Root biomass and root fractal analyses of an open Eucalyptus forest in a savanna of north Australia

Abstract

Below-ground biomass of a Eucalyptus savanna forest was estimated following trenching to depths of 2 m around 16 mature trees in a tropical savanna of north Australia. Correlations among below-ground and various components of above-ground biomass were also investigated. In addition, root morphology was investigated by fractal analyses and a determination of an index of shallow-rootedness was undertaken. Total root biomass was 38.4 t ha^–1, including 1 t ha^–1 of fine roots. About 77–90% of total root biomass was found in the upper 0.5 m of soil. While fine-root biomass density was approximately constant (0.1 kg m^–3) in the top soil, irrespective of distance from a tree stem, coarse-root biomass showed large variation with distance from the tree stem. Significant positive correlations among total root biomass, total above-ground biomass, diameter at breast height, leaf biomass and leaf area were obtained. It is likely that total root biomass can be reasonably accurately estimated from aboveground biomass and fine-root biomass from tree leaf area. We present equations that allow the prediction of belowground biomass from above-ground measures of tree size.

Root morphology of two evergreen and two deciduous species was compared by the use of three parameters. These were the fractal dimension (d), which describes root system complexity; a proportionality factor (α), which is the ratio of the cross-sectional area before and after branching; and two indices of shallow-rootedness (ISR). Roots were found to be amenable to fractal analyses. The proportionality factor was independent of root diameter (D_r) at any branching level in all tree species examined, indicating that branching patterns were similar across all root sizes. The fractal dimension (d) ranged from 1.15 to 1.36, indicating a relatively simple root structure. Mean d was significantly different between E. tetrodonta (evergreen) and T. ferdinandiana (deciduous); however, no significant differences were found among other pairs of species. Terminalia ferdinandiana had the highest ISR, while Planchonia careya (deciduous) had the lowest. In addition, differences in ISR between P. careya and the other three species were significant, but not significant among E. miniata, E. tetrodonta and T. ferdinandiana. There were clear relationships among above-ground tree stem diameter at breast height, stem base diameter, and horizontal and vertical proximal root diameter. By the use of mean values of and stem diameter, we estimated the total crosssectional area of root and root diameter-class distribution for each species studied.