Elevation reverses the effects of forest structure on folivory and leaf asymmetry

Centuries of timber production have homogenized many forests by reducing variation in canopy density and deadwood availability, with far‐reaching consequences for biodiversity and trophic interactions. Recent studies indicate that increasing structural heterogeneity through canopy gap creation and deadwood enrichment can promote biodiversity and support tree regeneration. These management practices may also influence tree performance, among others assessed by folivory and leaf fluctuating asymmetry, which indicate how well trees resist leaf damage and maintain developmental stability under environmental stress. However, it remains unknown whether such management enables trees to mitigate folivory and developmental instability, especially across macroclimatic gradients such as elevation. We conducted a large‐scale experiment in Germany across 11 pairs of forests: one structurally homogeneous control forest and one experimentally heterogenized forest, where canopy gaps and deadwood were created to increase between‐patch structural heterogeneity. The forests spanned an elevation gradient from 38 to 1143 m. Across all forest pairs, we sampled 19,656 leaves from 1404 European beech ( Fagus sylvatica L.) trees. We quantified folivory, leaf fluctuating asymmetry, microclimatic conditions (temperature, vapour pressure deficit) and biotic pressures (predation, parasitism, competition). Experimental enhancement of structural heterogeneity reduced folivory overall but increased leaf fluctuating asymmetry. Folivory increased with elevation, while leaf fluctuating asymmetry declined, producing an inverse relationship: At low elevations, structural enhancement reduced folivory but increased asymmetry, whereas at high elevations it increased folivory but reduced asymmetry. Microclimatic variables explained variation in both folivory and leaf asymmetry more consistently than biotic pressures. These findings extend the stress‐gradient framework by showing that identical management interventions can yield opposite outcomes depending on the macroclimatic gradient associated with elevation. Thus, integrating elevation and climate context into forest management seems to be crucial for maintaining the resilience of temperate forests under global change. Read the free Plain Language Summary for this article on the Journal blog.