Leaf trait variation across Italian forest endemics: drivers and divergence from widespread congeners

Despite their biogeographical and conservation relevance, the functional trait space explored by Mediterranean endemic plants still remains largely unknown, particularly for those of forest habitats (1). To what extent these taxa are functionally divergent from related widespread congeners from similar forest habitats is a central question potentially involved into the causes for their restricted distribution, ecological specificity and resource use attitudes. Leaf traits play a critical role in plant growth and survival, largely determining the functional space explored by plants in terms of resource use (2). Analyzing these traits in forest endemic plants and unveiling the range of interspecific variability can thus open a window on their relative position along the acquisition-conservation axis, in the framework of the global leaf economics spectrum (3). Accordingly, we analyzed six core leaf traits (LA, SLA, LMA, LDMC, Nmass, C:N ratio) across 45 endemic taxa (“paleo-“ and “neo-“ endemics) living in Italian forests, either woody or herbaceous, in distant angiosperm and conifer clades. Leaf traits were measured in the field on one representative population per species, mainly in the central, southern and insular regions. The influence of a set of environmental variables and the phylogenetic signal of traits were also examined to identify the major drivers of interspecific variation. Next, we performed species pairs contrasts in 27 pairs formed by the endemic taxon and a closely related widespread congener also of forest habitats, with allopatric, parapatric or sympatric distribution. Overall, trait variation across the endemics was remarkably wide, reflecting their diversity in terms of morpho-functional types, phylogenetic relationships and biogeographical contexts. Endemics were widely distributed along the resource use gradient associated with LA, LMA and Nmass. The herbaceous taxa showed a prevalence of the C and R strategies, while stress-tolerance was prevalent in the woody endemics. Most traits showed a phylogenetic signal based on various metrics used, with Pagel’s λ approaching the Brownian model for LA and LMA. Environmental factors variously influenced trait variation. LA was negatively affected by mean annual temperature and depended on forest type, while LDMC decreased with latitude and precipitation. LMA was positively associated with temperature and varied with ecoregion and forest type, while Nmass decreased with latitude and increased with precipitation. Species pairs analysis pointed to a generally negative effect of the endemic condition on LA, associated with increasing LMA and Nmass. Compared with widespread congeners, this pointed to lower acquisitive ability and stronger resource conservation attitude, also reflected in significant shifts in the CSR strategies. Differences in LA and LMA within allopatric and/or parapatric pairs were overall larger than in sympatric pairs, suggesting the role of vicariance and range separation in the divergence of these traits between related endemic-non endemic taxa of forest habitats.