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Figure 2.--Fungal proliferation in Norway spruce trees inoculated with Ophiostoma polonicum. Trees 1-6 of both clones
(194 and 582) are artificially drought stressed; trees 7-12 are controls. Upper graphs: Percent blue staining of the
sapwood cross-sectional area. Lower graphs: Percent cambial killing.
In a field experiment in young Pinus taeda, long-lasting water deprivation resulted in pre-dawn xylem water potentials
of the same magnitude as in the present study. Although resin flow was significantly reduced in the stressed pines, they
were also less attacked by Dendroctonusfrontalis than the irrigated controls. The conclusion was that drought of this extent
does not necessarily lower resistance (Dunn and Lorio 1993).
Like several other inoculation experiments in Norway spruce, the present one demonstrated that the invasive success
of O. polonicum is negatively correlated with the quantities of resin that accumulate in "reaction zones" around the points of
infection and ooze out from these sites (Christiansen and Horntvedt 1983, Christiansen 1985a, Christiansen and Ericsson
1986, Horntvedt 1988, Horntvedt and Solheim 1991). As in other conifers, this "secondary" resinosis is assumed to be due
mainly to localized, induced defence reactions (Berryman 1972).
During the early stages of invasion in conifers, the bark beetles encounter a first line of defence, i.e., constitutive or
primary resin oozing from severed ducts in the phloem. In our study there was no unequivocal difference between stressed
and control trees in this character, although the first of two measurements gave higher resin yields in control trees of Clone
194 than in stressed ones. However, in the end, these controls turned out to be less resistant to infection than the stressed
trees. In Norway spruce, the quantities of primary resin appear to be highly variable (Christiansen 1991). Despite its
apparent importance for the defence of certain pines (Cates and Alexander 1982), this constitutive resin appears to be less
important for others such as Pinus sylvestris (Schroeder 1990). It has been suggested that tree species that are repeatedly
exposed to attacks by multiple generations of beetles per year may rely more on a constitutive defences than those which
generally have only one critical period of attack (Matson and Hain 1983).
For some pine bark beetles (e.g., D. frontalis and D. brevicomis), studies suggest that drought has a negative effect on
resistance, and that this effect is cumulative both on a shorter and a longer time scale (Craighead 1925, Miller and Keen
196