Forecasting vole population outbreaks in forest plantations: The rise and fall of a major mammalian pest

Voles of the genera Microtus and Myodes feed on tree seedlings planted on cutover forest land in temperate and boreal forests of North America and Eurasia. This damage may have serious economic implications as well as limit regeneration of appropriate tree species in certain forest ecosystems. Prediction of vole population outbreaks and feeding damage to forest plantations, across even a limited geographic range, has yet to be achieved in North America. Thus, a major objective was a detailed analysis of changes in population dynamics of long-tailed voles (Microtus longicaudus), and to test three hypotheses (H) that vole populations would: (H₁) rise and fall in accordance with the abundance of herbaceous plants (grasses and forbs) during early vegetative succession after forest harvesting, (H₂) be positively associated with grass-seeded sites; and (H₃) incidence of feeding damage to seedlings would be positively associated with vole abundance. Voles were live-trapped for 6 years (2004-2009) from the time of harvesting on intensive sites, as well as surveyed over a range of extensive sites. Population numbers were related to habitat characteristics and tree damage in young forest plantations near Golden, British Columbia, Canada. Populations of long-tailed voles were low in the first two years after harvest with mean numbers <5-15/ha. Annual peaks of 49-84 voles/ha were recorded in 2006. In the fourth year (2007) after harvesting, numbers of voles declined on two of three sites, deepened in 2008 and reached extirpation in 2009. On the extensive sites, vole numbers increased 4.6-5.3 times from 1-2 to 3-6 years post-harvest before declining thereafter. Crown volume index of grasses and herbs, volume and abundance of downed wood, total species richness of vascular plants, and structural diversity of herbs were important habitat variables. Vole numbers were higher on those sites seeded with pasture grasses and forbs. There was a significant positive relationship of tree mortality and abundance of voles (Microtus) across a relatively wide geographic area. This study is the first relatively long-term analysis of changes in population dynamics of the long-tailed vole and the predictions of H₁ and H₂ seemed to be supported. The positive relationship (H₃) of the incidence of overwinter damage to trees and vole abundance is the first such analysis for forest plantations, on harvested sites, in North America. At 3-4 years post-clearcut harvesting is a critical time for population buildups of voles and subsequent damage to plantation trees. Seeded grass species clearly create optimum habitat conditions for voles, generating population densities up to 30-50 voles/ha, which is in the range of a “high” damage risk to seedlings. Risk ratings (voles/ha) for feeding damage to trees were low (<7), moderate (7-34), high (35-88), and very high (>88).