Assessment of forest structure and disturbance in a holm oak forest (Quercus ilex L.) in Sardinia with allometric functional approach

The contribution of natural forests to biodiversity preservation and climate regulation is inextricably linked to forest attributes. Forests with old-growth attributes represent land ecosystems where maximum naturalness is preserved due to high structural and functional complexity. In Europe, old-growth forests account for less than 1%, and they are especially rare in the Mediterranean Basin, one of the largest hotspots centers of biodiversity in Europe. This study focused on the forest structural assessment of one of the most ancient holm-oak (Quercus ilex) forests in the Mediterranean Basin: Montes Forest in the Supramonte of Oliena, Orgosolo and Urzulei (Sardinia). In June 2023 the forest was surveyed with 70 randomly placed relascopic assay plots (Bitterlich's Relascope) coupled with linear regeneration transects of 10 m2 each to get the full real diametric tree distribution. The scaling of the crown volume for the species was assessed by surveying total heights, depths, and canopy widths by laser tool Trupulse®. Data were interpreted by mean of the H-model, a tool that uses mechanical statistics and allometric principles to define the potential tree size distribution. These sites were paired with the sampling of microbial communities from tree bark and soil across different tree sizes, analyzed via DNA metabarcoding. The results revealed an H-parameter close to 1, denoting isometric scaling of canopy volume, similar to that found for trees growing in tropical forests, while the H-model exponent was of -2.4, which describes the slope of the tree size distribution corresponding to the maximum resource-use potential at the site. The real tree size distribution exponent in old-growth forest resulted higher than that the one measured in forest areas with same tree species but higher anthropogenic disturbance. These data, combined with information on bark- and soil-associated micro-biodiversity from sites with varying disturbance levels, will provide important insights into the relationship between forest structural attributes and biodiversity.