Effects of drought stress and high density stem inoculations with Leptographium wingfieldii on hydraulic properties of young Scots pine trees.

We examined drought-induced changes in susceptibility of potted Scots pine (Pinus sylvestris L.) trees to a bark-beetle associated fungus (Leptographium wingfieldii Morelet, from the bark beetle Tomicus piniperda L.). Five-year-old field-grown trees were transplanted to 50-l pots and grown for 1 year before the treatments were applied. Trees in the drought treatment were subjected to several successive, 3-week-long drought cycles, with predawn water potential dropping below -2 MPa at peak drought intensity. The experimental drought cycles were more severe than the natural drought episodes usually recorded in Scots pine stands. Trees were then mass-inoculated with L. wingfieldii at a density close to the critical threshold density of inoculations (400 m(-2)) above which tree resistance is overcome. Inoculation of well-watered trees resulted in induced reaction zones around the inoculation points and very limited damage (resinosis) in the sapwood. Drought alone had no long-lasting consequences on tree water relations, except for a decrease in hydraulic conductance in the youngest segments of the main stem. However, the combination of mass-inoculation and drought stress after inoculation resulted in a dramatic loss of stem hydraulic conductivity that was paralleled by conspicuous damage to the sapwood (resinosis, drying and blue staining). There was a close correlation between amount of visible sapwood damage and loss of hydraulic conductivity. The intensity of induced reactions in the phloem was unaffected by drought stress. We conclude that tree defence against L. wingfieldii is decreased during severe drought stress, resulting in changes in the spread and action of the fungus in the sapwood but not in the phloem.