Effects of climate and topography on height-diameter allometry of Pinus ponderosa

Climate is a potential driver of tree allometry. To explore the sensitivity of height-diameter allometry to climate change, we compiled height-diameter pairs of ponderosa pine (Pinus ponderosa) in Arizona, western United States, together with the climate and environmental data of this species' growing sites into a dataset, and selected power-law function as the basic allometric model. Then, the scaling parameters of basic model were re-parameterized to incorporate climate and environmental variables, and ordinary least squares and nonlinear mixed-effects (NLME) methods were used to fit and predict, respectively. The results showed that the NLME height-diameter allometric model performed best in this study. Tree height was positively affected by precipitation and competition from neighbors, while it was negatively affected by the elevation of tree living site. The relative importance analysis showed that competition-related variables were the most important environmental factors that had shaped height-diameter allometric relationship of ponderosa pine. In general, the height-diameter allometry of ponderosa pine was dominated by mechanical constraints, but the allometric variation was mainly caused by the differences of environment. Our results not only help to understand the response mechanism of tree resource allocation to environmental gradients but also contribute to forest management decision-making under future climate changes.