Tree: Understorey: Soil Interactions under native scattered paddock trees on the Northern Tablelands of NSW

Abstract

Scattered trees are a significant ecological resource across the agricultural landscape, yet their numbers are declining due to factors such as dieback, senescence, agricultural activity. Here, we report the findings of a study which examined the influence of 'Eucalyptus melliodora' trees on surface soil properties and vegetation composition in grazed and ungrazed paddocks in northern New South Wales. Four paired sites on granitic soils were examined in a split plot design involving grazed and ungrazed treatments. In each grazed or ungrazed plot, soil samples were collected and vegetation composition was assessed beneath the canopy and in the open areas (1.5 canopy radii away) from each of 5 trees. Results indicated that a significant difference occurred for soil C, N and pH, between the canopy and open areas, and between the grazed and ungrazed treatments. In general, C and N were higher in the grazed compared with ungrazed sites, although C was only significantly different in two of the four sites, and pH was generally higher in the ungrazed compared with the grazed sites. Throughout, these soil parameters were all higher under tree canopies. Vegetation composition also showed a significant difference between canopy and open, and grazed and ungrazed areas. Native grasses dominated the vegetation in both canopy and open, and grazed and ungrazed, however their composition and/or contribution frequently changed. Several C3 and C4 grasses were found to contribute to the groundcover of the canopy and open sites, but C3 grasses tended to be more prominent under the canopy in some sites. Analyses suggested that the tree, soil and vegetation factors were strongly related. This study confirms that individual scattered trees create a distinct mosaic of localised soil improvement, and that these effects are strongly associated with vegetation composition. These results illustrate the benefits of retaining trees for both landscape managers and graziers.