Non-native earthworms have long been present in the mid-Atlantic region and their abundance in forests differs across a gradient of forest age.  Non-native earthworms are more abundant in successional forests compared to mature forests.  The native orchid Goodyera pubescens and the Tulasnella fungi with which it forms mycorrhizae are abundant in forests with few, native, earthworms but rare in forests with abundant non-native earthworms.  Focusing on recruitment from seed, we hypothesized that abundant non-native earthworms would affect G. pubescens in three ways: ingesting seeds, thus affecting their viability, burying seeds too deep to access appropriate mycorrhizal fungi, and decreasing availability of Tulasnella spp. by altering availability of the carbon sources they need. We tested these hypotheses at the Smithsonian Environmental Research Center, MD. We used laboratory feeding experiments and mesocosms to examine the effects of two non-native and one native earthworm species on G. pubescens seed viability and burial in successional and mature forest soils.  We used litter decomposition and earthworm exclusion bags as part of a larger amendment experiment to examine earthworm effects on litter and soil carbon dynamics in the field.

Results/Conclusions

All three earthworm species ingested G. pubescens seeds and ingestion decreased seed viability by approximately 70%.  Seed ingestion and burial in mesocosms was moderate but earthworm species differed in their propensity to ingest and bury seeds and in how these varied among successional and mature forest soils.  In the field, abundant non-native earthworms stimulated a shift from oxidative to hydrolytic decomposition of leaf litter, indicative of bacterial dominated decomposition.  These differences in litter decomposition translated into differences in the biologically active, fast-decaying soil organic matter fraction (light fraction) and depended on earthworm abundance rather than differences among sites.  This suggests that non-native earthworms may be driving the decreased abundance of Tulasnella fungi needed by G. pubescens by reducing hydrolyzable carbon compounds in litter and soil. These results demonstrate that abundant non-native earthworms can have both direct and indirect effects on G. pubescens recruitment from seed and that native and non-native earthworms can influence native understory plant species in different ways.