LAUSR

Abstract Populus euphratica is a constitutive tree species of the mostly monospecific forests along Central-Asian rivers. In the hyper-arid climate along the Tarim River, Xinjiang, NW China, these trees are phreatophytes with continuous access to the groundwater. Those poplar stands are often moderately to completely defoliated by Lepidopteran larvae in spring. In a P. euphratica stand at the middle reaches of the Tarim River, which grew close to the river and to the water table, we tested whether severe defoliation significantly reduces the trees' radial stem increment and their production of above-ground wood biomass even at ample water supply by the river. Tree-ring analyses and allometric regressions revealed a drastic decrease in radial stem increment and annual above-ground wood production upon complete defoliation even in a period of ample water supply, which otherwise would have allowed maximum productivity. The loss of above-ground production by herbivory amounted to approximately 44% relative to the high production rate of the year before complete defoliation. During the recent decade, however, only the amount of river discharge – but not the estimated intensity of defoliation – was (marginally) significantly correlated with the stem increment. We conclude that in P. euphratica forests growing close to the river and to the groundwater, tree growth is mainly related to the amount of river discharge, but can be significantly reduced in years with complete defoliation. Therefore, defoliation events should be considered in assessing the productivity of these riparian forests.