© New Phytologist (2003) 157: 145 – 153 www.newphytologist.com 145 Research Blackwell Science, Ltd Influence of nonnative earthworms on mycorrhizal colonization of sugar maple (Acer saccharum) Beth Lawrence, Melany C. Fisk, Timothy J. Fahey and Esteban R. Suárez Department of Natural Resources, Cornell University, Ithaca, NY 14853, USA Summary • Exotic earthworms can modify or eliminate surface organic (Oe/Oa) horizons in cold-temperate forest ecosystems and have profound effects on the forest soil envir- onment, especially the rooting zone. • We examined the effects of earthworm colonization of northern hardwood forest soils on the abundance and morphology of mycorrhizal fungi associated with sugar maple ( Acer saccharum ). We compared mycorrhizal associations of areas of earth- worm invasion with those of reference (no-worm) areas in Arnot Forest, central New York, USA. • The organic horizon in reference areas had higher mycorrhizal colonization rates and higher colonized root length than did surface layers in areas with active earth- worm populations. Hyphal coils were more abundant and also formed a greater pro- portion of total fungal colonization in reference plots. Vesicles were more abundant and were a higher contribution to total colonization in earthworm plots, indicating a possible stress response to the presence of earthworms. • By affecting mycorrhizal colonization and morphology, earthworms may influence nutrient uptake capacity of dominant forest species. Our results suggest that a profound change in the mycorrhizal system will be one component of the potential ecosystem effects of invasion of new forest habitat by nonnative earthworms. Key words: mycorrhizas, arbuscular mycorrhizal (AM) fungi, earthworms, sugar maple ( Acer saccharum ), forest floor. © New Phytologist (2003) 157 : 145–153 Author for correspondence: Melany C. Fisk Tel: +1 828 262 6910 Fax: +1 828 262 2127 Email: fiskmc@appstate.edu Received: 11 June 2002 Accepted: 27 September 2002 Introduction Sugar maple (Acer saccharum) is a commercially important tree species in northern hardwood forests that forms a Paris- type symbiotic relationship with arbuscular mycorrhizal (AM) fungi (Yawney & Schultz, 1990; Cooke et al., 1993). The ubiquitous occurrence of AM and their importance to phosphorus nutrition and general plant health are now widely accepted. Our understanding of this mutualistic interaction is, however, largely derived from research on herbaceous annuals and pot cultures, with little work being done on woody perennials in natural ecosystems. During the past decade, concerns have arisen about the health of sugar maple in northeastern North America because many stands show symptoms of decline associated in part with inorganic nutrition of the trees (Paré & Bernier, 1989; Horsley et al., 2000). Coughlan et al. (2000) have suggested that sugar maple decline is linked to significant effects of altered pH on AM fungal community composition and colonization rates. Hence, further research is needed to achieve a more comprehensive understanding of mycorrhizal relationships of sugar maple in natural ecosystems. Invasion of forest habitats by nonnative earthworms changes decomposition and nutrient cycling in forest ecosys- tems (Scheu & Parkinson, 1994; Edwards & Bohlen, 1996; McLean & Parkinson, 1997; Burtelow et al., 1998). Northern hardwood forests of the north-eastern United States develop a thick (3 –10 cm) surface organic horizon (Oe/Oa, forest floor) that is not mixed with the underlying mineral soil. Fine roots of trees, activity of decomposer organisms, and recycling of plant nutrients are concentrated in this organic horizon (Bohlen et al., 2001; Yanai, 1992). Exotic earthworms such as the epi-endogeic (Lumbricus rubellus, Octolasion tyrteum) and anecic (L. terrestris) species common in hardwood forests of