The Iberian Peninsula comprises one of the largest boundaries between Mediterranean and Eurosiberian vegetation, known as sub-Mediterranean zone. This ecotone hosts many unique plant species and communities and constitutes the low-latitude (warm) margin of numerous central European species which co-occur with Mediterranean vegetation. Two of the main species found in this region are the Eurosiberian European beech (Fagus sylvatica L.) and the Mediterranean Pyrenean oak (Quercus pyrenaica Willd.). It remains unclear how the different physiological and adaptive strategies of these two species reflect their niche partitioning within a sub-Mediterranean community and to what extent phenotypic variation (intraspecific variability) is driving niche partitioning across Eurosiberian and Mediterranean species.

We quantified functional niche partitioning, based on the n-dimensional hypervolume to nine traits related to resource acquisition strategies (leaf, stem and root) plus relative growth rate as an additional whole-plant trait, and the environmental niche similarity between Pyrenean oak and European beech. Further, we analyzed the degree of phenotypic variation of both target species and its relationship with relative growth rates (RGR) and environmental conditions. Plant recruitment was measured for both target species as a proxy for the average fitness.

Species' functional space was highly segregated (13.09% overlap), mainly due to differences in niche breadth (59.7%) rather than niche replacement (25.6%), and beech showed higher trait variability, i.e., had larger functional space. However, both species shared the environmental space, i.e., environmental niches were overlapped. Most plant traits were not related to abiotic variables or RGR, neither did RGR to plant traits.

Both target species share similar environmental space, however, show notably different functional resource-use strategies, promoting a high complementarity that contributes to maintaining a high functionality in sub-Mediterranean ecosystems. Therefore, we propose that conservation efforts be oriented to preserve both species in these habitats to maximize ecosystem functionality and resilience.