Coastal Monitoring in the Mediterranean Basin: Issues and Technical Approaches

Extreme weather events, including prolonged droughts and heat waves, are increasingly causing forest dieback and tree mortality across many ecosystems, particularly in the Mediterranean region. In recent years, widespread decline of Quercus ilex L. (holm oak) has been reported in Southern Europe, notably in the Iberian Peninsula and Italy. As a dominant tree species, holm oak dieback has the potential to reshape understory shrub and herbaceous communities. Furthermore, Mediterranean vegetation is a major source of Biogenic Volatile Organic Compounds (BVOCs), whose emissions are highly sensitive to environmental conditions.

This study examines seasonal variations in holm oak canopy cover, understory species richness, and alpha diversity indices (Shannon–Wiener and Pielou) in two holm oak stands located in the Maremma Regional Park (Tuscany, Italy). The stands differ in crown defoliation intensity and are classified as high defoliated (HD) and low defoliated (LD). Seasonal field surveys were conducted between 2019 and 2023 to characterize temporal changes in vegetation composition. Plant species inventories were used to assess the biological spectrum within stands and estimate habitat explanatory factors using Ellenberg's indicator values. Additionally, relationships between vegetation dynamics and ecosystem-level BVOC emissions were evaluated.

During the study period, both stands experienced a 50 % reduction in holm oak canopy cover. This canopy loss increased light availability and was associated with a temporary rise in understory species richness in 2021. In contrast, a marked decline in species richness was observed in 2022, falling below 2019 levels in both stands. This reduction is likely linked to the accumulation of dead wood on the forest floor and the extreme temperatures recorded during that year. By 2023, species richness recovered to values comparable to those observed in 2020 and 2021. Notably, the Shannon-Wiener and Pielou indices did not significantly vary in the two stands, where the biological spectrum displayed a clear Mediterranean characteristic. However, both stands exhibited a progressive increase in geophytes and therophytes, suggesting worsening water stress conditions. From October 2020, the LD stand was mainly characterized by scapose hemicryptophytes and phanerophytes and caespitose phanerophytes, whereas the HD stand was dominated by nano-phanerophytes. BVOC measurements closely mirrored vegetation changes, showing a clear reduction in monoterpene emissions associated with increasing holm oak defoliation and mortality. Elevated Ellenberg indicator values for light and temperature further confirmed the Mediterranean imprint of the vegetation and revealed signs of anthropogenic disturbance in the HD stand. This was reflected in the higher abundance of nitrophilous and medicinal herbaceous species, such as Atropa belladonna L. and Datura stramonium L., whose historical introduction and subsequent expansion may have been facilitated by canopy opening and tree decline.

Overall, although diversity indices remained statistically stable, holm oak dieback induced notable shifts in species composition, particularly in the HD stand. The pronounced canopy reduction and increasing dominance of nano-phanerophytes suggest that part of the forest is undergoing a transition toward shrubland. Additionally, the observed decline in BVOC emissions highlights the potential consequences of holm oak dieback for ecosystem functioning and atmospheric chemistry.