COS 14-4
Controls on water use efficiency in mesic forests across North America: Insight from foliar δ13C, δ18O and N

Monday, August 11, 2014: 2:30 PM
Bataglieri, Sheraton Hotel
Rossella Guerrieri, Earth Systems Research Center, University of New Hampshire, Durham, NH
Lucie Lepine, Earth Systems Research Center, University of New Hampshire, Durham, NH
Zaixing Zhou, Earth Systems Research Center, University of New Hampshire, Durham, NH
Andrew P. Ouimette, Earth Systems Research Center, University of New Hampshire, Durham, NH
Heidi Asbjornsen, Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH
Scott Ollinger, Earth Systems Research Center, University of New Hampshire, Durham, NH
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, gs) 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 gs to iWUE across sites and species. We hypothesized that iWUE would be driven primarily by changes in A rather than gat these mesic sites, with leaf nitrogen (N) and plant functional type (conifers vs. angiosperms) explaining species-specific responses. 

Results/Conclusions

iWUE and ci/ca 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 gs limits CO2 uptake and therefore N use in assimilating more C, leading to a higher ci/ca ratio and a lower iWUE.  However, iWUE and ci/ca 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 gs. 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.