Edition 5, June 2010
Earthworms Could Accelerate Loss Of North American Tree Species
See all stories in this edition
June 18th, 2010
 |
|
| Deciduous forest in Minnesota. Image source |
|
Earthworm invasion of North American forests can exacerbate the impacts of droughts and is likely to accelerate the loss of tree species sensitive to climate change.
Evan Larson and co-researchers used tree rings to assess the effects of earthworms on the sugar maple (Acer saccharum) overstory of two deciduous forest stands in northern Minnesota.
The characteristic pattern of tree rings after earthworm invasion indicated that affected trees were more sensitive to drought than those growing in earthworm-free conditions.
This renders sugar maples more vulnerable to mortality under future climates if predictions of more frequent and severe droughts are realised, and would accelerate its predicted elimination from Minnesota over the next century from loss of suitable habitat.
European earthworms are “fundamentally altering the structure and function” of deciduous forests in northern North America, but have only recently been recognised as a serious threat.
The major sources of invasive earthworms are the release of unused fishing bait and intentional introductions for bait.
Earthworm invasion of previously earthworm-free forests can eliminate the litter layer (on average 10 cm thick in the forests studied) and reduce the abundance and diversity of understory vegetation.
The earthworms mix the litter layer with the top soil layers, which are then subject to erosion and compaction.
This reduces water infiltration, causes loss of the fine root mass of plants, disrupts mycorrhizal communities and probably increases nutrient leaching.
The loss of litter increases run-off rates, which “likely increases the frequency in occurrence of drought conditions at earthworm-invaded areas so that tree growth may be limited by low moisture availability even during periods where the average climate conditions are relatively wet.”
The researchers found that tree growth in earthworm-invaded areas was correlated with rainfall, which is expected for trees with poor water status.
References
Larson ER, Kipfmueller KF, Hale CM, Frelich LE and Reich PB. 2010. Tree rings detect earthworm invasions and their effects in northern Hardwood forests. Biological Invasions 12(5): 1053-1066.