Establishment of oak seedlings in historically disturbed sites: Regeneration success as a function of stand structure and soil characteristics

The establishment and development of oak (Quercus spp.) seedlings are frequently impaired by site heterogeneity resulting from historical anthropogenic disturbances in Mediterranean basin woodlands. In particular, the alteration of forest structure and soil biotic and abiotic components can compromise the natural regeneration of Persian oak (Quercus brantii Lindl.) in western Iran. This study aimed to investigate how soil properties, soil mesofauna diversity and stand structural indices differ between well and poorly regenerated areas. We sampled 105 plots in an oak woodland subjected to past disturbances but protected from human activities for the last 20 years. These 100-m2 plots were distributed into two groups: i) poorly regenerated plots (PRP) with only 0 or 1 newly established oak seedling, and ii) well regenerated plots (WRP) with ≥3 seedlings. In each plot, we characterized the forest structure using three structural indices, and determined the soil properties and mesofauna diversity. Structural indices were higher in WRP than in PRP for height (0.83 and 0.23), diameter differentiation (0.74 and 0.36) and species mixture (0.90 and 0.21). Also, richness and diversity of soil mesofauna were lower in PRP than in WRP.A canonical correspondence analysis (CCA) ordination clearly separated WRP, which were characterized by a clumped spatial distribution of mature oak trees on fertile soils, from PRP that exhibited a uniform spatial distribution of trees established on soils with a higher sand content (63 and 59%) and a higher bulk density (1.7 and 1.3).The uniform spatial distribution of trees, which was less favourable to oak seedling establishment, was inherited from intensive past human disturbances. To favour oak regeneration, we recommend applying management actions in cooperation with land-users, in particular the conservation of small patches of tree clumps to act as islands of fertility and seed sources.