Evapotranspiration and water use efficiency in relation to climate and canopy nitrogen in U.S. forests

Abstract

Understanding relations among forest carbon (C) uptake and water use is critical for predicting forest‐climate interactions. Although the basic properties of tree‐water relations have long been known, our understanding of broader‐scale patterns is limited by several factors including (1) incomplete understanding of drivers of change in coupled C and water fluxes and water use efficiency (WUE), (2) difficulty in reconciling WUE estimates obtained at different scales, and (3) uncertainty in how evapotranspiration (ET) and WUE vary with other important resources such as nitrogen (N). To address these issues, we examined ET, gross primary production (GPP), and WUE at 11 AmeriFlux sites across North America. Our analysis spanned leaf and ecosystem scales and included foliar δ13C, δ18O, and %N measurements; eddy covariance estimates of GPP and ET; and remotely sensed estimates of canopy %N. We used flux data to derive ecosystem WUE (WUEe) and foliar δ13C to infer intrinsic WUE. We found that GPP, ET, and WUEe scaled with canopy %N, even when environmental variables were considered, and discuss the implications of these relationships for forest‐atmosphere‐climate interactions. We observed opposing patterns of WUE at leaf and ecosystem scales and examined uncertainties to help explain these opposing patterns. Nevertheless, significant relationship between C isotope‐derived ci/ca and GPP indicates that δ13C can be an effective predictor of forest GPP. Finally, we show that incorporating species functional traits—wood anatomy, hydraulic strategy, and foliar %N—into a conceptual model improved the interpretation of Δ13C and δ18O vis‐à‐vis leaf to canopy water‐carbon fluxes.

Decadal-Scale Reduction in Forest Net Ecosystem Production Following Insect Defoliation Contrasts with Short-Term Impacts of Prescribed Fires

“…During the growing seasons following insect defoliation, little difference in foliar N levels or assimilation rates of canopy oak foliage occurred, but overall canopy N content was lower following oak mortality compared to the pre-disturbance period [61,63]. However, foliar N content of sub-canopy and understory species increased following defoliation, and thus little net change in the total amount of N in foliage occurred and ecosystem WUE was not significantly different pre-and post-disturbance [37,57,63]. Increased foliar N levels and enhanced maximum assimilation rates in the remaining pine needles were observed following relatively intense prescribed burns in the PNR, but not during low-intensity burns [25,64].…”

“…At the oak stand, complete defoliation of the canopy and understory transferred 6.5 g N m −2 in frass and green leaf fragments to the forest floor by mid-summer, approximately twice the amount of N that would normally occur in litterfall during the fall months [36,37]. Canopy foliar N concentrations and total N content were lower in the second flush of foliage following defoliation, and ecosystem WUE was reduced by approximately 42% during the growing season compared to pre-disturbance years [37,57]. During the growing seasons following insect defoliation, little difference in foliar N levels or assimilation rates of canopy oak foliage occurred, but overall canopy N content was lower following oak mortality compared to the pre-disturbance period [61,63].…”

“…Following the cessation of herbivory or other disturbances that damage foliage early in the growing season (including experimental defoliation), canopy and understory oak species typically produce a second flush of leaves driven by the reallocation of nonstructural stored carbon (NSC) [36,37,54,55]. In late spring in 2007 at the oak stand, the second leaf flush following complete defoliation totaled approximately half of spring leaf production and had a lower mean nitrogen content than pre-or post-disturbance periods (1.7% vs. 1.9% nitrogen (N) in canopy foliage) [36,37,54,57]. Both pitch and shortleaf pines readily produce new shoots and needles from epicormic meristems following insect defoliation or fires [25,58].…”

Systematic variation in evapotranspiration trends and drivers across the Northeastern United States

“…Indeed, at the regional scale, the lack of a consistent regional decline in AET (Figure ) implies that the direct CO 2 effect is small relative to changes driven by climate, such as temperature and precipitation, unexamined climatic drivers such as radiation (Dai et al, ; Wild, ) and wind (McVicar et al, ; Pryor et al, ), and the negative feedback between AET and VPD (Huntington, ). Furthermore, physiological relationships at the leaf level often do not scale linearly to the canopy or regional level (Guerrieri, Lepine, Asbjornsen, Xiao, & Ollinger, ; Wullschleger, Gunderson, Hanson, Wilson, & Norby, ). To the extent that WUE is increasing, we would expect it to offset the increases in ET that are hypothesized in a warmer climate with a longer leaf‐on season, particularly in watersheds dominated by deciduous forests.…”

“…wind (McVicar et al, 2012;Pryor et al, 2009), and the negative feedback between AET and VPD (Huntington, 2008). Furthermore, physiological relationships at the leaf level often do not scale linearly to the canopy or regional level (Guerrieri, Lepine, Asbjornsen, Xiao, & Ollinger, 2016;Wullschleger, Gunderson, Hanson, Wilson, & Norby, 2002). To the extent that WUE is increasing, we would expect it to offset the increases in ET that are hypothesized in a warmer climate with a longer leaf-on season, particularly in watersheds dominated by deciduous forests.…”