Restoration thinning and influence of tree size and leaf area to sapwood area ratio on water relations of Pinus ponderosa

Ponderosa pine (Pinus ponderosa Dougl. ex P. Laws) forest stand density has increased significantly over the last century (Covington et al. 1997). To understand the effect of increased intraspecific competition, tree size (height and diameter at breast height (DBH)) and leaf area to sapwood area ratio (A_L:A_S) on water relations, we compared hydraulic conductance from soil to leaf (k₁) and transpiration per unit leaf area (OL) of ponderosa pine trees in an unthinned plot to trees in a thinned plot in the first and second years after thinning in a dense Arizona forest. We calculated k₁ and Q_L based on whole-tree sap flux measured with heat dissipation sensors. Thinning increased tree predawn water potential within two weeks of treatment. Effects of thinning on k₁ and Q_L depended on DBH, A_L:A_S and drought severity. During severe drought in the first growing season after thinning, k₁ and Q_L of trees with low A_L:A_S (160-250 mm DBH; 9-11 m height) were lower in the thinned plot than the unthinned plot, suggesting a reduction in stomatal conductance (g_s) or reduced sapwood specific conductivity (K_S), or both, in response to thinning. In contrast k₁ and Q_L were similar in the thinned plot and unthinned plot for trees with high A_L:A_S (260-360 mm DBH; 13-16 m height). During non-drought periods, k₁ and Q_L were greater in the thinned plot than in the unthinned plot for all but the largest trees. Contrary to previous studies of ponderosa pine, A_L:A_S was positively correlated with tree height and DBH. Furthermore, k₁ and Q_L showed a weak negative correlation with tree height and a strong negative correlation with A_S and thus A_L:A_S in both the thinned and unthinned plots, suggesting that trees with high A_L:A_S had lower g_s. Our results highlight the important influence of stand competitive environment on tree-size-related variation in A_L:A_S and the roles of A_L:A_S and drought on whole-tree water relations in response to thinning.