Effects of associating Quercus robur L. and Alnus cordata Loisel. on plantation productivity and water use efficiency

•N transfer from alder increases growth and water use efficiency in Q. robur stands.•N-fixing species affects the balance between A and gs in favor of A.•N interactions between species change through time.•The beneficial effects of N disappear with the declining of alder.•Our study shows the importance of the intercropping with a suitable N-fixing species. Mixed-species plantations containing non-nitrogen (N)-fixing species and N-fixing species have the potential to increase the productivity and the ecophysiological performance of the target tree species in comparison to monoculture. However, there are few and conflicting data on the effects of fixed N transfer on species water use efficiency in the short, medium and long term. In this paper, we evaluate the productivity and intrinsic water use efficiency (WUEi), i.e., the ratio of photosynthesis (A) to stomatal conductance (gs), in Quercus robur L. (pedunculate oak) stands growing in monoculture and in combination with Alnus cordata Loisel. (Italian alder) in a ratio of 25% (stand F25%) and 50% (stand F50%). We analyzed δ13C in tree rings as a proxy of the inter-annual changes in WUEi, while the influence of the stomatal conductance was explored through δ18O data. Our results indicate a considerable increase in cumulative basal area (BAI) and in WUEi in the two mixed stands during the period 1990–2002, largely resulting from an increase in N availability, due to the N-fixing species that induced an enhancement of A. After 2002, the differences between the three stands in terms of N% in tree rings, BAI and WUEi leveled off, when natural mortality or management practices decreased the competitive ability of Italian alder. Our study shows the importance of intercropping with a suitable N-fixing species to ensure high levels of productivity and water use efficiency of the target tree species in a Mediterranean-type agroforestry system, but also the need to understand how species interaction changes in time and space with a view to setting the management criteria to maximize tree performance.