Deciduous and Evergreen Leaf Habit Structures Intraspecific Trait Expression along an Elevational Gradient

BACKGROUND AND AIMS: Evergreen and deciduous leaf habits impose different construction costs and carbon returns, which should lead to divergent intraspecific trait responses along climatic gradients. We tested whether leaf habit predicts systematic shifts in leaf economic traits across an elevational gradient capturing substantial variation in growing-season length on Mount Washington, USA. METHODS: We measured six traits on the most abundant woody species at 19 elevations: leaf mass per area (LMA), leaf area (LA), leaf thickness (LT), foliar nitrogen (N), foliar carbon to nitrogen ratio (C:N), and carbon isotope discrimination (δ13C). Trait coordination was evaluated using principal component analysis. We then modelled intraspecific trait-environment relationships using two complementary approaches: Bayesian multi-level generalized linear mixed-models and a meta-analytic meta-regression of species' trait-environment slopes. KEY RESULTS: The first principal component followed the leaf economics spectrum, with strong loading by LMA, LT, N, and C:N, while the second aligned primarily with LA and δ13C. Deciduous and evergreen species formed two distinct clusters, with deciduous species exhibiting traits associated with a fast-return economic strategy (lower LMA and C:N, higher N). Deciduous taxa showed strong intraspecific trait shifts with elevation and growing degree days, indicating a shift toward leaves with greater potential carbon return per unit leaf mass at higher elevations with shorter growing periods. Evergreen responses were weaker and less consistent. Both modelling frameworks converged on significant differences across leaf habits for traits involved in carbon acquisition. CONCLUSIONS: Leaf habit structures intraspecific trait expression with elevation along the slope of Mount Washington. Trait coordination among deciduous species led to increasingly fast-return leaves with altitude, consistent with offsetting annual construction costs by promoting greater carbon gain during a shortened growing period. These findings highlight how constraints on leaf lifespan can impose demands that fundamentally structure intraspecific responses to environmental change, sometimes leading to divergent patterns of trait expression among leaf habits.