Abstract
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 (δ15N) and C isotopic discrimination (Δ13C) 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 δ15N 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 Δ13C were stronger in a global data base of foliar Δ13C samples than observed in P. balsamifera. These results suggest that processes influencing foliar δ15N and Δ13C in P. balsamifera are partially normalized across its climatic range by the habitat it occupies or by the physiology of the species itself.
Highlights
Spatial variation in foliar chemical traits represent phenotypic responses to environmental and genotypic factors
In a model of foliar δ15N with mean annual temperature (MAT), log(MAP), and log[N] as model effects, the addition of P. balsamifera observations (n = 755; enlarging the global data set by 7%) exhibited little influence on model estimates compared with using the original global data set (n = 9828) (Table 1)
Foliar δ15N in P. balsamifera generally overlapped with foliar δ15N measurements in the global database, though within the restricted range of MAT and mean annual precipitation (MAP) exhibited by the P. balsamifera samples (Fig. 2)
Summary
Spatial variation in foliar chemical traits represent phenotypic responses to environmental and genotypic factors. The existence of broad relationships between foliar isotopic composition and environmental factors suggest that these foliar chemical traits index the availability of nitrogen and water to plants[22]. C3 plants vary in foliar Δ13C due to variation in carboxylation efficiency and stomatal response to environmental conditions[22] It is an outstanding question if these sources of variability among species strengthen or weaken the global to continental scale variability in foliar δ15N and Δ13C imposed by climate. We measure 755 specimens of the widely-distributed tree species Populus balsamifera for foliar [N], δ15N and Δ13C Using these data, our objective was to explore the extent to which large gradients in climate-associated variation in foliar δ15N and Δ13C observed globally e.g., refs 10, 28, are represented in measurements of a single species. To evaluate the extent to which patterns in P. balsamifera chemical traits correspond to patterns in plant functioning, we subsequently used structural equation modeling to compare the effects of climate variables and foliar [N] on Δ13C
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