EFFECTS OF SOIL TRENCHING ON OCCURRENCE OF ECTOMYCORRHIZAS ON PSEUDOTSUGA-MENZIESII SEEDLINGS GROWN IN MATURE FORESTS OF BETULA-PAPYRIFERA AND PSEUDOTSUGA-MENZIESII
Sw. Simard et al., EFFECTS OF SOIL TRENCHING ON OCCURRENCE OF ECTOMYCORRHIZAS ON PSEUDOTSUGA-MENZIESII SEEDLINGS GROWN IN MATURE FORESTS OF BETULA-PAPYRIFERA AND PSEUDOTSUGA-MENZIESII, New phytologist, 136(2), 1997, pp. 327-340
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.