Laboratory for
Climatology and Remote Sensing

Abstract:

Temperate grasslands play globally an important role, for example, for biodiversity conservation, livestock forage production, and carbon storage. The latter two are primarily controlled by biomass production, which is assumed to decrease with lower amounts and higher variability of precipitation, while increasing air temperature might either foster or suppress biomass production. Additionally, a higher atmospheric CO2 concentration ([CO2]) is supposed to increase biomass productivity either by directly stimulating photosynthesis or indirectly by inducing water savings (CO2 fertilization effect). Consequently, future biomass productivity is controlled by the partially contrasting effects of changing climatic conditions and [CO2], which to date are only marginally understood. This results in high uncertainties of future biomass production and carbon storage estimates. Consequently, this study aims at statistically estimating mid-21st century grassland aboveground biomass (AGB) based on 18 years of data (1998–2015) from a free air carbon enrichment experiment. We found that lower precipitation totals and a higher precipitation variability reduced AGB. Under drier conditions accompanied by increasing air temperature, AGB further decreased. Here AGB under elevated [CO2] was partly even lower compared to AGB under ambient [CO2], probably because elevated [CO2] reduced evaporative cooling of plants, increasing heat stress. This indicates a higher susceptibility of AGB to increased air temperature under future atmospheric [CO2]. Since climate models for Central Europe project increasing air temperature and decreasing total summer precipitation associated with an increasing variability, our results suggest that grassland summer AGB will be reduced in the future, contradicting the widely expected positive yield anomalies from increasing [CO2].

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 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.