Dynamics in radial sap flow and stem water reserves under varying environmental conditions

Knowledge of hydraulic strategies is essential to understand the response of forest ecosystems to changing environmental conditions. Species-specific regulation mechanisms of water fluxes and water storage in the xylem are important drivers of drought tolerance in trees. Differences in the regulation may also occur along the radial profile of the xylem leading to a differential drought response of water fluxes within the tree xylem. To this end, we investigated sap flow and stem water content in two xylem depths, variations in stem radius and water potentials of Abies alba, Fagus sylvatica, Picea abies and Quercus petraea in a mature forest stand in SW-Germany during three years with varying environmental conditions.

Generally, hydraulic strategies varied between the four investigated species: A. alba was generally the most water-saving species, while drought tolerance was highest in Q. petraea and lowest in P. abies. Under moist conditions F. sylvatica was the most water-spending species, whereas sap flow was strongly reduced under drought. Overall, sap flow of all four species responded more pronounced to high vapor pressure deficits (VPD) than to decreasing soil moisture. We found a varying contribution of stem water storage to daily sap flow between the four species, which might be a crucial trait explaining drought tolerance.

A dynamic radial shift of sap flow in ring-porous Q. petraea was observed, which was tightly linked to corresponding stem water content: under high VPD sap flow in the inner xylem (10 mm beneath the cambium) exceeded sap flow in the outer xylem (20 mm beneath the cambium), while the inverse was observed under low VPD. Such a differential response within different xylem depths might enhance drought tolerance of ring-porous Q. petraea.

Moreover, the relationship between declining stem water reserves (expressed as tree water deficit) and water potentials was analyzed by a generalized additive model incorporating VPD and soil moisture. The model was trained with measured water potentials and subsequently used to predict water potentials from tree water deficits. Model predictions represented well measured values, if environmental variables were considered, and thus our modelling approach might be a useful tool in the future to predict water potentials on a high temporal scale without excessive measurement intensities.

In summary, our study demonstrated that availability of stored stem water influences species-specific response of sap flow to drought conditions and that regulation mechanisms may vary along the radial profile. Further investigations are needed to determine if differential responses in different xylem depths are species-specific or part of a general protection mechanism, e.g. to preserve hydraulically more important xylem vessels from cavitation. Furthermore, predicting water potentials from tree water deficits might be a useful and cost-efficient approach to gain insights into stomatal regulation processes without the need to reach the tree canopy.