Integration of tree hydraulic processes and functional impairment to capture the drought resilience of a semiarid pine forest

Drought stress imposes multiple feedback responses in plants. These responses span from stomata closure and enzymatic downregulation of photosynthetic activity to structural adjustments of xylem biomass and leaf area. Some of these processes are not easily reversible and may persist long after the stress ended. Despite a multitude of hydraulic model approaches, simulation models still widely lack an integrative mechanistic description on how this sequence of tree physiological to structural responses may be realized, which is also simple enough to be generally applicable.
Here, we suggest an integrative, sequential approach to simulate drought stress responses. Firstly, a decreasing plant water potential triggers stomatal closure alongside a downregulation of photosynthetic performance and thus effectively slows down further desiccation. A second protective mechanism is introduced by increasing the soil-root resistance, represented by a disconnection of fine roots after a threshold soil water potential has been reached. Further decreases in plant water potential due to residual transpiration and loss of internal stem water storage are consistently leading to a loss in hydraulic functioning reflected in sapwood loss and foliage senescence. ... mehrThis new model functionality has been used to investigate responses of tree hydraulics, carbon uptake and transpiration to soil- and atmospheric drought in an extremely dry Aleppo pine (Pinus halepensis L.) plantation.
Using the hypothesis of a sequential triggering of stress-mitigating responses, the model was able to reflect carbon uptake and transpiration patterns under varying soil water supply and atmospheric demand - especially during summer - and responded realistically regarding medium-term responses such as leaf and sapwood senescence. We could show that the observed avoidance strategy was only achieved when the model accounted for a very early photosynthesis down-regulation, and the relatively high measured plant water potentials were well reproduced with a root-to-soil disconnection strategy that started before major xylem conductance losses occurred. Residual canopy conductance was found to be pivotal in explaining dehydration and transpiration patterns during summer but it also disclosed that explaining the water balance in the driest periods requires water supply from stem water and deep soil layers. In agreement with the high drought resistance observed at the site, our model indicated little loss of hydraulic functioning in Aleppo pine, despite the intensive seasonal summer drought.