Influence of Invasive Earthworms on above and Belowground Vegetation in a Northern Hardwood Forest
Hopfensperger, Kristine N., Leighton, Gavin M., Fahey, Timothy J., The American Midland Naturalist
ABSTRACT.-Scientists are beginning to have a firm grasp on the dramatic influence invasive earthworms can have on nutrient cycling in northern hardwood forests, yet a concrete understanding of their effects on plant communities is still needed. Towards this effort, we examined both the above and belowground plant communities, along with soil organic matter, pH, texture and light in Arnot Forest located in south central New York. Two transects were aligned parallel to previously defined gradients of earthworm density and were sampled in the spring and summer of 2008. We found plots codominated by multiple earthworm species decreased belowground seed abundance and species richness. Aboveground plant cover and diversity along with soil organic matter was lower along the transect with multiple earthworm species. In addition, we found earthworm density increased with soil pH, but did not detect relationships with soil texture or light intensity. Our results were consistent with previous findings of earthworms negatively affecting aboveground plant communities and soil organic matter, and increasing pH. Our study demonstrates the potential for exotic earthworms to dramatically alter forest soil seed banks; however, future research is needed to investigate complexities of earthworm community dynamics (e.g., number and life form of earthworm species) and to tease apart mechanisms responsible for changes in the belowground plant community.
A growing body of literature has found invasive earthworms in northern hardwood forests to cause remarkable changes in soil structure, nutrient cycling and plant communities. Perhaps the most striking is turning these ecosystems from important global carbon sinks into carbon sources (Alban and Berry, 1994; Bohlen et al, 2004a) through increased heterotrophic respiration (Li et al, 2003; Fisk et al, 2004). Earthworm consumption of soil organic matter and enrichment of the mineral soil can lead to an increase in microbial biomass and faster bacteria-dominated nutrient cycling (Bohlen et al, 2004a; Groffman etal., 2004).
Recently, research has begun to focus on the effects of exotic earthworms on forest plant communities. When earthworms shift the soil system from a fungal-dominated soil to a bacterial-dominated soil, crucial mycorrhizal-plant root relationships are lost (Wardle, 2002). The loss of mycorrhizae can lead to negative effects on plant root function (Lawrence et al, 2003), plant growth (Gundale, 2002) and plant community assemblages (Holdsworth et al., 2007) . In addition, an increase in earthworm diversity may cause a decrease in plant species diversity due to different earthworm species occupying multiple soil niches. For example, Hale et al. (2006) found lower plant diversity in areas wifh a combination of both epigeic (residing in fhe upper soil organic layer) and endogeic (mix the organic and mineral soil layers) worm species than in areas with only endogeic worm species. Very litde literature exists on earthworm effects on soil seed banks and the research fhat does exist is focused in agricultural or grassland ecosystems (Decaens et al, 2003; Jimenez and Decaens, 2004; Regnier et al, 2008).
With a firm understanding of how non-native earthworms influence nutrient cycling in northern hardwood forests, we proposed to investigate the effect of earthworms on belowground seed banks and provide additional information concerning earthworm effects on aboveground plant dynamics and soil characteristics. Our study took place in Arnot Forest, NY where previous studies suggest fhat earthworms have reduced soil carbon, redistributed organic matter and significandy altered soil carbon, nitrogen and phosphorus cycles (Bohlen et al, 2004b; Groffman et al, 2004; Suarez et al, 2004, 2006). We hypothesized fhat earthworms would have a negative effect on belowground seed abundance and species richness due to earthworm activities, such as seed burial and/or consumption. …