The effects of lactation on seedling damage by mountain beaver

The mountain beaver is a semi-fossorial rodent of the Pacific Northwest and is among a variety of herbivores that retard plant growth and cause tree seedling deformities and mortality. Douglas-fir seedlings are planted in the Pacific Northwest from February through March, a period coinciding with mountain beaver parturition. Previous research suggested that in spring, lactating females depend more on conifers than do non-lactating females and males. We conducted experiments to determine if female reproductive condition influenced seedling damage, and if physiological stage of the seedling affected damage. Dormant and flushing trees were offered to 6 pregnant and 6 non-pregnant females in 2002 and 2003. We found no difference between female condition and damage in 2002, but there was a significant difference between type of tree and damage (F6,79 = 6.75, P < 0.001). In 2003, we found a difference in seedling damage (F3,95 = 16.41, P < 0.001), with tree type (P < 0.001) and female condition (P = 0.02) contributing to the model. More flushing trees were damaged in both years than dormant trees, once bud break occurred. Statistical analyses of fructose (F5,23 = 12.07, P < 0.001) and glucose (F5,23 = 12.86, P < 0.001) concentration data indicate that tree type (dormant or flushing) was a significant effect (P < 0.001). The interaction between tree type and week sampled was also significant in both the glucose (P = 0.002) and fructose response (P = 0.009). Both fructose and glucose concentrations were the lowest in new flushing trees, and mountain beaver did not appear to be selecting flushing trees for their needle sugar content. Water concentration also varied between tree type but was not affected by the sampling time (F5,20 = 35.46, P < 0.001). New and dormant growth tissues had similar water concentrations that were greater than old growth tissue. Mountain beaver are dependent upon a constant water source, although it does not appear that damage