Abstract
Abstract. Drought periods can have important impacts on plant productivity and ecosystem functioning, but climatic conditions other than the lack of precipitation during droughts have never been quantified and have therefore not been considered explicitly in both experimental and modeling studies. Here, we identify which climatic characteristics deviate from normal during droughts and how these deviations could affect plant responses. Analysis of 609 years of daily data from nine Western European meteorological stations reveals that droughts in the studied region are consistently associated with more sunshine (+45 %), increased mean (+1.6 °C) and maximum (+2.8 °C) air temperatures and vapour pressure deficits that were 51 % higher than under normal conditions. These deviations from normal increase significantly as droughts progress. Using the process-model ORCHIDEE, we simulated droughts consistent with the results of the dataset analysis and compared water and carbon exchange of three different vegetation types during such natural droughts and droughts in which only the precipitation was affected. The comparison revealed contrasting responses: carbon loss was higher under natural drought in grasslands, while increased carbon uptake was found especially in decidious forests. This difference was attributed to better access to water reserves in forest ecosystems which prevented drought stress. This demonstrates that the warmer and sunnier conditions naturally associated with droughts can either improve growth or aggravate drought-related stress, depending on water reserves. As the impacts of including or excluding climatic parameters that correlate with drought are substantial, we propose that both experimental and modeling efforts should take into account other environmental factors than merely precipitation.
Highlights
Discrete climate events such as heat waves, droughts and storms, can have a disproportionate impact on ecosystems relative to the temporal scale over which they occur
There was an interaction with month (p < 0.001), caused by a slight positive These records contain daily minimum (Tmin) during summer (Fig. 1a)
Relative humidity was reduced by on average 12 % during droughts (p < 0.001, Fig. 1c), with the strongest decreases observed during late spring and early summer
Summary
Discrete climate events such as heat waves, droughts and storms, can have a disproportionate impact on ecosystems relative to the temporal scale over which they occur. A recent example is the European summer heat wave of 2003, which saw harvest losses surpass 10 billion dollars (Schar and Jendritzky, 2004), and which, together with the high number of premature human deaths (Vandentorren et al, 2004) served as a catalyst for increased public, political and scientific awareness (IPCC, 2007). Both the frequency and the intensity of extreme events is set to increase disproportionately under climate change (Meehl and Tebaldi, 2004), a direct consequence of the nature of probabilistic distributions (Schar et al, 2004). Droughts are reported to be increasing in frequency and intensity (Briffa et al, 2009), and are thought to have generally larger impacts on primary productivity than heat
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