Population dynamics of understory Tsuga heterophylla, western hemlock, in the Cascade Mountains, Oregon

Recruitment and survivorship of Tsuga heterophylla, western hemlock-juveniles in relation to substrate preference as well as seed rain were exam-ined in old-growth forests, Oregon. While T. heterophylla seeds fell ontothe forest floor mainly from October to March, some seeds may be shed in anymonth, based on 32 months of sampling. Seed removal, probably through preda-tion, is extensive on all substrates. The pattern of recruitment variedwidely from year-to-year among five substrates for the 3672 seedlings ofwestern hemlock which emerged during the 27 months of censuses. Significantlyhigher recruitment occurred on Pseudotsuga menziesii logs in which the barkhad sloughed off and sapwood had rotted versus other rooting substrates. Sur-vival appears to be much lower for the juveniles of western hemlock 2 yearsafter germination.
Roots of 1-5+ year old seedlings of T. heterophylla that had establishedon logs in three decay classes or on mineral soil were compared for numbersand kinds of ectomycorrhizae. The roots of about half the seedlings survivingthe first growing season (2-7 months) contained no mycorrhizae. Nonmycorrhizalseedlings were most frequent on the least decayed logs. Mycotrophy appeared
to be advantageous however to western hemlock; first year mycorrhizal seedlingshad shoots 60% longer and roots 47% longer than first year nonmycorrhizalseedlings. All seedlings > 2 years old contained mycorrhizae.
Factors suppressing the growth of T. heterophylla (40-200 cm tall) wereseparated into aboveground and belowground components. Incident light onwestern hemlock juveniles was increased through formation of gaps in thecanopy; available nutrients were increased by trenching around individuals.Growth of the previously suppressed juveniles was increased by trenching andto a much lesser extent by altering the canopy. The combination of the twomanipulations did not show a strong synergistic effect.