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 (Leptographi um wingfieldii Morelet, from the bark beetle Tomicus piniperda L.). Five-ye ar-old field-grown trees were transplanted to 50-1 pots and grown for 1 yea r before the treatments were applied. Trees in the drought treatment were s ubjected to several successive, 3-week-long drought cycles, with predawn wa ter potential dropping below -2 MPa at peak drought intensity. The experime ntal drought cycles were more severe than the natural drought episodes usua lly recorded in Scots pine stands. Trees were then mass-inoculated with L. wingfieldii at a density close to the critical threshold density of inocula tions (400 m(-2)) above which tree resistance is overcome. Inoculation of w ell-watered trees resulted in induced reaction zones around the inoculation points and very limited damage (resinosis) in the sapwood. Drought alone h ad no long-lasting consequences on tree water relations, except for a decre ase in hydraulic conductance in the youngest segments of the main stem. How ever, the combination of mass-inoculation and drought stress after inoculat ion resulted in a dramatic loss of stem hydraulic conductivity that was par alleled by conspicuous damage to the sapwood (resinosis, drying and blue st aining). There was a close correlation between amount of visible sapwood da mage and loss of hydraulic conductivity, The intensity of induced reactions in the phloem was unaffected by drought stress. We conclude that tree defe nce against L. wingfieldii is decreased during severe drought stress, resul ting in changes in the spread and action of the fungus in the sapwood but n ot in the phloem.