EFFECTS OF EROSION ON ECTO-MYCORRHIZAL AND VA-MYCORRHIZAL INOCULUM POTENTIAL OF SOIL FOLLOWING FOREST-FIRE IN SOUTHWEST OREGON

The Longwood Complex wildfire in the Siskiyou Mountains of southern Or egon in August 1987 created an opportunity to study erosion and its ef fects on mycorrhizal fungus inoculum potential of a forest soil on ste ep slopes. As measured by the erosion-bridge method, most erosion occu rred in a single, intense storm in December after the fire and amounte d to an estimated 2 to 4 cm of surface soil. Captured eroded soil had a higher pH and P and Mg levels than residual soil. Seedlings of Liboc edrus decurrens and Pseudotsuga menziesii were planted on eroded plots with additions of captured eroded soil (ET) or pasteurized eroded soi l (PET) transferred to the planting holes. After one growing season, L ibocedrus seedlings formed nearly 4 times the vesicular-arbuscular myc orrhizae in ET treatments and more than twice as much in PET treatment s than in controls. Survival and basal area growth were significantly better in ET than in the other treatments, and both ET and PET produce d more seedling shoot growth than did controls. Pseudotsuga seedlings did not differ in measured characteristics between treatments; ectomyc orrhiza formation was slight, evidently the result of reduced inoculum potential resulting from the fire.