Abstract: Outbreaks of forest defoliating insects are becoming more frequent and severe across the globe. The responses of non-structural carbohydrates (NSC) in trees to leaf loss are crucial for their growth and survival. Dioecious plant species play a significant role in maintaining forest stability, yet few studies have investigated sexrelated effects of defoliation on growth and NSC. In this study, we investigated sexrelated responses of growth and NSC to varying levels of defoliation (0, 50%, and 100% leaf removal) in Populus cathayana, an ecologically and economically important woody species widely distributed in China. We measured plant biomass, NSC concentrations and pools in different organs.  Biomass and NSC concentrations and pools decreased with increasing defoliation intensity, with such trend being more pronounced in roots (especially coarse roots) than in shoot.  Female NSC pools in leaves, coarse roots, fine roots and wholeplant were always greater than male pools. Biomass and NSC concentrations and pools in males decreased more than for females with increasing defoliation intensity. MaleP. cathayana suffered stronger negative effects on growth and NSC than the females under defoliation, which suggested that females had a higher tolerance to defoliation than males. Our results contribute to understanding the mechanisms of tree carbon balance across sexes, with implications on silviculture of poplar plantations.

Key words: dioecy, non-structural carbohydrates, tolerance, carbon balance.