Graduate Thesis Or Dissertation
 

The influence of induced host moisture stress on the growth and development of western spruce budworm and Armillaria ostoyae on grand fir seedlings

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  • 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.
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