Impacts of droughts and extreme temperature events on gross primary production and ecosystem respiration: a systematic assessment across ecosystems and climate zones

Abstract

Abstract.Extreme climatic events, such as droughts and heat stress induce anomalies in ecosystem-atmosphere CO 2 fluxes, such as gross primary production (GPP) and ecosystem respiration (R eco ), and, hence, can change the net ecosystem carbon balance.However, despite our increasing understanding of the underlying mechanisms, the magnitudes of the impacts of different types of extremes on GPP and R eco within and between ecosystems remain poorly predicted. Here we aim to identify the major 5 factors controlling the amplitude of extreme event impacts on GPP, R eco , and the resulting net ecosystem production (NEP).We focus on the impacts of heat and drought and their combination. We identified hydrometeorological extreme events in consistently downscaled water availability and temperature measurements over a 30 year time period. We then used FLUXNET eddy-covariance flux measurements to estimate the CO 2 flux anomalies during these extreme events across dominant vegetation types and climate zones. Overall, our results indicate that short-term heat extremes increased respiration more strongly than 10 they down-regulated GPP, resulting in a moderate reduction of the ecosystem's carbon sink potential. In the absence of heat stress, droughts tended to have smaller and similarly dampening effects on both GPP and R eco , and, hence, often resulted in neutral NEP responses. The combination of drought and heat typically led to a strong decrease in GPP, whereas heat and drought impacts on respiration partially offset each other. Taken together, compound heat and drought events led to the strongest C sink reduction compared to any single-factor extreme. A key insight of this paper, however, is that duration matters most: 15 for heat stress during droughts, the magnitude of impacts systematically increased with duration, whereas under heat stress without drought, the response of R eco over time turned from an initial increase to a down-regulation after about two weeks.This confirms earlier theories that not only the magnitude but also the duration of an extreme event determines its impact. Our study corroborates the results of several local site-level case studies, but as a novelty generalizes these findings at the global scale. Specifically, we find that the different response functions of the two antipodal land-atmosphere fluxes GPP and R eco can 20 also result in increasing NEP during certain extreme conditions. Apparently counterintuitive findings of this kind bear great potential for scrutinizing the mechanisms implemented in state-of-the-art terrestrial biosphere models and provide a benchmark for future model development and testing.

Development of Future Heatwaves for Different Hazard Thresholds

“…On the other hand, the duration of a heatwave can determine whether and when impacted systems can recover. For instance, longer‐lasting heatwaves can lead to stronger ecosystem impacts (von Buttlar et al, ). Furthermore, an increase in the duration of heatwaves can increase the mortality risk (Anderson & Bell, ).…”

Increased probability of compound long-duration dry and hot events in Europe during summer (1950–2013)

“…Considering such extremes as independent of one another can result in an underestimation in the probability of their cooccurrence , as well as in the risk of low crop yields , and the probability of wildfires (Gudmundsson et al 2014, Ruffault et al 2016. Additionally, both the duration and magnitude of dry and hot periods are required to explain the severity of ecosystem impacts (von Buttlar et al 2017). Underlying the significance of these studies are findings of increased concurrences of drought and heat waves within the US (Mazdiyasni and AghaKouchak 2015) and India (Sharma and Mujumdar 2017) along with projected increases in the likelihood of dry and hot summers globally .…”

Large‐Scale Droughts Responsible for Dramatic Reductions of Terrestrial Net Carbon Uptake Over North America in 2011 and 2012

“…Starting from July, GPP declined sharply (about 0.24 Pg C/year) owing to the drought, while Reco decreased less (about 0.08 Pg C/year) and later. This is a typical pattern during droughts (Schwalm et al, ; von Buttlar et al, ). The ensemble mean of FLUXCOM and TBMs indicates that the response of Reco to drought was lagged and lasted longer in comparison with that of GPP.…”