Laboratory for
Climatology and Remote Sensing

Abstract:

Grünländer der gemäßigten Breiten liefern wichtige Ökosystemdienstleistungen die von der Biomasseproduktion abhängen. Letztere nimmt mit geringeren Niederschlägen und höheren Niederschlagsvariabilitäten ab, während höhere Lufttemperaturen fördernd wirken können. Zusätzlich sollen höhere atmosphärische CO2-Konzentrationen ([CO2]s) die Biomasse durch direkte Stimulation der Photosynthese und Wassereinsparung (CO2-Düngeeffekt) erhöhen. Somit ist die Biomasseproduktivität durch teils gegenläufige Auswirkungen wechselnder klimatischer Bedingungen und [CO2] gesteuert. Die Studie untersucht diese Einflüsse des globalen Wandels auf Grundlage eines Freiluftexperiments zur CO2-Anreicherung (~20% über Umgebungs-[CO2]; 18 J.) um die oberirdische Sommer-Biomasse (AGB) Mitte des 21. Jhdt. statistisch vorherzusagen. Ein informationstheoretisches Screening lieferte die wichtigsten Prädiktoren, basierend auf Lufttemperatur- und Niederschlagsmessungen. Die AGB-Produktion wurde für verschiedene Klimaregime, abgeleitet aus den Beobachtungen des Versuchszeitraums, geschätzt. Wir fanden, dass die zukünftige AGB-Produktion hauptsächlich von der Niederschlagsmenge abhängt, gefolgt von Lufttemperatur und Niederschlagsvariabilität. Variablere Niederschläge reduzierten die AGB und umgekehrt. Die AGB-Produktion unter trockenen Bedingungen verringerte sich mit steigenden Lufttemperaturen weiter. Im Kontrast zu weithin erwarteter Ertragssteigerungen durch erhöhte [CO2]s, führen solche Bedingungen zu AGB-Vorhersagen unter denen der aktuellen AGBs. Da Klimamodelle für Sommer in Mitteleuropa steigende Lufttemperaturen und abnehmende Niederschlagsmengen mit zunehmender Variabilität projizieren, deuten unsere Ergebnisse trotz steigender [CO2]s auf eine reduzierte zukünftige Grünland-Sommer-AGB hin.

Abstract:

The increase of atmospheric greenhouse gas concentrations resulting from anthropogenic activities is the major driver of global climate change. A net sink effect caused by stimulated plant photosynthesis through rising atmospheric carbon dioxide concentrations ([CO2]) and, consequently, an increased net primary productivity (NPP) of C3 plants (termed the CO2 fertilization effect, CFE) is widely assumed. However, knowledge about the magnitude and persistence of the CFE under future climates including more frequent weather extremes is still lacking. Therefore, we have investigated the CFE in dependence to its accompanying environmental conditions in the long-term time series (16 years) of a temperate grassland grown under “Free Air Carbon dioxide Enrichment” in Gießen, the GiFACE. Our results reveal that the CFE is strongest under local average environmental conditions and reduced or disappeared under wetter, drier and/or hotter conditions. This is in contrast to predictions of an increased CO2 fertilization under drier and warmer conditions. The results are of utmost importance because respective weather and climate conditions are projected in climate scenarios. Consequently, the state-of-art biogeochemical models might overestimate the future NPP sink capacity of temperate C3 grasslands. Since temperate grasslands represent an important portion of the Earth’s terrestrial surface, and therefore the global carbon cycle, atmospheric [CO2] might increase faster than currently expected.

Abstract:

The increase in atmospheric greenhouse gas concentrations from anthropogenic activities is the major driver of recent global climate change1. The stimulation of plant photosynthesis due to rising atmospheric carbon dioxide concentrations ([CO2]) is widely assumed to increase the net primary productivity (NPP) of C3 plants—the CO2 fertilization effect (CFE). However, the magnitude and persistence of the CFE under future climates, including more frequent weather extremes, are controversial. Here we use data from 16 years of temperate grassland grown under ‘free-air carbon dioxide enrichment’ conditions to show that the CFE on above-ground biomass is strongest under local average environmental conditions. The observed CFE was reduced or disappeared under wetter, drier and/or hotter conditions when the forcing variable exceeded its intermediate regime. This is in contrast to predictions of an increased CO2 fertilization effect under drier and warmer conditions. Such extreme weather conditions are projected to occur more intensely and frequently under future climate scenarios. Consequently, current biogeochemical models might overestimate the future NPP sink capacity of temperate C3 grasslands and hence underestimate future atmospheric [CO2] increase.

Abstract:

The increase in atmospheric greenhouse gas concentrations from anthropogenic activities is the major driver of global climate change. The rising atmospheric carbon dioxide (CO2 ) concentrations may stimulate plant photosynthesis and, thus, cause a net sink effect in the global carbon cycle. As a consequence of an enhanced photosynthesis, an increase in the net primary productivity (NPP) of C3 plants (termed CO2 fertilization) is widely assumed. This process is associated with a reduced stomatal conductance of leaves as the carbon demand of photosynthesis is met earlier. This causes a higher water-use efficiency and, hence, may reduce water stress in plants exposed to elevated CO2 concentrations ([eCO2 ]). However, the magnitude and persistence of the CO2 fertilization effect under a future climate including more frequent weather extremes are controversial. To test the CO2 fertilization effect for Central European grasslands, a data set comprising 16 years of biomass samples and environmental variables such as local weather and soil conditions was analysed by means of a novel approach. The data set was recorded on a “Free Air Carbon dioxide Enrichment” (FACE) experimental site which allows to quantify the CO2 fertilization effect under naturally occurring climate variations. The results indicate that the CO2 fertilization effect on the aboveground biomass is strongest under local average environmental conditions. Such intermediate regimes were defined by the mean +/- 1 standard deviation of the long-term average in the respective variable three months before harvest. The observed CO2 fertilization effect was reduced or vanished under drier, wetter and hotter conditions when the respective variable exceeded the bounds of the intermediate regimes. Comparable conditions, characterized by a higher frequency of more extreme weather conditions, are predicted for the future by climate projections. Consequently, biogeochemical models may overestimate the future NPP sink capacity of temperate C3 grasslands. Because temperate grasslands represent an important part of the Earth’s terrestrial surface and therefore the global carbon cycle, atmospheric CO2 concentrations [CO2 ] might increase faster than currently expected.