Variation in Demography of Juvenile Tsuga Heterophylla Across the Substratum Mosaic

SUMMARY (1) Recruitment and survival of juvenile Tsuga heterophylla were followed on different substrata on the west slope of the Cascade Range, central Oregon, U.S.A. (2) Whilst seeds fell mainly from October to March, some were shed in all but six of thirty-two consecutive months. (3) Almost all juveniles occurred on decaying logs even though fallen timber may cover only 10-30% of the forest floor. (4) Recruitment on all substrata varied widely from year to year, as expected in a conifer with mast years of seed production. (5) Cohorts emergent in different months during one calendar year often showed greatly different survivorship curves even on the same rooting substratum. (6) Mortality was much higher for juveniles during the first 2 yr after emergence than in subsequent years. Microtine rodents probably account for most seed and seedling deaths. (7) The extent of decay of woody rooting-substrata does not influence percentage emergence, although survival of juveniles was most prolonged on Pseudotsuga menziesii logs with rotten heartwood. (8) The age-class structures of juvenile populations were functions of the fraction of the forest floor covered by fallen wood in each decay class.