Abstract

Background: Nitrogen and water are important limiting factors to forest productivity. At the plant level, there is contrasting empirical evidence about the trade -off between water use efficiency (WUE) and nitrogen use efficiency (NUE). Methods: A study was conducted on 3 -year -old loblolly pine (Pinus taeda L.) trees sampled at two contrasting sites (Virginia Piedmont (VA) and North Carolina Coastal Plain (NC)) in the southeastern United States. We investigated the leaflevel relationship between water and nitrogen use efficiency (i.e., WUE(ins )versus PNUE). Both parameters were measured in the fall, three years after outplanting. Results: WUEins and PNUE were higher at VA than NC. At both sites, WUEins increased from August to November, while PNUE showed a consistent decline for the same period only at NC, with no clear pattern observed at VA. The Pearson's coefficient of correlation (r) between WUEins and PNUE was negative (r=0.50) at VA in August, positive (r=0.77) at NC in October, and insignificant for the other measurement dates. Conclusions: Regardless of the site, at the young stage of stand development, WUEins and PNUE were mostly uncoupled, although it can be speculated that a transient trade -off between these two variables can be expected depending on soil water rather than nitrogen availability.