Relationship between photosynthetic-water and nitrogen use efficiencies in young Pinus taeda L. trees at two contrasting sites

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 leaf-level relationship between water and nitrogen use efficiency (i.e., WUEins 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.