Controls on water use efficiency in mesic forests across North America: Insight from foliar δ13C, δ18O and N

Background/Question/Methods

Water use efficiency (WUE), defined as the ratio of carbon assimilation (A) to water loss via transpiration, is the key physiological parameter that explicitly links water and carbon (C) cycling in forest ecosystems. Most studies examining the influence of climatic factors on forest-WUE have focused on water-limited Mediterranean ecosystems. Much less is known about the dynamics of WUE in mesic forests, where factors that regulate C uptake (e.g. light, energy, and nutrient availability) likely exert a stronger control on WUE than water availability. Here we explored the variation in the intrinsic WUE (iWUE, i.e., the ratio between A, and stomatal conductance, g_s) for two dominant tree species at five different forested Ameriflux sites across eastern North America, along a temperature (T) gradient, ranging from 6 to 20°C, with mean annual precipitation being similar at all sites. iWUE was derived from foliar δ¹³C while foliar Δ¹⁸O was used to examine the qualitative contribution of A and/or g_s to iWUE across sites and species. We hypothesized that iWUE would be driven primarily by changes in A rather than g_s at these mesic sites, with leaf nitrogen (N) and plant functional type (conifers vs. angiosperms) explaining species-specific responses. 

Results/Conclusions

iWUE and c_i/c_a ratio did not vary significantly across the large latitudinal gradient, irrespective of the forest ecosystem types. Surprisingly, the difference in air T across sites did not clearly affect Δ¹⁸O either, suggesting that other factors, e.g., VPD, soil moisture and species-specific water uptake strategies might contribute to explaining foliar Δ¹⁸O dynamics. Contrary to the expected pattern, we observed a negative relationship between iWUE and leaf N for both conifers and angiosperms across sites. A possible explanation could be that A is stomatally rather than N limited i.e., reduction in g_s limits CO₂ uptake and therefore N use in assimilating more C, leading to a higher c_i/c_a ratio and a lower iWUE.  However, iWUE and c_i/c_a ratio were in general poorly related to Δ¹⁸O, suggesting that A might be constrained by other factors, e.g., nutrient availability (e.g., P), sun light quality, rather than by g_s. Ongoing analysis of stable isotopes continues to provide important insight regarding species-specific ecophysiological strategies affecting forest ecosystem functioning. Future work includes i) using isotope data through data assimilation to improve GPP and WUE predicted by the PnET model and ii) exploring relationships between leaf δ¹³C and N and remotely sensed-derived NDVI across the latitudinal gradient.