The reasons why endemic plant species have a more or less restricted range with respect to widespread congeners are a central issue in ecology and biogeography (Lavergne et al., 2004). While many studies have focused on the relationships and conservation of endemic plants, only a few investigations have delved into the factors determining their inherently limited ability to spread over wider territories (Hobohm, 2014). It is well established that a multiplicity of historical, ecological and biological circumstances, often variously combined together, can result in a limited dispersal capacity and a restricted distribution range of a given species (Song and Li, 2023). Understanding these factors is increasingly important to help predicting the effects of global changes on endemic plants worldwide, and support strategies for their conservation (Damschen et al., 2012; Erfanian et al., 2021; Erlandson et al., 2021; Hobohm, 2014). In the Mediterranean biodiversity hotspot, narrow endemics are a significant proportion of the rich native flora (Quézel and Médail, 2003). It was highlighted that these taxa are most often ecologically divergent with respect to widespread congeners, being restricted to harsh habitats such as steep rocky slopes with open vegetation and low species richness (Thompson, 2020; Thompson et al., 2005). Niche specialization and adaptation in response to stressful conditions and selective pressures have been generally assumed to be major drivers for their origin and persistence, allowing them to colonize more extreme habitats and to escape from competition and anthropogenic disturbance (Coppi et al., 2014; Hand et al., 2017; Médail and Verlaque, 1997; Totté et al., 2015).

Niche specialization and ecological divergence are less likely to occur under the more favorable conditions of forest habitats. This explains, at least in part, the minor proportion of narrow-ranged endemics in temperate European forests (Quézel and Médail, 2003; Rackam and Grove, 2001). Stronger interspecific competition and, often, more intense disturbance in woodlands are further factors contributing to the low proportion of forest endemics in Europe (Bruchmann, 2011; Hobohm, 2014). In contrast, the Mediterranean flora includes substantial endemic woody and herbaceous plants within forest habitats, found in closed interiors, open canopies, gaps, and edges (Selvi et al., 2023). Despite the conservation relevance, rarity, and isolated phylogenetic position of many of these taxa, their biology and ecology remain largely unknown. Recently, Selvi et al. (2023) provided baseline statistics for over 130 taxa with narrow range across Italian forests, mainly herbaceous but also woody, either “paleo-” or “neo-” endemics, highlighting the current lack of information about their functional characteristics and ecological strategies.

The functional trait approach for the investigation of Mediterranean endemic taxa has been poorly adopted so far, although it can provide insights into the factors involved in their restricted range in terms of plant fitness and resource use strategies. Evidence from mainly non-forest taxa indicates that these plants have a smaller stature than widespread congeners, yet they do not significantly diverge in specific leaf area, leaf nitrogen content, leaf dry matter, or maximum photosynthetic rate (Hand et al., 2017; Lavergne et al., 2004). Overall, this pointed to a lower competitive ability, based on smaller stature, but similar attitudes in the acquisition and use of resources in the taxa analyzed in the above studies. However, whether these findings can be generalized to endemics specialized for a different macro-habitat, such as temperate forests, remains still unclear. Hence, little inference can be made on the factors behind their restricted distribution in terms of competitive ability for light, nutrients and water.

Leaf traits play a critical role in plant growth and survival, largely determining the functional space explored by plants in terms of resource use (Garnier et al., 2015; Niinemets, 2020; Niklas et al., 2023; Wright et al., 2004). Analyzing these traits in forest endemic species and unveiling the range of interspecific variability between them can provide insights into their ecological strategies and position within the leaf economic spectrum (Wright et al., 2004). Moreover, investigating the dependency of traits on environmental and main habitat factors (latitude, altitude, climatic variables, forest type, dominant tree species and others) on the one hand, and on phylogenetic constraints on the other, is key to infer the influence of the evolutionary history of the taxa and the current conditions of their environment on their variation (Garnier et al., 2015).

Several forest endemics in the Mediterranean have widespread (non-endemic) related taxa inhabiting similar habitats across larger continental areas. This circumstance offers the opportunity to compare trait divergence in endemic and non-endemic species pairs, excluding the effect of ecological specialization and under similar constraints of competitive interactions with other species of the canopy and understory communities. The question of whether endemics and widespread congeners differ in traits, resource use strategies and competitive ability can thus be addressed specifically for forest species, unlike in previous studies (Hand et al., 2017; Lavergne et al., 2004; Thompson, 2004; Thompson et al., 2005; Totté et al., 2015). If present, functional divergence and differences in resource use within these species pairs cannot be directly related to clear differences in the light regimes, since both taxa, endemic and non-endemic, share a similar adaptation to shade, a major driver of leaf trait variability (Liu et al., 2016; Poorter et al., 2019; Valladares et al., 2016). Instead, trait divergence may result from evolutionary and biogeographic processes within each species pair, potentially resulting in different resource use attitudes. For this purpose, accounting for the distribution pattern of species in multiple pairs, e.g. whether allopatric, parapatric or sympatric, can help to unravel the role of geographic separation and vicariance in trait variation.

To bring light on the above issues, we measured six core leaf traits capturing plant strategies related to resource use (leaf area, specific leaf area, leaf mass per area, leaf dry matter content, nitrogen mass and C:N ratio) in 45 endemic forest taxa of vascular plants in distant angiosperm and conifer clades, either woody or herbaceous, most of which have not been examined for functional traits so far. These taxa are restricted to parts of the Italian peninsula, especially the southern regions and the islands of Sicily and Sardinia, which are well known centers of plant endemism of high biogeographical relevance, significantly contributing to the diversity of the Mediterranean flora (Médail and Quézel, 1999). Remarkable examples of narrow-ranged and phylogenetically distinctive forest endemics in these areas are the umbellifers Petagnaea gussonei, Cryptotaenia thomasii and Heptaptera angustifolia, the composite Rhaponticoides centaurium, the borage Aegonychon calabrum or the shrub Rhamnus persicifolia. This allowed us to open a window on the range of interspecific variation in leaf traits across Mediterranean endemic forest plants, to infer their position in the Leaf Economic Spectrum and to get insights into their resource use strategies and competitive abilities. Moreover, exploring the influence of a set of unrelated environmental and geographical variables for each sampled species/population and the phylogenetic signal of each trait made it possible to evaluate the role of either ecological or evolutionary constraints on trait variation across the taxa. In addition, expanding trait analysis to twenty widespread and closely related taxa of the same functional type and from similar forest habitat allowed phylogenetically independent comparisons to unveil the levels and patterns of variability in 27 species pairs, also in relation to the model of distribution of the two species in each pair, e.g. whether allopatric, sympatric or parapatric. Based on previous studies, we could test the hypothesis that endemics show no or little divergence from congeners in traits related to resource use such as leaf area, leaf mass per area and nitrogen content, as well as in terms of CSR strategies. Finally, we could explore whether trait divergence is overall lower in sympatric species pairs (with overlapping ranges) than in allopatric or parapatric pairs (with separate ranges), due to geographic and/or ecological vicariance in distinct ecoregions. Overall, this study contributes to a better knowledge of Mediterranean endemic plants and to the recent perspective of integrating the functional approach to the investigation of biodiversity, considering the influences of environment and evolution under environmental change (Liu et al., 2024).

A first evidence from this study was the unexpectedly ample variability in the leaf traits across the examined forest endemics, substantially reflecting their diversity in terms of functional types, phylogenetic relationships, biogeographical contexts and forest habitat preferences. Plotting the endemics in the global spectrum based on leaf economic traits and plant height (Díaz et al., 2016) showed their wide and essentially continuous distribution in the functional space mainly defined by Nmass, LA and LMA. As these traits are among those that better reflect the species position along the acquisitive-conservative axis (Garnier et al., 2015), endemics resulted broadly spaced along the resource use gradient, though sharing common adaptive features to the forest macrohabitat such as their shade-tolerant character in the case of understory species. Accordingly, our findings support that the acquisitive-conservative axis emerges even in relatively homogeneous environments, due to the phylogenetic and biogeographic divergence between species (Gorné et al., 2022). Based on the mean values reported at the global scale (Díaz et al., 2022), endemics showed relatively high values of LA and Nmass, while slightly lower for LMA. However, woody and herbaceous taxa showed significant differences in all examined traits, in line with previous findings (e.g. An et al., 2021; Matsuo et al., 2024). The higher LA and Nmass and lower LMA displayed by the herbaceous taxa supported that understory species have more acquisitive traits compared with species in environments with growth limiting factors (Cornelissen et al., 2003; Poorter et al., 2009). Overall, this was supported by the dominance of the C and R components in these taxa, pointing to their relative ability for fast use of resources for growth. Stress tolerance was instead poorly implemented, unlike in other Mediterranean endemics mostly found in open and water-limited habitats (Hand et al., 2017). The latter strategy was more implemented by the woody endemics, which showed more resource-conservative trait values (e.g. lower LA, higher leaf mass per area and lower Nmass), in line with existing evidence for many angiosperms of forest habitats (Matsuo et al., 2024). Stress tolerance was especially implemented in the two endemic conifers with high density needle-like leaves from the Sicilian mountains with nutrient poor soils and intense summer drought (Pignatti et al., 2017).

Leaf area was the most variable trait, varying by 3140-fold among species between Abies nebrodensis and the large hemicryptophyte R. centaurium ( Supplementary Figure 2A, B ). Leaf area is globally one of the most variable leaf traits (Wright et al., 2017), and dependent on functional groups, clades, biomes and habitats (Garnier et al., 2015; Price et al., 2014; Wright et al., 2017). In the forest endemics examined here, it appeared influenced by phylogenetic relationships, approaching a Brownian model of trait evolution based on Pagel’s λ, as well by environmental variables related to climate and forest habitat. The negative relationship with temperature (MAT) is likely associated with the frequent water limitation of warm areas of the Mediterranean regions, while the cooler conditions of beech forests and other hygrophilous forest communities, especially at lower latitudes, supported larger leaf size in the endemics. Overall, these findings fit global patterns that large-leaved species predominate in humid and warm environments while leaf size decreases in species of warm but arid sites and at higher latitudes (Li et al., 2024; Wright et al., 2017). Remarkably, however, species pairs analysis pointed to a substantial negative effect of the endemic condition on this trait, especially in the herbaceous taxa. Consequent functional differences with respect to widespread congeners from similar forest habitats could be reduced photosynthesis rates as well as effects on leaf water balance and temperature regulation, all directly dependent on LA (Cornelissen et al., 2003; Díaz et al., 2016; Falster and Westoby, 2003). Notably, the divergence in LA resulted positively related to the level of geographic separation between endemics and non-endemic congeners, thus stronger in allopatric than in parapatric and, even more, sympatric species pairs. Allopatric speciation has thus likely contributed to a significant evolutionary divergence in this key trait, likely in concomitance with adaptive processes to ecological constraints in distinct biogeographical regions. Our results are in line with recent evidence that this mode of speciation, more than others, promotes leaf area divergence among allopatric congeneric species living under different habitat conditions (Vargas et al., 2023).

Leaf mass per area varied about ten-fold among the examined endemics and was on average slightly lower than the global mean of 72.4 g m^-2 reported in Díaz et al. (2022). Considerably higher values occurred in the woody than in the herbaceous taxa, as already known from a previous study (Poorter et al., 2009). Environmental factors and, to some extent, phylogenetic relationships between taxa had a significant influence on LMA, in line with data from a wide range of Mediterranean species investigated by De la Riva et al. (2018). Ecoregion and, even more than for LA, type of forest habitat resulted important drivers of variation, in apparent contrast with previous evidence for remarkably small differences between different types of forest worldwide (Poorter et al., 2009). Mean annual temperature and evergreen forests had both a positive effect on LMA, showing the more resource-conservative behavior of endemics from warmer areas and drought-adapted forests in the southern and central Apennines and the Tyrrhenian regions. This supports general evidence that species with tendentially high LMA are those from habitats where either drought, nutrient limitation or both strongly hamper growth, including more or less open woodlands (Poorter et al., 2009). Also in the forest endemics examined here, therefore, LMA increases with the relative abundance of epidermal and sclerenchyma tissue and with reduced intercellular air spaces in the mesophyll, as an adaptation to life in sites with limited water availability (De la Riva et al., 2016; Fan et al., 2024).

Similarly to LA, divergence in LMA was stronger within pairs formed by allopatric and parapatric taxa than in those including sympatric ones, suggesting the above histo-anatomical adjustments to have originated in concomitance with species range separation and adaptive vicariance.

A relevant finding was the consistent increase of LMA in the endemics, compared with the congeneric species. Combined with reduction in LA and the smaller stature, this clearly points to a lower acquisitive ability and a stronger implementation of a resource conservation attitude than in widespread, closely related taxa. Based on the assumption that LMA is an indicator of the entire plant strategy (Reich, 2014; see also Butrim et al., 2024), the studied endemics appear inherently more inclined to implement a slower return-on investment behavior and higher persistence. When restricting the analysis to plants of forest habitats, therefore, we lend no support to the hypothesis that Mediterranean endemics do not differ from widespread congeners in leaf traits related to resource acquisition or conservation (Hand et al., 2017; Lavergne et al., 2004). Decrease in the C dimension in the endemics also provided evidence for lower resource acquisition ability, though this was associated with a parallel decrease in stress-tolerance and an increase in the “ruderal” component in the herbaceous endemics. The weak increase in Nmass, usually considered as an indicator of plant photosynthetic capacity, respiration and growth rate (Khan et al., 2022; Wright et al., 2004), was in line with the enhanced ruderal strategy. As for LA and LMA, a phylogenetic signal was detected for Nmass, supporting recent findings that aboveground tissue element concentrations in plants, particularly N, Ca, K, and B, is phylogenetically conserved (Furey and Tilman, 2023). Hence, this trait appeared under the complex influence of interacting factors, including latitude and related environmental variables such as precipitation, ecoregion and, to a lesser extent, type of forest habitat. Lower values in the southern endemics from the Sicilian, Sardinian and south Tyrrhenian ecoregions fit evidence that leaf N globally declines with decreasing latitude (Reich and Oleksyn, 2004), though in the present study this increase resulted coupled with precipitation (MAP) and not with temperature (MAT). In addition, the higher Nmass values in endemics from open and evergreen forests support that species in drier or sunnier sites tend to have higher nitrogen concentration in their leaves, likely reflecting a higher photosynthetic efficiency (Fan et al., 2024). Unlike for morpho-functional traits, differences in Nmass were overall larger in parapatric and sympatric species pairs, suggesting this trait to be more influenced by environmental variables, forest habitat and possibly other local site conditions, than by allopatric speciation.

By providing novel trait values data for 45 endemic taxa of Mediterranean forests and 20 widespread congeners, this work contributes to a better understanding of the functional space exploited by globally rare plants from a major biodiversity hotspot, their resource use attitudes and ecological strategies. Overall, the ample variation in leaf traits across these endemics pointed to a significant differentiation and continuous distribution along the resource acquisitive-conservative gradient defined by LA, LMA and Nmass. Depending on the trait, interspecific variability was influenced by different drivers, including functional group, eco-geographical, climate-related factors and forest habitat, as well as phylogenetic constraints especially for LA and LMA. Herbaceous taxa showed more acquisitive traits increasing resource capture and use efficiency, while the woody taxa were overall more stress-tolerant and with more resource conservative trait values (e.g. higher leaf mass per area). Compared with widespread congeners from similar forest habitats, however, endemics were characterized by lower LA and higher LMA, as well as smaller stature, pointing to their lower acquisitive ability and stronger resource conservation attitude. These features may be involved in their inherently reduced capacity to spread outside their restricted range and often ecological niche over wider territories. Moreover, differences in LA and LMA within allopatric and parapatric pairs were overall larger than in sympatric pairs, suggesting the role of vicariance and range separation in the divergence of morpho-functional leaf traits. Analyzing other traits related to persistence, reproduction and dispersal will further advance our understanding of the functional and ecological space exploited by Mediterranean forest endemic plants, which will help to implement actions for their conservation under the increasing pressures to their habitat.