Abstract:
The objective of this study was to determine how the
balance of carbon to nitrogen in a grand fir ecosystem affects
the chemistry of emerging grand fir foliage and the growth of
western spruce budworm larvae. Forest plots in the grand fir
zone of eastern Oregon were thinned, thinned and fertilized,
fertilized without thinning, or left as a control to determine
how increased nitrogen availability alters the efficacy of
carbon-based chemical defenses at different light levels.
Thinning did not significantly alter any of the foliar
chemical fractions measured while fertilization increased the
concentration of foliar nitrogen and free amino acids.
Thinning increased tree vigor (g wood produced per m²
foliage), but fertilization only increased vigor in the
thinned plots. Fertilization increased the weights of
western spruce budworm pupae; thinning had no effect.
Male and female pupal weights correlated with foliar free
amino acid concentration and the ratios of foliar free amino
acids to foliar nitrogen, available carbohydrates, and lignin,
but stepwise regression analysis showed that foliar free amino
acid concentration alone explained most of the variation
in pupal weights.
The correlation of foliar free amino acid concentration
with male and female pupal weights, and the lack of
correlation of any indices of carbohydrate availability
suggest that changes in available nitrogen rather than changes
in the carbon/nitrogen balance were associated with changes in
larval growth. This can be attributed to either a lack of
defensive capability in the emerging foliage or a failure to
measure or manipulate the variables responsible for
controlling foliar defense. However, larval growth is only
one aspect of plant susceptibility to insects; changing the
carbon/nitrogen balance in the grand fir ecosystem may
ultimately affect the susceptibility of grand fir to western
spruce budworm by changing the balance between plant growth
and levels of plant herbivory.
