Physiological and growth responses to pruning season and intensity of hybrid poplar

Pruning is one of the silvicultural practices recommended for poplar plantations in order to produce clearwood. Common wisdom proposes pruning during the summer season, however little is known on trees' physiological responses to pruning season and intensity. This study aimed at determining the effects of pruning season and intensity on growth and physiology of four hybrid poplar clones. Using 3 years old trees, we compared nine combinations of pruning season (fall, spring and summer) and intensity (unpruned, 1/3 and 2/3 of crown length pruned). We found that pruning increased net photosynthesis of residual foliage, nitrogen foliar concentrations and reduced total non-structural carbohydrates reserves of roots. Leaf carbon isotopic ratios (δ13C) of pruned trees decreased compared to unpruned trees, reflecting their greater stomatal conductance and suggesting that pruned trees could have greater drought stress resistance. Two growing seasons after pruning, 1/3 pruned trees had reduced height growth except for those pruned in summer. Regardless of pruning season, pruning 2/3 of the live crown reduced both breast height diameter and height 2 years after treatments. Summer pruning increased foliar nitrogen concentrations, stomatal conductance and decreased leaf carbon isotopic ratios (δ13C) more than for trees pruned in fall or spring, suggesting better capacities to recover from pruning. We concluded that removal of 1/3 of the lower crown in summer was the best option to produce clear wood without compromising growth rates and physiological processes in hybrid poplar.