Wood density variation in naturally regenerated stands of Pinus ponderosa in northern Arizona, USA

In the southwestern United States, land managers are implementing large-scale forest restoration projects involving treatments designed to improve forest health, protect ecosystem services, and reduce the risk of catastrophic wildfire in overstocked ponderosa pine (Pinus ponderosa Dougl. ex P. & C. Laws.) forests. A better understanding of wood properties is necessary to improve the currently limited markets for the woody byproducts generated by these treatments. Therefore, our objective was to investigate variations in ponderosa pine wood density across the northern Arizona landscape. We sampled trees from 18 naturally regenerated stands and used X-ray densitometry to quantify the radial and axial variation within and among trees. We modeled within-stem wood density patterns using generalized additive models, and investigated the effects of climatic variation using response function analyses. Additionally, we tested the effects of site-level predictors on whole-tree wood density. We found high variability in radial wood density profiles among trees in our stands compared with that observed in studies of other species grown in plantations — perhaps due to high genetic variation within naturally regenerated stands. Wood density was negatively correlated with precipitation at annual and most quarterly intervals, except for a positive correlation with late-summer monsoon precipitation. The high wood density variation we found among trees highlights the need for further investigation of controls over wood properties in natural nonplantation forests.