Variation in height-diameter allometry of ponderosa pine along competition, climate, and species diversity gradients in the western United States

Height-diameter (H-D) allometry plays an important role in exploring forest growth dynamics and estimating forest biomass and carbon storage. Ponderosa pine (Pinus ponderosa) is one of the most economically and ecologically valuable tree species in North America. However, our understanding about the H-D allometry of ponderosa pine and its variation along environmental gradients is still limited. To quantify the region-scale variation in H-D allometry, 87715 pairs of height and diameter from 4034 plots in seven states in the western United States were used to compile a dataset together with neighborhood competition, species diversity, climatic and topographic variables. We used the power-law function as the basic H-D allometric model and generalized it to incorporate competition, climate, diversity and topography. Then, the nonlinear mixed-effects (NLME) modeling method was used to further fit the H-D allometry. The results showed that the allometric exponents in seven states changed near the theoretical 2/3 exponent, indicating that tree allometry was plastic rather than fixed, and its allometric variation was related to the difference of growth environment. The quality of the multi-level (state, inventory year, site and plot) NLME allometric model was significantly better than that of the basic and the generalized allometric model. Further study showed that tree height was positively correlated with DBH dominance (DOMI, a neighborhood-based competition index), mean annual precipitation (MAP) and mean annual temperature (MAT), however it was negatively correlated with species mingling (MING). Moreover, we also found that the variations in H-D allometry across climate gradients were far weaker than those in competition gradients, which indicated that competition dominated tree allometric relationship. It could be proved that the allometry of ponderosa pine should be driven by two opposite factors: competition from neighbors drove trees to invest more in height, whereas tree height tended to be shorter under arid and cold conditions. Therefore, the large-scale variation in H-D allometry of ponderosa pine was mainly explained by neighboring competition and partly by climate, diversity, or topography. Large amounts of data, wide environmental gradients and statistically reliable H-D allometric model help us make convincing conclusions about allometric variation. Our results reveal the response mechanism of tree resource allocation to environmental gradients, which is bound to be beneficial to make rational and scientific forest management decision under future environmental changes.