Responses of antinomic foliar traits to experimental climate forcing in beech and spruce saplings

Global warming is predicted to have a strong impact on mountain ecosystems. Subalpine sylvopastoral systems are very sensitive to climate change, which puts their future sustainability at stake. These ecosystems are mostly dominated by spruce and beech, and therefore their regeneration abilities are critical in this context. The main objective was to characterize the short-term responses, of foliar traits in beech and spruce saplings through phenotypic plasticity with regard of actual scenarios of climate change. Therefore, we transplanted saplings from a cold environment at 1350 m a.s.l. in the Swiss Jura mountains to three recipient sites at lower altitudes along an altitudinal gradient, in the experimental framework of a space-for-time substitution approach and measured morpho-anatomical foliar traits. The results revealed for beech an increase of xeromorphism through the increase of the cuticle thickness, vein network and smaller stomata, associated, surprisingly, to a higher leaf area. This antinomic response allowed beech to grow in warmer conditions while coping with an increase of evaporative demand during summer. Spruce did not present as much plasticity as compared to beech due to its inherent xeromorphic traits. Our findings further suggest a strong correlation between the timing of the leaf development, extreme conditions and tree growth. These contrasting strategies may lead to the competitive advantage of beech over spruce under climate change.