Growth dynamics, climate sensitivity and water use efficiency in pure vs. mixed pine and beech stands in Trentino (Italy)

Understanding to what extent the species mixture modifies the growth of trees and their responses to climate, in comparison with monospecific stands, is important to support forest adaptation and mitigation strategies. Trees are able to record and store information useful to identify, assess and reconstruct the environmental factors influencing their growth and adaptation capacity. This paper aimed at (i) defining the tree growth trends, then analysing possible correlations with the climatic trends (Standardised Precipitation-Evapotranspiration Index; SPEI), and (ii) evaluating the intrinsic Water Use Efficiency (WUEi) of two important tree species, occurring in pure and mixed forest stands through the support of isotopic analysis. Three sites with similar pedo-climatic conditions were identified in the Italian Alps. The first two are characterized by monospecific stands, respectively dominated by European beech (Fagus sylvatica L.) and Scots pine (Pinus sylvestris L.). The third site refers to a mixed stand of both the previous species. In particular, in order to assess the annual changes in WUEi, we analysed 13C in tree rings. The influence of the stomatal conductance was also investigated through 18O. Our results indicated that: (i) Basal Area Increments (BAI) consistently increased in all stands (except for pure stand of Scots pine), in the 1994-2003 period; (ii) SPEI highlighted a drought period between 1991 and 2007; and (iii) the WUEi values were generally higher in pure than in mixed stands. The divergence between BAI and SPEI values in the 90s and early 00s may be explained as consequence of moderate thinning. Indeed, our findings showed how past forest management (namely thinning) might be more influent on tree growth than current climatic oscillations (temperature and precipitation).