Leaf litter and the small-scale distribution of carabid beetles (Coleoptera, Carabidae) in the boreal forest

Management practices favoring conifers at the expense of deciduous tree spe cies, and the eradication of deciduous trees, especially aspen Populus trem ula, from managed forests have resulted in population declines in several s pecies in Fennoscandia. In addition to species depending on decaying wood o f deciduous trees, earlier evidence suggests that leaf litter, especially t hat of aspen, is favored by many carabid species. We ran a four-year experi ment in order to compare carabid assemblages of unchanged forest Boor with artificially created leaf-litter plots id central Finland. A total of 18 pl ots (5 m in diameter) were established in three forest stands without aspen a few kilometers apart. Each stand had 3 litter plots (litter added) and 3 control plots. Pre-treatment samples were compared with those collected af ter litter addition. The litter addition affected the carabid-assemblage structure by increasing the catches of some species and decreasing the catch of one species. The n umber of carabid species was similar in control and litter plots. The litte r effect was smaller than variation among forest stands and year-to-year fl uctuations. There was a strong temporal constancy among the plots: "rich" p lots remained "rich" from year to year and similarly "poor" plots remained "poor". The significant influence of leaf litter on carabid abundance can be attrib utable to both abiotic factors (microenvironmental conditions, especially h umidity and temperature), and biotic ones (changes in niche structure, impr oved food supply). Leaf litter seems to have an effect on carabid distribut ion patterns, and deciduous trees scattered among conifers are likely to be of importance on carabid fauna in boreal forests.