Linking root water uptake, plant-hydraulic traits and transpiration dynamics of beech and spruce

Transpiration, a major water flux of the hydrological cycle, is limited by plant’s control on stomatal conductance. Plants increase stomatal conductance to take up carbon from the atmosphere for photosynthesis, i.e., during periods of high radiation, and decrease stomatal conductance to limit water loss, i.e., during periods of high vapor pressure deficit. At the same time, stomatal conductance is thightly linked to leaf water potential, which in turn is affected by the water availability for the plant through its root distribution across the gradient of water potentials in the soil. This soil-plant hydraulic system differs across species depending on the species-specific hydraulic traits, such as the stomatal sensitivity to changing leaf water potentials or the distribution of roots in the soil. Furthermore, the ability to store and release water from its tissues, here referred to as hydraulic capacitance, directly affects soil-plant hydraulics by enabling plants to source water from their internal water storage instead of the soil. Thereby, hydraulic capacitance can act as a hydraulic buffer during periods of low water availability in the root zone or high water demand from the atmosphere, particularly when plant internal water storage is high. Because plant water storage and hydraulic capacitance are rarely considered in soil-plant hydraulic models and can not directly be measured in the field, we still lack a comprehensive mechanistic understanding on how capacitance potentially affects stomatal regulation across species and environmental conditions.

In this study, we extended a soil-plant hydraulic model that simulates water fluxes across the soil-plant system utilizing well-constrained concepts of water flow in porous media, to include plant water storage and hydraulic capacitance. The model serves to better understand the sensitivity of soil-plant hydraulics towards plant water storage and capacitance. Soil-plant hydraulic simulations were validated with data from the ‘WaldLab forest experimental site’ in Zürich, Switzerland, where we have been measuring water fluxes and potentials in soils, roots, stems and stomata of beech (Fagus sylvatica) and spruce (Picea abies) trees for the past four growing seasons, including periods of limited water availability. The measurements together with the hydraulic simulations yield novel insights into species-specific water use strategies and hydraulic traits.

Our results show the different stomatal behaviour of beech and spruce, with beech generally allowing leaf water potentials to drop further than spruce. Both species showed shifts to deep root water uptake during soil drying, but the higher uptake from deeper and wetter soils was not enough to compensate for the lower water availability in the shallower, drier soils. We observed higher water storage capacity and hydraulic capacitance in spruce. However, despite higher capacitance, spruce were more conservative in their water use and did typically not allow high transpiration rates and low leaf water potentials.