Ectomycorrhizal populations on a chronosequence of disturbed areas in the central Cascades

Graduation date: 1981A greenhouse bioassay was used to investigate effects of natural and manmade disturbances on native ectomycorrhizal populations of Douglas-fir and western hemlock on a steep southeast slope in the\ud west central Cascade Mountains. Total and mycorrhizal root tips were counted on seedlings grown in soils collected from (a) two 100+ year old forests, (b) a 36-40 year old forest established after wildfire, (c) a recent clearcut in which residue had not been\ud burned, (d) a recent clearcut which had been broadcast burned, and (e) a 20 year old Douglas-fir plantation. With Douglas-fir, the greatest number of total and ectomycorrhizal root tips were produced on seedlings grown in soils from the unburned\ud clearcut. The least number of tips were produced in soils from the plantation and one of the old forests. The natural burn,\ud the other old forest, and clearcut and burn plots were intermediate in total and mycorrhizal root production. The pattern of root tip formation on western hemlock was quite different. Plots with no recent burn history (the clearcut with no burn, the natural burn, and the two old growth forests) produced equivalent numbers of total and mycorrhizal root tips. Total root\ud and mycorrhizal tip formation was depressed in soils from the clearcut and burn and from the young plantation. Soil variables measured, particularly nitrogen, carbon, and\ud moisture, were highly correlated with both root dry weight and total mycorrhizal tips of Douglas-fir and western hemlock and occurrence of Cenococcuni geophilum Fr., a mycorrhizal fungus common to both seedling species. Differences among soils of the disturbance units obscured the relationship between root size, root tip numbers, and\ud mycorrhizae. Data from the greenhouse bioassay, however, particularly regarding occurrence of Cenococcum, and the litter experiment indicated that differences in these parameters were at least in part due to differences in mycorrhizal response. Differential responses of Douglas-fir and western hemlock species also suggested\ud that the mycorrhizal fungi, save for Cenococcum, were different for each seedling species