Abstract
Remote sensing of sun-induced chlorophyll fluorescence (SIF) has been suggested as a promising approach for probing changes in global terrestrial gross primary productivity (GPP). To date, however, most studies were conducted in situations when/where changes in both SIF and GPP were driven by large changes in the absorbed photosynthetically active radiation (APAR) and phenology. Here we quantified SIF and GPP during a short-term intense heat wave at a Mediterranean pine forest, during which changes in APAR were negligible. GPP decreased linearly during the course of the heat wave, while SIF declined slightly initially and then dropped dramatically during the peak of the heat wave, temporally coinciding with a biochemical impairment of photosynthesis inferred from the increase in the uptake ratio of carbonyl sulfide to carbon dioxide. SIF thus accounted for less than 35% of the variability in GPP and, even though it responded to the impairment of photosynthesis, appears to offer limited potential for quantitatively monitoring GPP during heat waves in the absence of large changes in APAR.
Highlights
Over the past decade, land ecosystems have removed around one quarter of the carbon emitted by human activities annually, another quarter being removed by the oceans[1]
Even though the relationship between chlorophyll fluorescence and leaf photosynthesis is not unique[14], tower- and satellite-based sun-induced chlorophyll fluorescence (SIF) measurements have been shown to scale with gross primary productivity (GPP) as it changes during the season and/or with eco-climatological factors that govern the distribution of global biomes[15,17,18]
The explanatory power of these SIF-GPP relationships derives from the underlying significant changes in absorbed photosynthetically active radiation (APAR) across season and latitude[19], which together with the light-use efficiency (LUE), determines GPP29
Summary
Land ecosystems have removed around one quarter of the carbon emitted by human activities annually, another quarter being removed by the oceans[1]. Whether land ecosystems will continue to significantly remove CO2 from the atmosphere or whether human emissions will eventually outpace sinks, is highly uncertain[3,4], as different carbon cycling models produce widely differing future source/sink estimates[5] This uncertainty has important practical consequences as the warming relative to pre-industrial times is approximately linearly dependent on cumulative CO2 emissions, leaving a finite amount of allowable CO2 emissions in order to constrain warming below some threshold[6,7]. The magnitude of the required reduction measures critically depends on the strength of the terrestrial and oceanic sinks, which are projected to decline with realised reductions in atmospheric CO2 concentrations[8] To this end, a robust monitoring system is required which allows quantifying the CO2 uptake by land ecosystems at global scale with high spatial and temporal resolution and in response to climate variability and extremes[9] and helps reduce uncertainties in Earth system models[10]. SIF may decrease in concert with GPP as excess energy is increasingly dissipated via non-photochemical quenching (NPQ), or may increase if NPQ mechanisms become ineffective www.nature.com/scientificreports/
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