MYCORRHIZAL OCCURRENCE IN WILLOWS IN A NORTHERN FRESH-WATER WETLAND

Willows growing in a north-central Michigan [USA] wetland were ectomycorrhizal throughout the 1978 growing season on each of 2 sites (1 water-saturated, 1 relatively drier). Each site was dominated by a mix of sedges (Carex spp.) and willows (Salix spp.). On both sites P was added at 2 levels (20 or 200 kg P ha-1) to simulate potenial inputs from the disposal of secondarily-treated municipal wastewater. The intensity of willow root infection was determined by scoring root tips on a 0 to 4 scale based on mantle, intercellular hyphae, and root epidermal cell characteristics. Infection on the wet site remained uniformly heavy in July and Aug. when P was added, but it declined significantly during Aug. for control plants at both sites. When water levels increased during Sept., the intensity of mycorrhizal infection increased on control plants on the wet site. On the drier site, mycorrhizal intensity decreased on controls and on plants exposed to the highest P levels. Between Aug. and Sept. samplings, mycorrhizal intensity increased considerably on dry-site controls but did not change on plants exposed to added P.