|Abstract or Summary
- This greenhouse study evaluates the influence of separately and
simultaneously imposed water stress, western spruce budworm (Choristorneura
occidentalis Freeman) defoliation, and inoculation with the root pathogen, Armillaria
ostoyae (Romagn.) Herink, on the growth and biochemical features of Abies .grandis
(Dougl.) Lindi. Seedling biomass, plant moisture status, bud phenology, and
allocation patterns of phenolics, carbohydrates, and key nutrients (nitrogen,
phosphorus, potassium and sulfur) are reported.
Hypotheses are developed and tested on the impacts of water-stress,
defoliation, and root inoculation, on western spruce budworm growth and
development, and Armillaria ostoyae-caused mortality and infection.
Western spruce budworm larvae fed on water-stressed seedlings had higher
survival rates, grew faster, and produced larger pupae than those fed on well-watered
seedlings. There is no clear reason for the positive insect response, but
changes in foliage nutrient patterns and phenolic chemistry are indicated.
Insect caused defoliation has been earlier reported to enhance successful
colonization of Armillaria spp. on deciduous trees in the forests of the northeastern
United States. The positive response of the fungus was attributed to a weakened tree
condition. Conversely, although this study conclusively found water-limited trees to
have increased susceptibility to A. ostoyae, defoliation significantly lowered
Armillaria-caused infection and mortality. The decline in infection success is
attributed to defoliation-caused reduction in plant water stress and an alteration of
root carbohydrate chemistry.
One and/or two years of defoliation did not appear to weaken the
physiological condition of seedlings. Conversely, water-stressed seedlings that were
also defoliated produced more buds, had an earlier bud phenology, contained higher
total reserve carbohydrates, and had little Armillaria-caused mortality.
The study suggests that during drought, short-term defoliation may be
beneficial to grand fir and its associated forest community. Also, the additive
effects of simultaneously occurring A. ostoyae and western spruce budworm may
not be as severe as conventionally believed. Ecological and forest management
implications are explored.