Stomatal responses of Eucalyptus spp. under drought can be predicted with a gain-risk optimization model.

Abstract

The frequency and severity of drought events are expected to increase due to climate change, with optimal environmental conditions for forestry likely to shift. Modeling plant responses to a changing climate is therefore vital. We tested the process-based gain-risk model to predict stomatal responses to drought of two Eucalyptus hybrids. The process-based gain-risk model has the advantage that all the parameters used within the model are based on measurable plant traits. The gain-risk model proposes that plants optimize photosynthetic gain while minimizing a hydraulic cost. Previous versions of the model used hydraulic risk as a cost function; however, they did not account for delayed or reduced hydraulic recovery rates from embolism post-drought. Hydraulic recovery has been seen in many species, however it is still unclear how this inclusion of a partial or delayed hydraulic recovery would affect the predictive power of the gain-risk model. Many hydraulic parameters required by the model are also difficult to measure and are not freely available. We therefore tested a simplified gain-risk model that includes a delayed or reduced hydraulic recovery component post-drought. The simplified gain-risk model performed well at predicting stomatal responses in both Eucalyptus grandis × camaldulensis (GC) and Eucalyptus urophylla × grandis (UG). In this study two distinct strategies were seen between GC and UG, with GC being more resistant to embolism formation, however it could not recover hydraulic conductance compared with UG. The inclusion of a delayed or reduced hydraulic recovery component slightly improved model predictions for GC, however not for UG, which can be related to UG being able to recover lost hydraulic conductance and therefore can maintain stomatal conductance regardless of hydraulic risk. Even though the gain-risk model shows promise in predicting plant responses, more information is needed regarding hydraulic recovery after drought.