Modeling the effect of physiological responses to green pruning on net biomass production of Eucalyptus nitens

Green pruning of Eucalyptus nitens (Deane and Maiden) Maiden increases instantaneous rates of light-saturated CO2 assimilation (A), and changes patterns of total leaf area and foliage distribution. We investigated the importance of such changes on the rate of recovery of growth following pruning. A simple process-based model was developed to estimate daily net biomass production (G(d)) of three-year-old plantation-grown trees over a 20-month period. The trees had been pruned by removal of 0, 50 or 70% of the length of green crown, equivalent to removal of 0, 55 or 88% of leaf area, respectively, when the plantation verged on canopy closure. Total G(d) was reduced by only 20% immediately following the 50%-pruning treatment, as a result of both the high leaf dark respiration and low A in the portion of the crown removed compared to the top of the crown. Pruning at the time of canopy closure preempted a natural and rapid decline in G(d) of the lower crown. Although leaf area index (L) was approximately 6.0 at the time of pruning, high light interception (95%) occurred with an L of 4.0. The 50%-pruning treatment reduced L to 3.5, but the physiological responses to pruning were sufficient to compensate fully for the reduction in intercepted radiation within 110 days of pruning. The 70%-pruning treatment reduced L to 1.9, and reduced G(d) by 77%, reflecting the removal of branches with high A in the mid and upper crown. Physiological responses to the 70%-pruning treatment were insufficient to increase G(d) to the value of unpruned trees during the study. Model sensitivity analysis showed that increases in A following pruning increased G(d) by 20 and 25% in the 50- and 70%-pruned trees, respectively, 20 months after pruning. Changes in leaf area/foliage distribution had a greater effect on G(d) of 50%-pruned trees (47% increase) than did changes in A. However, the reduction in photosynthetic potential associated with the 70%-pruning treatment resulted in only small changes in leaf area/foliage distribution, which consequently had little effect on G(d). The effects of physiological processes occurring within the crown and in response to green pruning on G(d) are discussed with respect to pruning of plantations.