EFFECTS OF SOIL TRENCHING ON OCCURRENCE OF ECTOMYCORRHIZAS ON PSEUDOTSUGA-MENZIESII SEEDLINGS GROWN IN MATURE FORESTS OF BETULA-PAPYRIFERA AND PSEUDOTSUGA-MENZIESII

Seedlings of Pseudotsuga menziesii (Mirb.) France were grown for six-1 6 months in untrenched and trenched treatments in three 90-120-yr-old mixed forests dominated by Betula papyrifera Marsh. and P. menziesii i n the southern interior of British Columbia. Each forest was character ized by mesic conditions and low light intensity (PAR < 200 mu mol m(- 2) s(-1)) in the understorey. The objective of the study was to evalua te the influence of overstorey tree roots on (i) ectomycorrhizal funga l composition, richness and diversity, and (ii) photosynthesis and gro wth of understorey P. menziesii seedlings. Seventeen ectomycorrhizal m orphotypes were recognized on seedlings in the untrenched treatment, a nd nine in the trenched treatment over the three sites. Six types occu rred in both treatments, of which on average Rhizopogon vinicolor Smit h type was 20 times more abundant and Thelephora type six times less a bundant in the untrenched as in the trenched treatment. Of types that formed strands or rhizomorphs, eight occurred in the untrenched treatm ent, where they occupied on average 23% of root tips, and only four oc curred in the trenched treatment over 4% of the root tips. Mean richne ss, diversity, and evenness of ectomycorrhizal associates per seedling were approx. twice as great in the untrenched as in the trenched trea tment. Net photosynthetic rate of P. menziesii seedlings was greater i n the untrenched than in the trenched treatment in July and August, bu t not in September 1994. Height, diameter and biomass of seedlings did not differ between treatments, but height:diameter ratio was greater in the untrenched treatment at time of harvest. The effect of trenchin g on seedling performance was attributed mainly to differences in ecto mycorrhizal colonization patterns because trenching had no significant effect on soil nutrient concentrations (total C, total N, NH4-N, avai lable N, exchangeable Ca, exchangeable Mg, exchangeable K), C:N ratio, soil pH or light availability. Nor was there a significant difference in soil water in August, when seedlings in untrenched plots had highe r net photosynthetic rates than those in trenched plots. Results sugge st that influence of overstorey trees and pattern of ectomycorrhizal f ormation are important to P. menziesii seedling performance in deeply shaded forest environments.