Continent-Wide Decrease of Stomatal Conductance in Vegetation During Large Droughts of the Recent Decade

Abstract:

Severe droughts in the Northern Hemisphere caused widespread decline of agricultural yield, reduction of forest carbon uptake, and increased CO₂ growth rates in the atmosphere during the past decade. Plants respond to droughts by partially closing their stomata to limit their evaporative water loss, at the expense of carbon uptake by photosynthesis. Here we present new evidence on this drought response of terrestrial vegetation derived from year-to-year changes in the ¹³C/¹²C stable isotope ratio in atmospheric CO₂. Observations from more than 50,000 flask samples from the NOAA Global Greenhouse Gas Reference Network suggest a strong decrease in stomatal conductance in vegetation that is highly correlated (see green line in the figure) with reductions of net carbon uptake over the Northern Hemisphere. This correlation is driven by severe drought conditions over areas several million km² in size in Europe (2003, 2006), Russia (2010), and the United States (2002). This spatially integrated vegetation drought response at this scale can not be measured from laboratory experiments or field studies and the atmosphere thus offers a unique perspective on large-scale vegetation drought dynamics. A widely used stomatal conductance parameterization used in our study as well as many current climate models underestimate this observed decrease in carbon and water exchange during droughts (black and blue lines in the figure). The global δ¹³C record could provide a new opportunity to improve interannual drought dynamics in coupled vegetation-atmosphere models for CO₂.