Abstract

Abstract. Extreme climatic events, such as droughts and heat stress, induce anomalies in ecosystem–atmosphere CO2 fluxes, such as gross primary production (GPP) and ecosystem respiration (Reco), 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 Reco within and between ecosystems remain poorly predicted. Here we aim to identify the major factors controlling the amplitude of extreme-event impacts on GPP, Reco, 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 CO2 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 they downregulated GPP, resulting in a moderate reduction in 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 Reco 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: for heat stress during droughts, the magnitude of impacts systematically increased with duration, whereas under heat stress without drought, the response of Reco over time turned from an initial increase to a downregulation after about 2 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 on the global scale. Specifically, we find that the different response functions of the two antipodal land–atmosphere fluxes GPP and Reco can 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.

Highlights

  • Extreme climatic events such as heat or drought are key features of Earth’s climatic variability (Ghil et al, 2011) and occur on a wide range of timescales (Huybers and Curry, 2006)

  • In this study we evaluated and corroborated the current understanding and hypotheses about the response of ecosystem CO2 fluxes to extreme climatic events

  • Our approach first defines extreme values in the climate data to detect extreme events of varying length and calculates the difference between CO2 fluxes during these events compared to nonextreme reference periods

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Summary

Introduction

Extreme climatic events such as heat or drought are key features of Earth’s climatic variability (Ghil et al, 2011) and occur on a wide range of timescales (Huybers and Curry, 2006). Extreme climatic events directly propagate into the terrestrial biosphere, affecting ecosystem functioning (Reichstein et al, 2013; Frank et al, 2015) and land surface properties (e.g., soil moisture), which in turn triggers ecosystem– atmosphere feedback loops (e.g., Seneviratne et al, 2010; Green et al, 2017). Drought in conjunction with severe heat reversed several years of ecosystem carbon sequestration in Europe in 2003 (Ciais et al, 2005), and strong land–atmosphere feedbacks exacerbated the event while it was occurring (Fischer et al, 2007). Ecosystem impacts of extreme climatic events are often nonlinear and interact with concurrent climatic conditions. Warm spring conditions and corresponding earlier vegetation activity likely contributed to exacerbating drought impacts through reduced initial soil moisture at the onset of summer drought (Wolf et al, 2016)

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