Wildfire,clearcutting, and vole populations: Balancing forest crop protection and biodiversity

Early successional forest habitats that develop after wildfire may provide ideal conditions for population build-ups and subsequent fluctuations by Microtus voles. Regeneration of burned-over forest land may be hindered by consumption of planted trees by voles. A high abundance of voles, occurring in the second growing season after a wildfire, may result in serious feeding damage to seedlings leading to major plantation failures. A wildfire occurred near Golden, British Columbia, Canada in the spring of 2011 and may have initiated the successional conditions to generate a vole population outbreak with consequent feeding damage to tree seedlings. We tested the hypotheses (H) that (H₁) abundance of herbaceous plants (grasses and forbs) will be greater, and (H₂) abundance of voles and incidence of feeding damage to tree seedlings will be higher, in burned than unburned (control) sites. Microtus voles and other forest-floor small mammals were live-trapped for four years (2011–2014) in replicated sites of a wildfire (burned plantation), control (unburned) plantation, and a new control clearcut. Abundance of total herbs and grasses, incidence of feeding damage, and mortality to tree seedlings by voles were measured in all sites. Mean abundance of total herbs and grasses were similar among treatments during the post-wildfire period. Mean annual peak numbers of Microtus in clearcut sites ranged from 18 to 30 per index-line. Annual peak numbers in the burned and control plantation sites ranged from 5 to 8 voles per index-line and were essentially stable at these numbers. Vole populations in the control and burned plantation sites were dominated by the meadow vole (Microtus pennsylvanicus Ord), and those in the clearcut sites by the long-tailed vole (Microtus longicaudus Merriam). Mean mortality of trees was significantly higher in the control clearcut sites at 30% compared with 13% in the burned plantation sites. These results did not support either H₁ or H₂. Wildfire, at least in this case, did not generate high populations of voles and significant damage to tree seedlings. Mean abundance of total small mammals was similar in burned and control plantation sites, but less than in clearcut sites. Populations of the deer mouse (Peromyscus maniculatus Wagner) increased after wildfire and those of the southern red-backed vole (Myodes gapperi Vigors) declined. Populations of the northwestern chipmunk (Neotamias amoenus J.A. Allen) and Sorex spp. did not show a clear preference for any of the treatments. Disturbance by clearcutting or wildfire seemed to reduce species richness and diversity, but all eight species of small mammals were present in each of the treatments.     

Voles,trees, and woody debris structures as habitat: Balancing forest crop protection and biodiversity

Relatively homogeneous early successional habitats develop after clearcutting and wildfire that voles of the genera Microtus and Myodes may colonize and generate population fluctuations. In these habitats, vole populations may reach pest status by their feeding on newly planted tree seedlings. Strategic management of excess woody debris into piles and windrows helps diversify new clearcuts by enhancing populations of forest-floor small mammals, including voles, and some of their predators.This study tested the hypotheses (H) that (H₁) abundance of voles and incidence of feeding damage to tree seedlings will be higher in windrow than dispersed (conventional) sites of woody debris, and (H₂) there will be a gradient of damage with the highest incidence immediately adjacent to windrows. A third hypothesis (H₃) predicts that feeding damage to trees will increase in relation to windrow size. Microtus voles and red-backed voles (Myodes gapperi) were live-trapped for three years (2010–2012) in replicated sites with woody debris dispersed and in windrows at three study areas in the southern interior of British Columbia, Canada. Incidence of feeding damage and mortality to tree seedlings by voles was measured in all sites. Mean abundance of M. gapperi, Microtus, and total voles were all significantly (P ≤ 0.04) higher (up to 3.4 times) in windrow than dispersed sites, and hence the abundance part of H₁ was supported. Mean annual percentage of trees damaged by voles was significantly (P ≤ 0.03) higher in windrow than dispersed sites over the two winters and for cumulative incidence of damage, and hence the tree damage part of H₁ was supported. Mortality of trees followed this pattern but was not formally significant. Trees planted immediately adjacent to a windrow had significantly (P < 0.01) greater feeding damage than seedlings planted further away, and hence H₂ was supported. There were significant linear relationships between mean percentage of trees killed (r = 0.67; P < 0.01) and mean number of total voles, and also with mean volume of woody debris per meter length of windrow (r = 0.98; P < 0.01). Thus, H₃ was also supported. To minimize overall mortality of trees, it is likely worthwhile to not plant trees near windrows. Slightly reduced stocking (<5% net forest area) and potential loss of some trees to voles need to be balanced alongside biodiversity and conservation considerations provided by woody debris structures.