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MILD DROUGHT ENHANCES THE RESISTANCE OF N©RWAY SPRUCE TO
A BARK BEETLE-TRANSMITTED BLUE-STAIN FUNGUS
ERIK CHRISTIANSEN and ANNIE MARIE GLOSLI
Norwegian Forest Research Institute, N-1432 A.s, Norway
INTRODUCTION
Outbreaks of the spruce bark beetle, Ips typographus (L.), one of the most serious pests of the Eurasian spruce
forests, are frequently triggered by large-scale windfelling. Although correlative evidence indicates that long-lasting drought
incites and aggravates such epidemics (cf. Christiansen and Bakke 1988), very little experimental work has been carried out
to test this assumption.
Ideally, the defensive capacity of the trees should be tested using prescribed numbers of bark beetle attacks. In Pinus
contorta this has been done successfully by screening the branch-free part of the stem, and inducing a defined number of
Dendroctonus ponderosae attacks under the screen (Raffa and Berryman 1983). Without screening, additional attacks by
"wild" beetles in the neighbourhood will occur if the beetles possess aggregation pheromones.
Unlike mountain pine beetle attacks on lodgepole pine, I. typographus attacks often extend far into the living crown
of Norway spruce, Picea abies, making screening impractical. For this reason we chose to assay host resistance using
prescribed loads of artificial mass-inoculation with the blue-stain fungus, Ophiostoma polonicum Siem., a close associate of
the spruce bark beetle.
Spores of O. polonicum are transmitted both externally and internally by L typographus (Furniss et al. 1990), and the
fungus is consistently isolated from the advancing front of blue-stain in successfully attacked trees (Solheim 1986, 11992). O.
polonicum can kill healthy Norway spruce when artificially inoculated under the bark (Horntvedt et al. 1983, Christiansen
1985a, Solheim 1988). Similarly, other spruce species and even Pseudotsuga menziesii may succumb to mass-inoculation
with this fungus (Christiansen and Solheim 1990). Parallel to the "threshold of successful attack" for bark beetles
(Thalenhorst 1958), a "threshold of successful infection" exists for mass-inoculation with this fungus (Christiansen 1985b).
Drought is generally accompanied by hot weather, and the two factors are not easily separated when one attempts to
find the causes for large-scale infestations. We hypothesize that (1) elevated temperatures act directly on the various life
stages of the beetles to favour population build-up, and/or that (2) drought affects the physiology of the trees, making them
more susceptible to attack or more suitable as food. Here, we report an experiment designed to shed light on aspects of
Hypothesis 2: Norway spruce trees were artificially drought stressed, and their defensive capability was assayed and compared
with unstressed, control trees.
In ordinary forest stands, trees will vary considerably in their susceptibility to beetle/fungus attack. This necessitates
large numbers of experimental trees to obtain statistically significant results. In the present experiment, however, we utilized
clones of spruce trees and thus permined work with relatively few trees.
METHODS
Experimental Trees and Their Treatment
The Norway spruce clones used in this study grew in a multi-clonal stand at Hogsmark Experimental Farm in ,As,
Akershus, Norway, and had been produced as follows: Seeds from selected trees were sown in 1951 and cuttings from the
Mattson, W.J., Nieme1_i, P., and Rousi, M., eds. 1996. Dynamics of forest herbivory: quest for pattern and principle.
USDA For. Serv. Gen. Tech. Rep. NC-183, N.C. For. Exp. Sta., St:.Paul, MN 55108.
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