Water-stress-induced breakdown of carbon–water relations: indicators from diurnal FLUXNET patterns

Journal Article · 20 April 2018 · Biogeosciences (Online)

DOI: https://doi.org/10.5194/bg-15-2433-2018 · OSTI ID:1503311

Nelson, Jacob A. ^[1]; Carvalhais, Nuno ^[2]; Migliavacca, Mirco ^[1]; Reichstein, Markus ^[3]; Jung, Martin ^[1]

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1. Max Planck Society, Jena (Germany). Max Planck Inst. for Biogeochemistry
2. Max Planck Society, Jena (Germany). Max Planck Inst. for Biogeochemistry; Univ. of Lisbon, Lisbon (Portugal)
3. Max Planck Society, Jena (Germany). Max Planck Inst. for Biogeochemistry; Michael Stifel Center Jena for Data-Driven and Simulation Science, Jena (Germany)

Understanding of terrestrial carbon and water cycles is currently hampered by an uncertainty in how to capture the large variety of plant responses to drought. In FLUXNET, the global network of CO₂ and H₂O flux observations, many sites do not uniformly report the ancillary variables needed to study drought response physiology. To this end, we outline two data-driven indicators based on diurnal energy, water, and carbon flux patterns derived directly from the eddy covariance data and based on theorized physiological responses to hydraulic and non-stomatal limitations.Hydraulic limitations (i.e. intra-plant limitations on water movement) are proxied using the relative diurnal centroid ( $C_{\mathrm{ET}}^{*}$), whichmeasures the degree to which the flux of evapotranspiration (ET) is shiftedtoward the morning. Non-stomatal limitations (e.g. inhibitions of biochemical reactions, RuBisCO activity, and/or mesophyll conductance) are characterized by the Diurnal Water–Carbon Index (DWCI), which measures the degree of coupling between ET and gross primary productivity (GPP) within each day.As a proof of concept we show the response of the metrics at six European sites during the 2003 heat wave event, showing a varied response of morning shifts and decoupling. Globally, we found indications of hydraulic limitations in the form of significantly high frequencies of morning-shifted days in dry/Mediterranean climates and savanna/evergreen plant functional types (PFTs), whereas high frequencies of decoupling were dominated by dry climates and grassland/savanna PFTs indicating a prevalence of non-stomatal limitations in these ecosystems. Wholly, both the diurnal centroid and DWCI were associated with high net radiation and low latent energy typical of drought. Using three water use efficiency (WUE) models, we found the mean differences between expected and observed WUE to be -0.09 to0.44µmolmmol^-1 and -0.29 to -0.40µmolmmol^-1 for decoupled and morning-shifted days, respectively, compared to mean differences -1.41 to -1.42µmol mmol^-1 in dry conditions, suggesting that morning shifts/hydraulic responses are associated with an increase in WUE,whereas decoupling/non-stomatal limitations are not.

• Research Organization: Oregon State Univ., Corvallis, OR (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Biological and Environmental Research (BER)
• Grant/Contract Number: FG02-04ER63911; FG02-04ER63917
• OSTI ID: 1503311
• Journal Information: Biogeosciences (Online), Vol. 15, Issue 8; ISSN 1726-4189
• Publisher: European Geosciences UnionCopyright Statement
• Country of Publication: United States
• Language: English