The role of the northern pocket gopher in a primary successional landscape at Mount St. Helens

Background/Question/Methods   The Northern Pocket Gopher (Thomomys talpoides) has been shown to strongly influence ecosystem dynamics in western North America through mound building and herbivory, resulting in altered plant communities and biogeochemical processes. Pocket gophers colonized the Pumice Plain of Mount St. Helens in 1992, 12 years after the 1980 eruption, and have expanded their population since that time. The objectives of the research are to determine the effects of pocket gophers on plant community diversity as well as on soil nutrient fluxes and pools in a primary successional system. I compared plant species percent cover and abundance on mounds, near mounds and in undisturbed areas. I also created artificial gopher mounds and compared rates of soil CO2, NO3-N, NH4-N, and PO4-P flux, and soil %N and %C composition between artificial mounds and undisturbed areas.

Results/Conclusions   Young (1 year old) gopher mounds exhibited decreased species density and richness compared to near-mound, and undisturbed areas, while older (2 year old) mounds were not significantly different in species richness from near-mound and undisturbed areas. One year old artificial gopher mounds exhibited decreased rates of soil CO2 flux compared to undisturbed areas, while recently-created mounds (2 weeks old) showed no difference in soil CO2 flux rates. Additionally, there was no difference in NO3-N, NH4-N, and PO4-P flux rates or %C and %N between artificial mounds and undisturbed areas. These results indicate that gopher disturbances create conditions that are suitable for a subset of plants found in the surrounding community. They also suggest that the relationship between gopher disturbance and soil nutrient fluxes are likely mediated by interactions with plant communities and soil microbe assemblages, resulting in soil nutrient pools and fluxes that are relatively resilient to physical disturbance.