Shoot allometry of Jatropha curcas

Abstract

The South African government has banned planting of Jatropha curcas L. (Jatropha), potentially a multipurpose tree and biofuel source, owing to insufficient knowledge about the species. Use of allometry as a non-destructive method of monitoring growth and biomass attributes of Jatropha was investigated. The objectives were to examine: reliability of allometry between above-ground variables and basal diameter and crown depth of Jatropha; effects of below-ground interspecies competition and tree spacing on allometry; and validity of these relationships with independent data. The study site was Ukulinga Research Farm, South Africa. Destructive sampling was carried out in March 2008, and tree height and basal diameter were measured periodically during March 2005 to April 2007. Regression analysis and analyses of covariance were used to analyse the data. The height–diameter equation developed by destructive sampling was validated using independent data. Highly significant allometric regressions resulted from using basal diameter (r ≥ 0.89) and crown depth (r ≥ 0.94). Stem diameter had linear relationships with wood and foliage biomass percentages (r = 0.91). Height–diameter equations were equivalent across competition and tree spacing treatments. Predicted and measured tree heights were linearly related (r > 0.97). It could be concluded that above-ground allometry of Jatropha was very reliable and not significantly affected by either below-ground interspecies competition or tree spacing. The site-specific allometric equations are useful for accurate and non-destructive estimations of Jatropha growth under various growing and (non-pruning) tree management conditions. The equations presented here are, however, not universally applicable.

Keywords: Above-ground allometry; basal diameter; crown depth; interspecies competition; Jatropha; tree spacing

Southern Forests 2009, 71(4): 279–286