ECTOMYCORRHIZAL COLONIZATION OF QUERCUS-RUBRA SEEDLINGS IN RESPONSE TO VEGETATION REMOVALS IN OAK AND PINE STANDS

Ectomycorrhizal (ECM) colonization of northern red oak (Quercus rubra L., NRO) seedlings in response to different degrees of overstory and u nderstory removal was investigated in NRO and red pine (Pinus resinosa Ait.) stands in northern Lower Michigan. The experimental design cons isted of two stand types (oak and pine), three blocks nested within st and type, four levels of canopy cover (clearcut, 25% (50% first year), 75%, and uncut), and two understory treatments (shrub removal and unt reated control). NRO acorns from a common seed source were sown in the spring of 1991 and the emerged seedlings were sampled to quantify the ir ECM during the first two growing seasons. Photosynthetically active radiation (PAR) transmittance was recorded during the second growing season. Soil moisture and temperature were also measured at two- to th ree-week intervals for the first two years. ECM colonization (%) was s ignificantly greater in the 50% canopy cover treatment (37.5) than in the clearcut (22.3) and uncut (20.8) treatments during the first growi ng season. In contrast, during the second growing season, percent ECM in the 25% canopy cover treatment (45.8) was significantly greater tha n in the clearcut treatment (20.4), but did not differ from the 75% co ver (40.4) and uncut (37.6) treatments. ECM number per gram dry root w as also significantly larger in the 25% canopy cover treatment (4595) than in the clearcut treatment (2588). Significantly more ECM (number per three lateral roots) were found in the untreated understory of the pine stand type (121) when compared to the shrub-removal treatments ( 103 and 107 for oak and pine stands, respectively) and untreated under story of the oak stand type (104) during the second growing season. Ou r results indicated that intermediate canopy levels stimulated the dev elopment of ECM, whereas complete removal of overstory and understory reduced such development. These results may aid forest managers in man ipulating the field mycorrhizal condition of oak seedlings through sil vicultural practices. (C) 1997 Elsevier Science B.V.