Non-native earthworms in boreal forests : Effects on plant growth, decomposition and nitrogen uptake

Sammanfattning

The effect of non-native burrowing earthworms on boreal forest vegetation, from which they have historically been missing, is not well known even though ecosystem-wide changes caused by introduced earthworms have been observed in other forest types. This study investigated the effect of earthworm introduction in boreal forests of varying fertility levels on plant new growth, microbial decomposition and plant leaf nitrogen content using material from a common garden experiment with two different densities of worm treatment and two control treatments, one using dead worms. New growth was measured on understory shrubs, microbial decomposition as mass loss of leaf litter contained in mesh bags, and nitrogen content through combustion of dried leaf material. New growth during the growing season increased with worm treatment in Empetrum sp. by up to 86% (p < 0.001) depending on treatment and fertility level, but not in other shrubs. Worm treatment had no significant effect on microbial decomposition mass loss over 11 weeks (p = 0.69). In the high density worm treatment, leaf nitrogen content increased by 24% (p < 0.05) in Empetrum sp. and by 43% (p < 0.001) in grasses and had no effect on the other plant species in the study. The results show that earthworm presence favours certain species in boreal forests, which could eventually lead to changes in community composition in ecosystems where worms establish. Since nitrogen content of some plants increased but microbial decomposition did not, the increased nitrogen content is likely due to the earthworms’ processing of organic material.