Dynamic simulation of the multilayer crown net photosynthetic rate and determination of the functional crown for larch (Larix olgensis) trees

Abstract

Pruning can encourage the early formation of knot-free wood with high economic value. However, few studies have assessed the costs and benefits of production in order to determine the optimal pruning design. The concept of a functional crown provides reasonable guidance for pruning, but there is no objective method of determining the functional crown. Semimonthly measurements of leaf traits and photosynthetic characteristics and the corresponding environmental conditions were conducted for young Larix olgensis plantations throughout the entire growing season. The dynamic crown net photosynthetic rate (An) was simulated by comprehensively considering the leaf mass per area, photosynthetically active radiation, air temperature (Tair), vapor pressure deficit and leaf position within the crown (relative depth into the crown, RDINC). The precision (P) was estimated to be 95.5%, indicating that our model performed well in predicting the dynamic crown An for young Larix olgensis plantations. The net primary productivity (NPP) of each whorl of the tree crown was calculated by numerically integrating the instantaneous An. The net carbon contribution from branches to the tree was obtained by subtracting the annual carbon increment from the NPP. The results showed that some live branches in the lower crown contributed negatively to the trunk, indicating that pruning should not focus only on dead branches. Thus, the lower boundary of the functional crown (LBFC) provides a valuable guideline for pruning treatments. Considering that the LBFC will shift upward as the tree grows, we suggest that advanced pruning techniques be applied to reduce costs and minimize labor.