Tree hydraulics under pressure

Stefan Mayr

Plant water relations are frequently under pressure as water transport is passively driven by transpiration, which generates negative pressure (i.e. negative water potential), transduced from leaves to the roots and to the soil. Negative water potentials also develop when the stomata are closed but soil water supply is limited.

Trees, due to long transport pathways from the soil to the crown, can reach remarkably low water potentials. The stability of water columns under these conditions is only possible due to optimized structures, enabling both, sufficient transport capacities (hydraulic efficiency) and prevention of interruptions, so-called embolism, in water columns (hydraulic safety). In the xylem, various structural features of the transport conduits, including their connections via bordered pits, contribute to maintaining hydraulic efficiency and safety.

The talk will give general insights into xylem hydraulics and then focus on two contrasting examples of hydraulic damage and the counter strategies of trees: (i) At the alpine treeline, trees are exposed to harsh conditions during winter. Winter drought leads to extreme water potentials, which, together with frequent freeze-thaw events, cause embolism. But trees are able to recover from winter embolism. (ii) Trees surviving forest fires often show structural and non-structural damages of the hydraulic system, which may cause dieback years after the fire. Some species are well protected, especially by good insulation properties of the bark.