Drought sensitivity of gross primary productivity in willow: effects from physiological versus structural responses

The increasing frequency and intensity of drought events make them a growing threat for plants that are sensitive to water scarcity. It is therefore important to understand how plants react to drought stress. The willow trees from the short-rotation coppices (SRC) on the DTU-Risø Campus in Denmark (DK-RCW) are particularly sensitive to water shortage as they are rainfed. We address the following question: how do extremely dry conditions affect the willows growth? We study the plants response mechanisms to periods of water scarcity and examine how these responses impact their gross primary productivity (GPP). There is a particular relevance to this in the current context of global warming, where the SRC are used to produce bioenergy and can store carbon to mitigate climate change.

Field measurements were carried out at the DK-RCW site to gather information on canopy structure (leaf area index). These results were integrated into a modelled relationship with carbon flux data from an eddy covariance flux tower located onsite and providing continuous CO₂ and H₂O flux data in more than 10 years. The simple empirical model was used to contrast the GPP’s sensitivity to stomatal and non-stomatal processes by comparison of extreme drought conditions (summer 2018 in Denmark) and wetter conditions (summers 2015 and 2021). These years represent the same stage of the rotational cycle.

This new model enables us to highlight two complementary responses to drought: the trees immediately react by adapting their physiology (stomatal resistance, increased sensitivity to vapour pressure deficit under drought), but also by changing the canopy structure as the drought increases (reduction of the leaf area index) and other responses on canopy photosynthetic capacity. High vapour pressure deficit and the reduction of the leaf area index both reduced the photosynthesis of willow trees under dry conditions. The simulated data imply limited drought recovery after the dry period had ended. For these reasons, the carbon uptake by the willow SRC is lower during droughts and thus limits the SRC productivity and carbon sink strength. We conclude from the very clear results from this case study that different drought response mechanisms must be considered when trying to understand and predict plant responses to extreme drought.