Abstract:
Douglas-fir and western hemlock growth and stocking were examined on two neighboring clear-cut watersheds in the Western Cascades of Oregon and related to the intensity of burning and logging disturbance, habitat type, soil type, aspect and the influence of percent cover of both invading Ceanothus species and residual Acer circinatum. The nitrogen content of the surface soil, both total and mineralizable nitrogen, and current and year old Douglas-fir foliar nitrogen contents as percent dry weight were determined. Total nitrogen content was determined by micro-kjeldahl techniques. Mineralizable nitrogen was obtained through anaerobic incubation at 40 C for seven days. Douglas-fir establishment and growth were not limited on severely burned or compacted sites. A strong preference for unburned sites was shown by western hemlock, however. Appreciable Douglas-fir stocking, basal diameters, basal areas and relative densities were supported by Rhma/Gash, Acci/Gash and Cola habitat types. Cola communities also contained the greatest western hemlock stocking and basal area values. Soil type influenced Douglas-fir but not western hemlock. Largest Douglas-fir basal diameters, basal areas and relative densities were found on lateritic soil types. Both Douglas-fir and western hemlock stocking were favored on northerly exposures, though the basal diameters of Douglas-fir were maximized on level and westerly aspects. The percent cover of snowbrush (Ceànothus velutinus), redstem ceanothus (C. sanguineus), deerbrush (C. integerrimus) and vine maple (Acer circinatum) was also examined in relation to site factors. Cover values of all Ceanothus species were highest on disturbed and! or burned sites. For the most part, the three Ceànothus species had varying preferences in terms of habitat type, soil type and aspect expressed as degrees of cover. Percent vine maple cover was only influenced by burning and habitat type, preferring sites undisturbed by burning and Rhma/Gash, Acci/Gash and Acci/Bene communities in which it was also an important component before logging. Nultiple regressions indicated that no site factor, shrub cover or nitrogen variable accounted for more than sixteen percent of the total explained variation in conifer growth or stocking. Site factors could explain the most variation. greater Douglas-fir basal diameters occurred on plots with snowbrush cover, while the presenèe of redstem ceanothus was associated with lower Douglas--fir basal diameters, basal areas and relative densities. The presence of Ceanothus species was generally not associated with increased soil or foliar nitrogen content, though Douglas-fir year old needles showed slightly higher mean values on sites with redstem ceanothus. Plots containing vine maple, however, contained greater quantities of mineralizable nitrogen. Nitrogen data indicates that though total soil nitrogen levels appear adequate for Douglas-fir nutrition, mineralizable and foliar nitrogen may be limiting at least during the growing season. hi1e partially ameliorated, results suggest that the effects from harvesting and slash burning have yet to subside, though factors including habitat type, soil type and aspect are becoming increasingly- important in determining the distribution of shrub cover as well as the growth and establishment of Douglas-fir and western hemlock.
