LAUSR

PREMISE Climate change may lead to carbon (C) stress (negative C balance) in trees. Since non-structural carbohydrates (NSC) are required during spring's metabolic reactivation, C stress might delay the budbreak timing. This effect can be anticipated to be greater in shade-intolerant than in shade-tolerant species, owing to the faster C economy of the former. METHODS We experimentally induced C stress by exposing saplings of six temperate tree species with contrasting light requirements to shade versus open conditions from summer to spring and recorded budbreak afterwards. As the levels of C reserves representing effectively C stress may differ among species, we quantified the degree of C stress through survival during the whole experimental period along with the whole-sapling NSC concentrations after budbreak. RESULTS Shade reduced NSC concentrations and increased the NSC's sugar fraction in all species. Shade-intolerant species exhibited higher mortality and generally lower NSC concentrations under shade than shade-tolerant species, indicating a trend for greater C stress severity in species of faster C economy. In shade-intolerant species, budbreak occurred earlier and faster in open conditions than in shade, while in shade-tolerant species the opposite was found. The effects of the light environments on budbreak were not greater in shade-intolerant than in shade-tolerant species. CONCLUSIONS Our study reveals a matching between budbreak responses to light and the light requirements of the species. This finding confirms a significant role of C metabolism in triggering budbreak, and it demonstrates that whether C stress anticipates or delays budbreak depends on the species light-requirements. (249 words). This article is protected by copyright. All rights reserved.