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Populus euphratica, a constitutive tree species of the Central-Asian riparian forests, forms extremely heterophyllous leaves. Along gradients of salinity and water supply from the soil in arid regions of NW China, we tested whether specific leaf area (SLA), a pivotal plant functional trait, is related to other morphological variables and to the productivity of the trees as well as to important physiological variables (foliar concentrations or ratios of sodium (Na+), nutrients and stable isotopes of carbon (delta C-13) and oxygen( delta O-18)). We evaluated our data at the leaf, tree and stand level.

Under the arid climate of the study area, we found large morphological variation among the leaves, which, however, converged to a range of SLA (4.7-12.5 m(2)kg(-1) across 95% of the total sample size) that is small when compared to temperate deciduous forest trees of more humid regions. Correlations between SLA and morphological variables (leaf area, number of teeth at the leaf margin, ratio of blade width to blade length) were weak. However, SLA exhibited a significantly negative correlation with foliar delta C-13, and a positive one, with tree height, annual above-ground wood production (AAWP) and (marginally) with stand density. SLA increased with increasing water supply from the soil within a given site but not across sites. Foliar Na+ was negatively related to potassium concentration and, at two out of three study sites, positively to delta O-18, but not to SLA, tree morphology, AAWP and stand density. AAWP was not related to nutrient concentrations. We conclude that in P. euphratica, a permanent phreatophyte in the study region, SLA constitutes an acclimatization not only to the water supply from the soil but also - and probably even more importantly - to the local transpirational demand; and that SLA is related to biomass production through optimization of the water use efficiency. (C) 2017 Elsevier GmbH. All rights reserved.