Decadal nitrogen addition increases divergence in intrinsic water-use efficiency between sapling and mature larch plantations

Abstract

Intrinsic water-use efficiency (iWUE) is the ratio of the photosynthetic assimilation rate to stomatal conductance and is tightly linked with tree growth and mortality. Increasing atmospheric nitrogen (N) deposition has been recognized to have a profound influence on ecosystem functioning, especially in typically N- and water-limited temperate forests. However, the effects of N deposition on leaf-derived iWUE estimates and the underlying physiological mechanisms remain poorly understood, as well as effects related to the developmental stage of temperate plantation forests. Using leaf stable carbon and oxygen isotope (δ13C and δ18O) approaches, we investigated iWUE of sapling and mature larch plantations subjected to decadal N addition in northern China. Results showed that saplings had significantly higher leaf iWUE compared to mature individuals. Long-term N addition significantly increased iWUE by 6.85% in the sapling stand, with no remarkable changes occurred in the mature stand. In saplings, the N-addition-induced increase in iWUE could be attributed to enhanced photosynthesis (illustrated by higher leaf N concentration), while leaf shading may cause lower leaf photosynthesis regardless of N level in mature trees. The higher and temporal stable leaf iWUE in saplings imply conservative water-use strategy, whereas the opposites in mature trees imply profligate water-use strategy. Thus, the divergent responses of iWUE to decadal N addition should be considered for sustainable forest development under increasing atmospheric N deposition conditions.