Impacts of droughts and extreme-temperature events on gross primary production and ecosystem respiration: a systematic assessment across ecosystems and climate zones
- 1Max Planck Institute for Biogeochemistry, Hans-Knöll-Straße 10, 07745 Jena, Germany
- 2Georg-August-Universität Göttingen, Wilhelmsplatz 1, 37073 Göttingen, Germany
- 3ETH Zürich, Rämistraße 101, 8092 Zürich, Switzerland
- 4TUM School of Life Sciences Weihenstephan, Technische Universität München, 85354 Freising, Germany
- 5Norwegian Institute of Bioeconomy Research, Høgskoleveien 8, 1431 Ås, Norway
- 6German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, Deutscher Platz 5e, 04103 Leipzig, Germany
- 7University of California Santa Barbara, Santa Barbara, CA 93106-3060, USA
- 8McMaster University, 1280 Main St W, Hamilton, ON L8S 4L8, Canada
- 9JRC, Institute for Environment and Sustainability, TP290 Via E. Fermi, 2749, 21027 Ispra, Italy
- 10Fondazione Edmund Mach di San Michele all'Adige, Via E. Mach, 1, 38010 S. Michele all'Adige, Italy
- 11University College Cork, College Road, Cork, T12 YN60, Ireland
- 12Oregon State University, Corvallis, OR 97331, USA
- 13CNR, Institute for Mediterranean Forest and Agricultural Systems, via Patacca 85, 80040 Ercolano (Napoli), Italy
- 14Université Laval, 2325, rue de l'Université, Québec, G1V 0A6, Canada
- 15Queen's University, 99 University Avenue, Kingston, Ontario, K7L 3N6, Canada
- 16Mazingira Centre, International Livestock Research Institute (ILRI), P.O. Box 30709, 00100 Nairobi, Kenya
- 17Free University of Bozen-Bolzan, Piazza Università 1, 39100 Bolzano (BZ), Italy
- 18Global Change Research Group, San Diego State University, San Diego, CA 92182, USA
- 19Department of Geography, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4RJ, UK
- 20Global Change Research Institute CAS, Bělidla 986/4a, 603 00 Brno, Czech Republic
- 21Department of Forest Ecology & Management, Swedish University of Agricultural Sciences, Skogsmarksgränd, 901 83 Umeå, Sweden
- 22Centre d'Ecologie Fonctionnelle et Evolutive CEFE, 1919, route de Mende, 34293 Montpellier 5, France
- 23Istituto di Biometeorologia – Sede di Firenze, Via Giovanni Caproni 8, 50145 Firenze, Italy
- 24Southwest Watershed Research Center, 2000 E. Allen Road, Tucson, AZ 85719, USA
- 25Australian National University, Acton ACT 2601, Canberra, Australia
- 26A.N. Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences, Leninsky pr., 33, Moscow, 119071, Russia
- 27University of Innsbruck, Sternwartestrasse 15, 6020 Innsbruck, Austria
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.