Colloquium: Friday, 4 April, 2:10 pm, EN 6085

Detection of terrestrial ecosystem responses to climate variability and extremes via integration of multiple-scale observations and modeling

Climate-carbon-cycle feedbacks have most often been discussed in the context of gradual changes of temperature (’global warming’), but the role of climate variability and extremes has received less attention. However, during the last decades our ability to monitor terrestrial ecosystem responses to climate variability and extremes has greatly improved. This statement will be illustrated in this presentation with one European and one Northern Hemispheric case study, both indicating that simplified reasoning concerning stimulation of carbon sequestration by enhanced productivity through moderate warming may be misleading. In both case studies, a strong integration of ecosystem observation with the eddy covariance technique, remote sensing, and atmospheric observations with process- and data-oriented modeling approaches is put forward.

The European case study analyzed the effects of the 2003 summer heatwave on ecosystem-atmosphere CO₂ and H₂O exchange and shows both strong direct but also lagged detrimental effects of this climate extreme on ecosystem productivity. Results indicate that the drop in productivity was not primarily caused by high temperatures (‘heat stress’) but rather by limitation of water (drought stress) and that, contrary to the classical expectation about a heat wave, not only gross primary productivity but also ecosystem respiration declined by up to more than 80 gC m^-2 month^-1. Through these effects, the carbon sink of at least five ‘normal’ years was undone, indicating the importance of both considering extreme events and carbon-water cycle interactions.

The other – Northern Hemispheric – case study looks at the effects of autumn warming on ecosystem carbon balance as opposed to spring warming which has been shown to enhance vegetation productivity and stressed that we have to consider above- and below-ground processes. We find that both photosynthesis and respiration increase during autumn warming, but the increase in respiration is greater, probably due to temperature lagging behind radiation. The integrated view of observations and model simulations suggests that northern terrestrial ecosystems may currently loose carbon dioxide in response to autumn warming, with a sensitivity of about 0.2 PgC per degree C, hence offsetting 90% of the increased carbon dioxide uptake during spring.