dense Callitris glaucophylla (Joy Thomps. & L.A.S. Johnson) stands affect landscapes across NW NSW, Australia, and strategies to reduce tree density to levels which maintain biodiversity values alongside agricultural production are currently being sought. We investigated soil chemical and groundcover patterns associated with individual small and large C. glaucophylla trees at six sites of variable management history and lithology in NW NSW, Australia. We posed two questions: (1) do individual C. glaucophylla trees impose patterns on soil and groundcover (soil extractable P, C, N, S and pH, litter biomass, litter P and pasture cover), and, (2) if patterns exist, do they differ between tree sizes?

Results showed that extractable P, C and pH decreased away from trees of both sizes, but significantly higher values were recorded adjacent to the stem of large trees. Litter biomass exhibited a strong site-related trend independent of soil variables. Positive correlations between litter and soil variables existed for some sites and not others, indicative of processes such as grazing which contribute to the transport of litter away from the tree. Irrespective of tree size ground-storey vegetation cover increased significantly away from the stem, presumably as a consequence of competition for soil water.

Further results indicated that single C. glaucophylla trees enrich soils in patterns analogous to other species in similar environments worldwide. However, localised soil improvements must be weighed up against the negative effects of decreased groundcover associated with trees, and the potential for the species to re-seed prolifically into managed paddocks. Future research will discern the impact of individual C. glaucophylla trees at higher densities, where soil patterning may be modified by intense within-stand competition.