The effect of thinning intensity upon understory growth and species composition in an Oregon Coast Range Pseudotsuga menziesii stand

Graduation date: 1975Cover, frequency, and biomass of under story vegetation were estimated on 14 study plots in a central Oregon Coast Range Pseudotsuga menziesii stand. The treatments included a control and three different thinning intensities. Basal area of active growing stock, defined as young-growth P. menziesii with diameter at breast height of 7.6 inches (19.3 cm) or greater, determines thinning intensity. Heavily thinned stands are maintained between 100 and 130 square feet per acre (23-30 m²/ha) of active growing stock, moderate thinnings between 130 and 160 square feet per acre (30-37 m²/ha), and light thinnings between 160 and 190 square feet per acre (37-44 m²/ha). The amount of canopy opening and of light reaching the understory were found to be negatively correlated with the log of active growing stock basal area. Despite much plot-to-plot variation in cover, several trends corresponding with thinning treatment were detected. Berberis nervosa and Eurhynchium oreganum are the dominant species on unthinned plots, whereas Pteridium aquilinum and E. oreganum predominate on all thinning treatments. Herbaceous species are most responsive to thinning, with highest cover in the moderate and heavy thinnings. Herbs showing the greatest increases with thinning are Pteridium aquilinum, Campanula scouleri, Collomia heterophylla, Festuca occidentalis, Hieracium albiflorum, and Trientalis latifolia. Further, a few species are confined to either thinned or to unthinned plots, but of these species, only Lotus crassifolius and Lupinus latifolius, both on thinned plots, occur with relatively high frequencies. No species that attains high cover or frequency on any treatment significantly decreases with thinning intensity. Trends in understory biomass are similar to those for cover and frequency, though sampling error was larger for the biomass estimates. In general, cover and frequency data on woody plants show no clear response to thinning. Biomass estimates, especially for the large shrubs, show a trend towards increased woody vegetation with thinning intensity. Data on stem density, however, best indicate that some large woody plants are responding to thinning. Large woody stems are scattered widely throughout the plots, but where the shrubs Acer circinatum, Corylus cornuta, and Holodiscus discolor, in particular, occur on thinned plots, they often are growing in clumps of many young, erect stems. Such clumps do not occur on unthinned plots. These findings suggest the hypothesis that the occurrence of these woody plants in the original forest was infrequent and that the clumps result vegetatively from the original stems or root systems in response to thinning. Understory competition does not prevent initial establishment of Pseudotsuga menziesii seedlings on the thinned plots. Although stocking levels for P. menziesii seedlings on the thinned plots are high, growth is slow, and few seedlings survive past the fifth year. Few tree seedlings are found on the unthinned plots