Water- and nutrient-use efficiency of a deciduous species, Vaccinium myrtillus, and an evergreen species, V-vitis-idaea, in a subalpine dwarf shrub heath in the southern Alps, Italy

Periodic measurements of gas-exchange rates and determinations of foliar N and P concentrations were used for evaluating instantaneous water-use effic iency and photosynthetic nutrient-use efficiency in two co-existing dwarf s hrubs of different growth form (V. myrtillus, deciduous, and V. vitis-idaea , evergreen) in a subalpine heath in the southern Alps of Italy. Those data were compared with cumulative assessments of water-use efficiency and phot osynthetic nutrient-use efficiency obtained by measuring leaf carbon isotop e discrimination in leaf tissues and by estimating nutrient resorption from senescing leaves. V. myrtillus presented higher dry-weight based rates of net photosynthesis (A(weight)) compared to V. vitis-idaea. A(weight) was po sitively correlated with foliar-nutrient status and intercellular-to-ambien t gradient in CO2 concentrations. A(weight) was, furthermore, negatively co rrelated with leaf specific mass. Instantaneous photosynthetic nutrient-use efficiency did not differ between the two species but the percentages of N and P pools resorbed from senescing leaves were somewhat higher in the dec iduous species. The evergreen species showed lower P concentrations in sene scing leaves which indicated a higher proficiency in resorbing phosphorus c ompared to the deciduous species. In addition, the evergreen species achiev ed a higher carbon gain per unit foliar N and P, due to a longer mean resid ence time of both nutrients. The two species did not differ from each other with respect to both instantaneous and long-term water-use efficiency. Thi s was consistent with the climatic pattern, showing no sign of water defici ency through the growing season. Current-year V. vitis-idaea leaves had a s ignificantly higher delta(13)C compared to previous-year leaves, possibly m irroring a long term acclimation of evergreen leaves, as far as they age, t o the habitat conditions in the understory where evergreen species are usua lly confined within mixed dwarf-shrub communities.