Recruitment into the seedling bank of an undisturbed Mediterranean pinewood: Increasing forest resistance to changing climates

Although typically considered shade-intolerant, Mediterranean pine species may form seedling banks due to the existence of facilitative interactions between seedlings and canopy trees. Consequently, seedling banks could be considered as part of the seedling pool that should restock managed Mediterranean pinewoods, thus increasing the resistance of these forests to climate change. The study investigated the characteristics of the seedling bank in a silviculturally undisturbed forest of Pinus nigra subsp. salzmanii exposed to increasingly arid conditions. The study site was located in the southernmost range limit of the species. The dynamics of the seedling bank was studied by comparing current seedling (height < 1.30 m, age < 2 yr) and pole tree (height ≥ 1.30 m and diameter at breast height < 12.5 cm) altitudinal distributions, using altitude as a proxy of aridity. Aridity was proved to have increased in the study area along the last four decades. These climatic trends were identified by means of Mann-Kendall tests for trend. Additionally, the effects of summer drought, light availability, soil compaction, litter layer and micro-slope as explanatory variables of the recruitment outcome were analysed by means of Generalized Linear Mixed Models and survival analysis. Altitude affected positively seedling establishment, but had no effect on the current abundance of pole trees, indicating a recent upwards displacement of the regeneration niche due to increasingly arid conditions. Water and light availability, the latter at intermediate levels of irradiance, affected tree recruitment positively, whereas the effects of soil compaction, litter layer depth and micro-slope were negative. Implementing extended rotation periods and maintaining intermediate values of tree cover are proposed as important management recommendations to achieve successful natural regeneration in Mediterranean pinewoods.