Competitive capability of Robinia pseudoacacia with native species in an old-growth broadleaf forest

Differences in shade tolerance are frequently important determinants of forest structure and dynamics. The main objective of this research was to analyze leaf trait variations among tree species growing in a broadleaf deciduous forest (Natural Reserve “Siro Negri”, Italy) in response to the intra-canopy light variations. We tested the relationship between shade-tolerance and leaf plasticity considering that different plastic responses may contribute to the range of environments that species inhabit. Our results support the overall trend that light-demanding species would be more plastic than shade-tolerant species. The measured phenotypic plasticity for the considered species is in the range of broadleaf deciduous species. Among them, Quercus robur and Robinia pseudoacacia (shade intolerants) have a higher phenotypic plasticity than Acer campestre and Corylus avellana (shade tolerant). Temperature and CO2 concentration increase might act as potent agents of natural selection among species favoring the more plastic species. In particular, air temperature increase might allow reproduction of Phythophora cinammomi, a pathogen largely affecting Q.robur while CO2 concentration increase might favor R. pseudoacacia because of its high growth rate. The conservative management of the forest carried out since the establishment of the Reserve has probably limited the presence of R. pseudoacacia. It is important to maintain this type of management since creation of gaps could allow regeneration of R. pseudoacacia over Q. robur. Forests with old-growth characteristics like the investigated forest are important reference sites for management approaches involving a broad range of ecosystem functions and services.