Small Rodent Abundance in Relation to Reforestation Measures and Natural Habitat Variables in Northern Sweden1

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ABSTRACTResults of an extensive survey, 1972–1975, aimed at establishing the influence of forest management practices and other environmental variables on small rodent abundance are discussed. The study area included several habitat types and, in total, 1,878 small quadrats were marked out in 47 regeneration areas. Most areas were sampled twice a year, in the spring and autumn, and more than 5,000 voles were trapped; the species Clethrionomys glareolus (Schreber) and Microtus agrestis (L.) were trapped in about equal numbers, while C. rufocanus (Sundevall) was rather sporadic. The cycle increase started earlier in C. glareolus than in M. agrestis and C. rufocanus.Data on environmental variables were collected partly by means of field observations and pardy from the literature. An analysis of the effect of environmental variables revealed considerable interaction making identification of primary factors difficult. The populations of C. glareolus and C. rufocanus generally decreased with an increase in time since clear‐cutting. For M. agrestis, highest numbers were found on sites in which clear‐cutting had been carried out 5–15 years previously. The influence of surface treatment, in particular, was found to be dependent on the population cycle. During increase and early peak phases, C. glareolus was found in smaller numbers in regenerated areas treated by burning or ploughing, compared with those where only normal patch scarification had been carried out. During the very gradual decline in numbers, conditions were the reverse. During increase and early peak phases, the highest catches of M. agrestis were made in scarified areas. This inverse relationship between the 2 species was also found in connection with many of the other environmental variables studied. M. agrestis was the most typical “regeneration phase” species, positively influenced by environmental factors such as moderately coarse grained soil texture, low soil acidity, moderate humus thickness, increasing moisture up to a certain level and a vegetation type with forbs predominating as opposed to the Vaccinium type. The pattern of displacement indicates that competition occurs between M. agrertis and C. glareolus.