Increased water-use efficiency and reduced CO2 uptake by plants during droughts at a continental scale

Abstract

Severe droughts in the Northern Hemisphere cause widespread decline of agricultural yield, reduction of forest carbon uptake, and increased CO2 growth rates in the atmosphere. Plants respond to droughts by partially closing their stomata to limit their evaporative water loss, at the expense of carbon uptake by photosynthesis. This trade-off maximizes their water-use efficiency, as measured for many individual plants under laboratory conditions and field experiments. Here we analyze the 13C/12C stable isotope ratio in atmospheric CO2 (reported as δ13C) to provide new observational evidence of the impact of droughts on the water-use efficiency across areas of millions of km2 and spanning one decade of recent climate variability. We find strong and spatially coherent increases in water-use efficiency along with widespread reductions of net carbon uptake over the Northern Hemisphere during severe droughts that affected Europe, Russia, and the United States in 2001-2011. The impact of those droughts on water-use efficiency and carbon uptake by vegetation is substantially larger than simulated by the land-surface schemes of six state-of-the-art climate models. This suggests that drought induced carbon-climate feedbacks may be too small in these models and improvements to their vegetation dynamics using stable isotope observations can help to improve their drought response.

Response of ecosystem intrinsic water use efficiency and gross primary productivity to rising vapor pressure deficit

“…Finally, our results show that increased iWUE driven by higher VPD usually corresponds with a reduced GPP (figures 1(e)-(h)), different from increased iWUE under elevated CO 2 that results from increased GPP. Thus, an increase in iWUE does not necessarily translate into increased carbon uptake as simulated by land surface models (Peters et al 2018) or increased tree growth as reported by tree ring data (Lévesque et al 2014, van der Sleen et al 2015, or in other words, whether higher iWUE corresponds to higher GPP depends on the driver of iWUE changes.…”

“…VPD-effects on iWUE could also partially explain the increasing decoupling of growth and iWUE as observed from tree rings (Andrer-Hayles et al 2011, Peñuelas et al 2011). A VPD-driven increase in iWUE should not increase tree growth, as overall it will likely be accompanied by lower GPP (Peters et al 2018). Finally, our results are relevant for drought monitoring and prediction indices that incorporate information about the difference between potential and actual evapotranspiration (Narasimhan andSrinivasan 2005, Otkin et al 2013), which is largely driven by stomatal conductance during dry periods (Novick et al 2016a).…”

Long‐term physiological and growth responses of Himalayan fir to environmental change are mediated by mean climate

“…The expected effect of elevated [CO 2 ] on tree photosynthesis and carbon availability (Huang, Bergeron, Denneler, Berninger, & Tardif, ) may differ across environmental conditions and warming (Lévesque, Siegwolf, Saurer, Eilmann, & Rigling, ; Lindner et al, ; Sarris, Siegwolf, & Kӧrner, ). For instance, a substantial number of studies reported declining long‐term tree growth and productivity (Barber, Juday, & Finney, ; Giguère‐Croteau et al, ; Girardin, Bouriaud, et al, ; Girardin, Hogg, et al, ; Lévesque et al, ; Nock et al, ; Silva & Anand, ) due to regional drought or warming‐induced drought (Liang, Leuschner, Dulamsuren, Wagner, & Hauck, ; Linares & Camarero, ; Peñuelas, Canadell, & Ogaya, ; Peters et al, ; Rahman, Islam, Gebrekirstos, & Bräuning, ). For montane and boreal forests, no growth enhancement related to elevated CO 2 was found (Dawes et al, ; Hättenschwiler, Miglietta, Raschi, & Korner, ; Klein et al, ; Sigurdsson, Medhurst, Wallin, Eggertsson, & Linder, ).…”

“…For instance, a substantial number of studies reported declining long-term tree growth and productivity (Barber, Juday, & Finney, 2000;Giguère-Croteau et al, 2019;Girardin, Bouriaud, et al, 2016;Lévesque et al, 2014;Nock et al, 2011;Silva & Anand, 2013) due to regional drought or warming-induced drought (Liang, Leuschner, Dulamsuren, Wagner, & Hauck, 2016;Linares & Camarero, 2012;Peñuelas, Canadell, & Ogaya, 2011;Peters et al, 2018;Rahman, Islam, Gebrekirstos, & Bräuning, 2019).…”

Information Needs of Next-Generation Forest Carbon Models: Opportunities for Remote Sensing Science

“…Respiration is not as well defined with model choice dependent on the spatiotemporal scale under scrutiny [32][33][34]; and 3. Latent heat and water interact at the leaf level via evapotranspiration and hydrological processes in forests [35,36], with research still advancing our knowledge of this complex interaction [37,38].…”