Trophic effects of rainfall on Clethrionomys rutilus voles:: an experimental test in a xeric boreal forest in the Yukon Territory

The Kluane forest is unusual in that it is less productive than other boreal forests because it lies in a rain-shadow zone. Densities of the boreal red-backed vole Clethrionomys rutilus are known to be food-limited in the Kluane region, and its food sources (mostly plants) could be rainfall-limited. We therefore tested the hypothesis that rainfall indirectly controlled vole densities in the Kluane region. Our predictions were that (i) food for voles would increase with additional rainfall and (ii) food-limited voles would in turn increase in numbers. Three sites in the Kluane forest were irrigated during the growing season for 5 years, and these were compared with three paired control sites without irrigation. Irrigation increased rainfall 91% above normal, on average. Neither understory plants, trees, invertebrates, nor the vole population reacted to irrigation. Only mushroom biomass increased. Hence, the above hypothesis must be rejected. The vegetation is not directly water-limited at these sites, and nitrogen limitation probably prevailed. However, mushroom biomass increased with irrigation and in turn should have increased nitrogen mineralization. It is therefore unclear why plant production and vole numbers did not increase with mushroom biomass on the irrigated sites.