Predicting soil depth using simple ground-based measurements of stem shape and taper in the butt swell section of individual Pinus radiata trees

Predicting soil depth using simple ground-based measurements of the tree stem has multiple benefits for precision (site-specific) forest management and estimating carbon stocks of plantation forests. Current methods of mapping soil depth rely on collecting a sufficient density of direct soil measurements, which is expensive and typically not feasible over extensive forest areas. The availability of detailed soil depth information under forest plantations is consequently sparse and this presents a significant impediment to precision forest management and the ability to estimate forest soil carbon stocks. In this study, we propose that the relationship between stem shape and taper in the butt swell of individual Pinus radiata trees and soil depth can be described in a simple empirical model. We demonstrate that shape and taper of the butt-swell section of the tree stem are as robust predictors of soil depth as individual tree height, and also have the advantage of being easy to measure from the ground. This finding has potential benefits for reducing the cost of soil data collection and improving fine-scale forest soil mapping.