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Exotic Earthworms & Northern Temperate Forests

Lead Scientist(s): Dr Peter M Groffman

Through their burrowing activities, earthworms reap dramatic physical changes on forest soil. Prior research has shown that by altering forests floor structure, earthworms can dramatically affect crucial nutrient cycling processes, such as nitrification. Presently, it is unknown how these soil structural changes will impact nutrient retention in invaded forests. Understanding earthworm effects on plant nutrient uptake, fine-root production and foliar chemistry will help predict the fate of invaded forests.

We propose that earthworm invasion of new forest habitats will alter ecosystem processes in the coming decades. To increase our understanding of earthworm impacts, we are testing four hypotheses. Our predictions are that earthworm invasion will:

  • Increase hydrologic and gaseous losses of nitrogen and decrease retention of exogenous nitrogen
  • Alter soil nutrient-supplying (N and P) capacity with consequences for plant nutrient demand and foliar nutrient contents and ratios
  • Change the vertical distribution, production and functioning of fine roots

We are using two types of experiments to explore our hypotheses- controlled introductions and gradient assessments. The first approach involves introducing earthworms into experimental forest plots that lack earthworms. By monitoring the soil before and after the earthworms are introduced, we will gain insight into how earthworm invasions impact the forest floor. The second approach involves comparing invaded forest stands with adjacent earthworm-free stands, viewing these stands as endpoints along a gradient of earthworm invasion.

The experiments are taking place in forests at two different geographic locations, one in eastern (Tompkins Farm, near Millbrook) and the other in central (Arnot Forest, near Ithaca) New York. We are documenting the obvious and expected decline in forest floor depth and redistribution of surface organic matter in the soil profile following earthworm invasion. This profile will allow us to determine how earthworms affect the forest's ability to retain exogenous N and provide nutrients to support plant growth.

Retention of atmospheric N will be assessed by tracing the movement of small additions of 15N and Br- through the top 30 cm of the soil profile. We are measuring pools of available N and P, and the biomass and distribution of roots. To determine whether the presence of earthworms affects foliar nutrient content and ratios, we are analyzing litter and foliar chemistry. Hydrologic nutrient losses are measured using zero-tension lysimeters and gaseous N losses are measured using soil cores.

Research Rationale: This research will hopefully provide new information on a critical but overlooked factor in the study and management of north temperate forests. The influence of earthworms on biogeochemistry in natural ecosystems, especially forests, has received little attention in North America. Our results will allow us to determine if earthworm invasion will be a factor influencing the long-term productivity of these forests and whether earthworms will influence the response of these forests to other factors, such as nitrogen deposition, climate change and exotic plant invasions.