Discussion group 4
Uncertainties, thresholds, time lags, and climate and ecological "surprises".

Chair: Mike Loik + Wolfgang Cramer

What are the major uncertainties in understanding and quantifying the effects, are effects governed by general gradual changes or by exceedance of thresholds, do thresholds determine resilience and reversibility, is it the amplitude of single events or the number of repetitions important (is one severe drought better or worse than 10 smaller), what does it take to recover, what do the models say and what type of studies do we need.

Questions proposed

David Briske: Role of competitive interactions in mediating ecosystem responses to climate change and response of plant traits/functional types to climate change

Norbert Lamersdorf: New challenge for forest ecosystems/trees: Recent de-acidification, still high N-input, enhanced CO₂ - may lead to enhanced growth rates but may also increase future susceptibility of trees to droughts

Franco Miglietta: GPP and Transpiration coupling: do we really understand the process? - In temperate climates, high air temperature events are often accompanied by reduced precipitation leading to drought in forests and agricultural ecosystems. Water stress can reduce photosynthesis under those circumstances through stomatal limitations. Thus, a close coupling between photosynthesis and transpiration or a constant ecosystem water use efficiency (WUE, the ratio between primary productivity and transpiration) can be maintained even under stressful conditions. But heat stress can also directly cause photosynthetic inhibition in the leaves through a series of biochemical mechanisms. This has been neglected when considering environmental stress effects at the landscape scale. In climatic zones, where the vegetation is not specifically acclimated to heat stress, short duration high temperature episodes can actually cause a substantial decrease in carbon uptake of terrestrial ecosystems, even in the absence of drought.

José M. Moreno: Interactions among disturbances (i.e, fire) and post-disturbance precipitation patterns in determining plant composition/structure

Bridget Emmett: Interactions with other environmental changes
Filip Moldan: Potential of time for space substitution in assessing the effects of increased/decreased precipitation

John Grace: What are the critical experiments to be done in order to settle the uncertainties?

Markus Reichstein: Detecting current changes in water balance at larges scales (remote sensing etc)

Andreas Ibrom: What are requirements for models that can generate hypotheses on “surprising” ecosystem responses to, e.g., changes in precipitation climate? Do such models exist, anyway?

Discussion group 1
Contrasting ecosystem responses to precipitation change at different climatic conditions (wet/dry & cold/warm).

Discussion group 2
Major factors regulating/controlling the effects of precipitation change on ecosystems: Our current state of understanding.

Discussion group 3
Contrasting effects of changes in precipitation amounts and distribution patterns.