Continental scale variability of foliar nitrogen and carbon isotopes in Populus balsamifera and their relationships with climate

Variation across climate gradients in the isotopic composition of nitrogen (N) and carbon (C) in foliar tissues has the potential to reveal ecological processes related to N and water availability. However, it has been a challenge to separate spatial patterns related to direct effects of climate from effects that manifest indirectly through species turnover across climate gradients. Here we compare variation along environmental gradients in foliar N isotope (δ(15)N) and C isotopic discrimination (Δ(13)C) measured in 755 specimens of a single widely distributed tree species, Populus balsamifera, with variation represented in global databases of foliar isotopes. After accounting for mycorrhizal association, sample size, and climatic range, foliar δ(15)N in P. balsamifera was more weakly related to mean annual precipitation and foliar N concentration than when measured across species, yet exhibited a stronger negative effect of mean annual temperature. Similarly, the effect of precipitation and elevation on Δ(13)C were stronger in a global data base of foliar Δ(13)C samples than observed in P. balsamifera. These results suggest that processes influencing foliar δ(15)N and Δ(13)C in P. balsamifera are partially normalized across its climatic range by the habitat it occupies or by the physiology of the species itself.