A new, proportional method for reconstructing historical tree diameters

Accurate methods of reconstructing historical tree diameters from increment cores are important because diameter is used in allometric equations to predict stand characteristics and to study stand dynamics. The conventional reconstruction method assumes that the pith is in the centre of the stem. This is often incorrect, as evidenced by a pith increment index quantifying the deviation between the geometric radius of the stem and the chronological radius of a core. I propose a new method which assumes that growth is proportional around the stem and, unlike the conventional method, cannot yield negative historical diameters. These methods were evaluated by calculating the deviations between reconstructed diameters and historical diameter measurements from 164 ponderosa pine (Pinus ponderosa Dougl. ex P. & C. Laws.) trees from permanent plots in Arizona and New Mexico. Deviations varied with pith increment index for the conventional method but not for the proportional method, and varied with tree age for both methods at one site. These methods could be used in tandem, with the proportional method applied where the increment from outer ring to pith is measured and the conventional method applied where this increment cannot be measured.