To provide a theoretical basis for developing intensive cultivation practices for Populus tomentosa plantations, the leaf phenology variation within the canopy and its relationship with transpiration of well-watered P. tomentosa tree in a dense plantation were investigated. The variation in canopy light interception, indicated by the ratio between net radiation under (Rn-u) and above (Rn-a) canopy, with leaf development was also studied. During the growing season, the achievement of maximum leaf number tended to be later in higher parts of the canopy. In the lower and middle canopy, the leaf number maximized earlier in the east-facing side than in the west-facing side, but this difference disappeared in the upper canopy. The Rn-u/Rn-a was stable in May, but declined and then varied steadily until late August. Generally, in May, the crop coefficient (Kcb) of the tree reached its highest level and was not correlated with leaf area (LA) in all layers (p > 0.05). However, it increased linearly (p < 0.001) with LA in the layers above a canopy height of 3 m from June to late August, and most of its variation was explained by LA in the 5–7 m layer. After late August, Kcb decreased linearly with decreasing LA in all layers (p < 0.001). Consequently, a temporal ecological strategy seems to be adopted by P. tomentosa leaves in different layers and azimuthal sides for efficient light acquisition. The contribution of the different canopy layers to tree transpiration can vary, with the leaves in the upper and all layers mainly controlling transpiration in summer and in spring and autumn, respectively.
Variability of tree transpiration across three zones in a southeastern U.S. Piedmont watershed
