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esulting seedlings were then rooted. In spring, 1970, rooted ramets of each clone were planted using 2x2 m spacing. The
planting was done in a west-facing, gently sloping agricultural field where underground drain pipes had been installed and the
clayey soil tilled before planting. The homogeneous soil and the exact planting array reduced phenotypical variation in the
stand to a minimum.
From each of two clones, #194 and #582, twelve ramets were selected. Trees of Clone 194 had been used in an
earlier mass-inoculation experiment with O. polonicum, and had proved relatively resistant to the infection (Christiansen,
unpubl.). Nothing was known about the susceptibility of Clone 582 as to its susceptibility.
The continuous rows of clones followed the slope of the field. The row of trees between Clone 194 and Clone 582
had been cut in early October 1992, and so had the two rows on their other side. Thus both rows had a distance of 4 m to
their nearest neighbours. Trees at both ends of the two rows were discarded as phenotypically aberrant, having much larger
crowns than trees inside the stand.
In late October, 1992, all branches below ca. 2.5 m were removed from all 24 trees. These branches were all dead
due to shading. The ground on both sides of the upper six trees in each row was then covered with plastic sheets to prevent
autumn and winter precipitation from percolating into the ground. Instead, this water was led down to the lower six trees in
the two rows. On 20 April t993 the plastic ground cover was removed. Ropes were strung at 2 m above ground between the
upper six trees of the rows, and non-transparent plastic tarpaulins were suspended from these ropes down to drains fastened
to the stumps of the tree rows that had been removed. The drains collected the rain falling through the canopy, and channelled
it down to the lower six trees in the rows.
This way the six upper trees in a row were deprived of most of the precipitation falling from October till the end of
the experiment. The lower six trees of the two rows served as unstressed controls. Because the early summer of 1993 was
very dry, these control trees were given an extra 47 mm of water, applied under the canopy by means of a garden sprinkler.
Monitoring of Drought Stress
Starting on 12 May 1993, drought stress was monitored by repeated measurements of pre-dawn xylem water potentials
(hereafter termed "WP"), using a pressure chamber. Freshly excised shoots taken 4-5 m above ground were used. To
detect a possible natural WP gradient along the slope, trees at the upper and lower ends of the rows were measured. By mid-
June WPs of the experimental trees were clearly different from those of the controls, and after a monitoring of all 24 trees on
23 June we decided that they had reached a satisfactory level of stress for inoculation with the fungus.
Fungal Inoculation
Prior to inoculation a 80 cm long template was attached around the stern between 1.2 and 2.0 m above ground. In the
template evenly spaced holes had been punched at a density of 4 per dm 2,and the points of inoculation were marked through
these holes. The choice of this particular dose was based on earlier experience with the susceptibility to infection by trees of
Clone 194 and of two other clones growing in the same stand. Inoculation occurred on 25 June. Bark plugs were removed
with a 5 mm cork borer. The inoculum, actively growing O. polonicum mycelium on malt agar, was placed in the cambiumholes,
whereupon the bark plug was returned to its original position (cf. Horntvedt et al. 1983).
Further Field and Laboratory Procedures
Around the time of inoculation, exudation of constitutive resin was estimated twice (cf. Christiansen and Horntvedt
1983): on each tree 10 thin plastic tubes were inserted into holes cut with a cork borer about 2.1 m above ground. After 24
hours the length of the resin column in the tube was measured.
On l August, about 5 weeks after inoculation, exudation of resin from the point of inoculation was recorded on a
subjective scale from 0 to 5, and used as an estimate of induced resinosis. On 14 September 1993, 81 days after inoculation
all trees were felled, and tree height and height of the lowest green whorl of branches were measured. The inoculated stem
section was cut out, and brought to the laboratory.
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