Beneficial effect of pine thinning in mixed plantations through changes in the understory functional composition

Areas dominated by Pinus halepensis in the Mediterranean Basin largely result from its plantation as a restoration tool and often exhibit low diversity and resilience. Given their extent and importance, increasing efforts are being made to overcome these limitations, mainly through management practices such as pine thinning. However, studies on the effects of thinning are few, rely mainly on species diversity, and often lead to contradictory results. Further clarification is needed, particularly regarding species functional traits, as they are the key drivers of ecosystem functioning and resilience, two primary targets of restoration projects.We performed a 35% pine thinning to decrease competition and improve the functioning of an ecosystem dominated by pines, at a restored quarry site in the southwest of the Iberian Peninsula. A functional approach was used for the first time to monitor the thinning effect in mixed plantations of P. halepensis and late-successional Mediterranean shrubs under a dry sub-humid climate. The effects of thinning on the understory plant diversity, functional composition and woody species growth were monitored for two years.While no changes were detected in species richness and diversity, thinning promoted key functional traits, increasing the density of groups with N-fixing ability, semi-deciduous drought strategy and therophyte life-form, mostly with anemochorous and barochorous dispersal. In the medium term, these changes are expected to contribute to improve nutrient cycling, ecosystem resilience to drought and fire, and biotic fluxes, which are important early limitations to plant establishment and functional recovery at the quarry site. Basal growth of planted evergreen sclerophylls increased, indicating a competitive relief. A moderate intensity pine thinning was useful to alleviate competition and improve ecosystem function in mixed stands with different pine densities and sizes.Our findings reinforce the benefits of a functional approach to monitor the effect of management practices on ecosystems, and may help researchers and managers to convert P. halepensis dominated areas into more diverse and resilient ecosystems, particularly in Mediterranean areas