View or print this publication - Northern Research Station - USDA ...

-0.2
0
5 82.
_j -0,4 e a a a
D4
E
I ® e ®
-0,6 ®
_4 e
.4 4.3
C; -0.8
o
.#
o
_4 _ .... t I 0 O 0 | . l
(1)
194
-0 .2
- 0.4 _
if,
3:
o
zx
a
zx
_I 0
-o. 6
04 o o
- o. 8 "
@
e
-1 0 | I I I I I I I ....I I |
1 2 3 4 5 6 7 8 9 i0 ll 12
Tree number
Figure 1.--Pre-dawn xylem water potential of Norway spruce trees on 23 June 1993. Trees 1-6 of both clones (194 and 582)
are artificially drought stressed; trees 7-12 are controls.
An earlier experiment in Norway spruce addressed possible after-effects of drought on the resistance to O. polonicum
infection (Christiansen 1992). In that case, three summers of severe artificial drought did not affect the resistance of the trees
during a fourth season of normal precipitation. Thus neither that experiment nor the present one supports a general hypothesis
that drought stress makes coniferous trees more susceptible to attack by bark beetles and their associated blue-stain fungi
(cf. Christiansen et al. 1987).
The drought stress was relatively mild in the present study, though possibly adequate to have influenced important
physiological processes in the trees (cf. Hsiao 1973), even including gas exchange (cf. Havranek and Benecke 1978). Such
changes may be thought to reduce resistance, but vascular wilts are characteristically found to be more severe in wet than in
dry soils (Schoeneweiss 1986).
A
195