Impact of holm oak dieback on alpha biodiversity in a Mediterranean forest

The impact of holm oak dieback on understory vegetation composition was investigated in the Maremma Park (Tuscany, IT). Seasonal inventories of shrub and herbaceous species were conducted from summer 2019 to summer 2022 on two stands of an area of about 600 m2, characterized by different degrees of holm oak crown defoliation (high and low defoliated, HD and LD). Species richness, Shannon-Wiener and Pielou indices (alpha diversity) were calculated. Species inventories were also used to assess the biological spectrum and estimate habitat explanatory factors using Ellenberg's indicator values (EV). We hypothesized that the reduction in canopy cover of holm oak trees would result in loss of alpha diversity. Canopy cover was halved from summer 2019 to summer 2022 in both areas, and the higher light availability due to canopy gap resulted in an increase in species richness for both stands in 2021. However, in 2022, species richness decreased in both areas, probably because of the large amount of dead holm oak wood on the ground. During the study, the Shannon-Wiener and Pielou indices did not significantly change between the stands; however, the LD stand maintained a higher variety of life forms than the HD stand (phanerophytes, hemicryptophytes, and therophytes). High temperature and light EV values suggested that both HD and LD areas are suited for species preferring warm conditions and high light intensity. Further, EV highlighted a certain anthropic disturbance of the HD stand, which counted more nitrophilous species than the LD stand. These results may be due to the presence of some medicinal herbaceous plants (e.g., Atropa belladonna L., and Datura stramonium L.) introduced by humans in the Middle Ages, whose diffusion may have recently been favoured by the holm oak decline. In conclusion, although no reduction in alpha diversity has been observed during the study period, holm oak dieback has altered species composition in the understory vegetation.