Abstract
Tree recruitment is a key process underlying stand dynamics and sustainability in managed forests. Woody plant cover is known to affect the regeneration success of Pinus nigra, suggesting the existence of facilitative plant-plant interactions. The regeneration patterns of this Mediterranean pine were analyzed across its distribution area, using data from 3226 plots of the Spanish National Forest Inventory. We aimed to test the hypothesis that seedlings establishment occurs under higher values of either canopy or shrub cover in the driest populations, as predicted by the stress-gradient hypothesis. Data were analyzed by means of Generalized Linear Models and multivariate methods. Results revealed that regeneration failure occurs on a regional scale, and that regeneration is facilitated by tree canopy cover of 55%–80%. A non-linear pattern of interaction along an aridity gradient was identified, with competition at the wettest site, high facilitation at the mid-dry sites, and low facilitation at the driest site. Evidence suggests that some shrub species may facilitate recruitment in the harsher areas. Collectively, our results reduce the possibilities of adapting forest management to drying climates by the application of alternative silvicultural prescriptions involving canopy cover.
Highlights
Water shortage has been widely reported as the main cause impeding the establishment of juvenile plants in Mediterranean ecosystems [1,2,3]
Studies focused on the recruitment dynamics of Mediterranean tree species have found increasing seedling mortality during the summer, compared with other periods of the year, and in open areas, compared with sites located under the canopy of woody plants [9,10]
This study identifies that P. nigra may have recruited insufficiently during the last decades in order to restock managed stands, this circumstance being more pronounced in the drier southern edge of the species distribution area
Summary
Water shortage has been widely reported as the main cause impeding the establishment of juvenile plants in Mediterranean ecosystems [1,2,3]. Studies focused on the recruitment dynamics of Mediterranean tree species have found increasing seedling mortality during the summer, compared with other periods of the year, and in open areas, compared with sites located under the canopy of woody plants [9,10]. The risks of excessive evapotranspiration, overheating, and photo-inhibition are ameliorated under the cover of tree canopies [2,10,11]. These positive plant-plant interactions between tree seedlings and woody species are examples of facilitation [12]
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