Reforestation with resprouter species to increase diversity and resilience in Mediterranean pine forests

• Sunlight was the parameter with the most marked oscillations among pine densities.
• Intense shade was beneficial for seedling survival.
• Different pine density conditions affected growth and water use of seedlings.
• Results are relevant for forest management and conversion to resilient communities.

Under Mediterranean conditions, drought and fire are prevalent disturbances that play a fundamental role in the distribution, organization and evolution of ecosystems. Reforestation programs were carried out in degraded areas by planting pines. Presently, the spontaneous regeneration of other species is scarce and species diversity is poor. Introducing woody resprouter species under pine forests would increase species diversity and ecosystem resilience to cope with different types of disturbances, such as wildfires. In this study, we investigated the survival, growth and functional responses of six Mediterranean trees and shrub resprouter species planted under pine forests. Experiments were done in a dry Mesomediterranean climate. Seedlings from a wide array of early to late successional trees and shrub species were outplanted in different pine densities, low, medium and high, which corresponded to 22%, 48% and 62% of the maximum sunlight received, respectively. The time course of survival and growth (height and basal diameter) were analyzed for 3 years after outplanting. Survival was higher than 78%, although differences between species and pine densities were found. Seedling growth also reflected different interactions among introduced seedlings and density of pines. Water use efficiency of species based on isotopic determinations (δ13C and δ18O) differed between densities and species according to functional groups. These results have relevant implications for the management and restoration of fire-prone Mediterranean pine forests.