Intra- and inter-specific leaf trait responses of understorey species to changes in forest maturity

Coppice forest management impacts understorey vegetation. This is particularly true for habitat specialists (priority elements in conservation) who prefer closed canopy conditions associated with mature forests. Most trait-based studies of forest understorey focus on interspecific differences, which neglect the role of intraspecific variability. Here, we tackled this issue by examining how leaf traits respond (intra- and inter-specifically) to different stages of stand maturity for habitat specialist and generalist species. We measured the leaf area (LA), leaf dry matter content (LDMC) and specific leaf area (SLA) for 1,932 individuals belonging to 71 understorey species in 18 beech coppice forests grouped in three stages of stand maturity (i.e., post-logging, n = 5; recovering, n = 5; old stands, n = 8). We assessed leaf trait variation across stages of forest maturity for the entire community, and for specialists and generalists separately using monotonic trend tests. Then, we disentangled the proportion of variation attributable to intraspecific variability and interspecific differences of specialists and generalists using variance decomposition. We found significant positive trends in community-level SLA and negative ones in LDMC with increasing stand maturity. Beech forest understorey specialists (but not generalists) explained these patterns. The contribution of intra- and inter-specific components was dependent upon the trait under consideration and habitat specialization. For specialists, intraspecific variability was significant for all leaf traits and could explain 15.2% of model variance for SLA, 4.2% for LA and 3.1% for LDMC along the stand maturity gradient. Interspecific differences were significant for LDMC (29.8% of variance explained) and SLA (7.6%). For generalists, intraspecific variability was not significant; interspecific differences were significant for LA and explained 26.2 % of variability along the stand maturity gradient. We revealed that the maturity of coppice stands have different effects on generalist and specialist species. Habitat specialists are responsive to subtle changes in forest stand age both intra- and interspecifically, with their prevalence being function- and trait-dependent. Interspecific differences explained most of the LDMC (resource conservation) variance, while intraspecific variability accounted most of the SLA (light capture) variance. Generalists contribute to community-level trait variation, yet these species were less responsive to changes in forest stand age. To assess the impact of forest management on understories and refine the effectiveness of monitoring plans, we recommend focusing on habitat specialists, ideally including both trait variability within- and among-species.