Contrasting effects of protected areas on understory plant diversity in broadleaved deciduous forests

Protected areas are essential for preserving forest ecosystems and maintaining their ecological integrity. Nonetheless, their role in conserving understory plant species in Italy remains understudied. We analysed 5,423 vegetation plots sampled in Italian broadleaved deciduous forests between 1980 and 2020. Using propensity score matching, we controlled for potential confounding factors such as elevation, climate, proximity to roads, and plot size, enabling an unbiased assessment of the effect of protection status on understory plant diversity. We compared alpha and gamma diversity between protected and unprotected areas, considering both the total number of species and separately for understory plant species associated with forest- or open-habitats. Differences in alpha diversity were assessed using generalized linear models, while gamma diversity was evaluated by quantifying the number of species exclusive to protected areas, exclusive to unprotected areas, and shared between the two. Protected areas generally tended to support higher levels of both alpha and gamma diversity, although patterns varied among habitat types. Alpha diversity was generally higher within protected areas, with statistically significant differences observed only in some habitats. Similarly, forest-associated species showed higher gamma diversity within protected areas than outside across all habitat types, whereas patterns for open-habitat species were more variable and did not consistently differ between protected and unprotected areas. Overall, our results suggest that Italian protected areas represent an important portion of understory plant diversity, particularly for forest-associated species. However, the presence of habitat-specific assemblages suggests that additional conservation efforts may still be needed beyond protected areas boundaries.