Kolloquien des Botanischen Instituts

Tree water use and transport were studied in Pinus halepensis trees growing in southern Israel, at the dry end of tree occurrence, where they are regularly challenged by seasonal drought. We found that the regulation of water use in P. halepensis is isohydric: stomata typically limit water potential to above -2.8 MPa. At -3.1 MPa, there is already 50% loss of xylem conductivity due to cavitation, leaving a narrow safety margin of ~0.3 MPa. This risky behavior, near or beyond the safety margin, allows prolonged CO2 uptake under drought. We demonstrated that this is possible because it is complemented by efficient cavitation reversal. The field study in southern Israel was complemented by provenance trials across a climatic gradient including Rome, Italy and Beit Dagan (central Israel); comparison to a coexisting oak species; a forest thinning experiment; and a greenhouse experiment; all revealing important adjustment mechanisms with implications to forestry in the Mediterranean region and beyond.