COMPARING EARLY DROUGHT RESPONSES OF ITALIAN ALDER AND HYBRID POPLAR IN A BIOENERGY FIELD TEST IN MEDITERRANEAN EUROPE

In the context of sustainable deployment of bioenergy plantations, the selection of trees species with low water demand and, possibly, nitrogen fixing capacity is of primary importance. Italian alder (Alnus cordata L.) is a fast-growing, nitrogen-fixing tree, endemic to restricted areas of the north Mediterranean Basin with interesting potentialities for bioenergy plantations, also in mixture with hardwoods for fine timber production. Italian alder is considered the most drought-adapted species amongst the Alnus genus. Although these potentialities, little is known about the drought adaptation of this species. An experimental plantation was recently established in central Italy, for comparing Italian alder with the hybrid poplar clone AF2, which is actually one of the most interesting new cultivar for short-rotation coppice (SRC) plantations in Italy, as well as in Europe. Alder seedlings and poplar cuttings were planted in March 2012 in an experimental site, within a network of the European Project AgroCop (www.agrocop.com), studying the intercropping of SRC with hardwood timber species. After a rainy spring, with the onset of summer drought, plant water status, leaf nitrogen concentration and biometric and eco-physiological parameters were periodically monitored. Plant water status was assessed measuring the pre-dawn and midday leaf water potential with a Scholander pressure chamber and the Relative Water Content (RWC). Gas exchange and fluorescence parameters were measured using a Licor 6400 portable gas exchange system and carbon isotope composition of leaf material was determined with a mass spectrometer (Isoprime, Cheadle, UK) coupled in continuous-flow with an elemental analyser. Both alder and poplar showed a strong drought-induced decrease of leaf water potential and concurrent reduction in stomatal conductance. However, alder showed a stronger decrease of leaf water potential than poplar and a lower water-use efficiency. This was associated with lower values of stomatal conductance and assimilation rate per unit of leaf area and of the apparent quantum efficiency of PSII. Alder showed a much higher number of leaves per plant and a faster leaf turnover than poplar. Our preliminary results indicate that alder and poplar showed contrasting water-use strategies. Further studies are in progress to finally demonstrate which of these species are more adapted to cope with the summer drought in Mediterranean areas.