LAUSR

The current expansion of forests in Europe is expected to be affected by transient colonization credits (i.e. species deficits compared to long-established forests), especially due to plant species with limited dispersal ability. We aimed to disentangle the role of forest connectivity and climatic aridity in the assembly of woody plant communities present in expanding Mediterranean forests by studying species with different types of dispersal. We used a space-for-time approach in recent (post 1980), middle-aged (1956–1980) and long-established forests (pre-1956) along an aridity and forest connectivity gradient in NE Spain. We recorded woody canopy and understory species and classified them as either vertebrate-, non-vertebrate- or human-dispersed species (i.e., crops). Species turnover between forests of different ages was limited to particular species and did not give rise to colonization credits determined by plant dispersal type. Connectivity and aridity drove understory richness in a similar way in forests of different ages. Within this general pattern, non-vertebrate-dispersed species were especially promoted by benign conditions. Sites with high forest connectivity had either the richest or the most impoverished understories given, respectively, low or high aridity. Thus, the rescue effect was modulated by aridity, which acted as a limiting factor. Mediterranean forest richness and composition are strongly dependent on climatic constraints and the ‘rescue effects’ (connectivity) originating in surrounding forest areas. These two factors shape the composition of expanding forests in different ways depending on the ability of species to disperse.