Improving estimates of biomass change in buttressed trees using tree taper models

Summary: Repeat censuses of tree plots are key tools for investigating forest carbon fluxes. Current measurement procedures for trees with buttresses or trunk irregularities - trees that account for a large fraction of tropical forest biomass -introduce substantial systematic error in plot-level estimates of biomass change. The diameters of buttressed trees are measured above the standard height of 1·3 m, and the measurement heights on individual trees are often moved upwards as buttresses grow. Because tree trunks taper (diameter decreases with height), biomass growth in buttressed individuals tends to be underestimated. Methods have been introduced to correct biomass growth estimates in individual trees for increases in measurement height; however, these methods change the distribution of effective measurement heights over time, introducing biases in plot-level estimates of biomass change. In this study, we developed and applied new methods to measure and model trunk taper, and to use taper models to correct estimates of AGB change for changing measurement heights. We measured trunk taper above buttresses in 190 stems on Barro Colorado Island, Panama (BCI), a site where more than half of forest biomass is in trees measured above standard height. We compared proposed taper models to see which best described our measured taper data, then used the best taper model to correct for changing measurement heights in the historical plot data. Specifically, for all diameter measurements taken above 1·3 m, we calculated equivalent diameters at 1·3 m and substituted these into allometric equations to examine biomass change over time. We found that measured taper was best fit by an exponential model with a rate parameter that varied with measured diameter (tree size), height of measurement (buttress height) and species. Whereas uncorrected data show a decrease in biomass of 0·21% year^-1 between 1985 and 2010 on BCI, taper-corrected data show an increase of 0·18% year^-1. The novel correction method presented here converts all measured diameters to one standard effective measurement height. This corrects for biases at the plot level and provides a stronger foundation for measuring biomass change in tropical forests.