THE INFLUENCE OF TIMBER HARVEST ON THE STRUCTURE AND COMPOSITION OF RIPARIAN VEGETATION IN THE COASTAL REDWOOD REGION A Report For the Save-the-Redwoods League

The effects of various timber management histories on the dynamics of riparian forests was analyzed to determine if the length of time since timber harvest and the width of the riparian buffer zone had a measurable effect on variables such as canopy cover, solar radiation, the ratio of hardwood to conifer dominance, and the occurrence of individual species. Ten sites were sampled in the coastal redwood forest type within a range of post-harvest age groups and riparian buffer strip widths. Data was collected using randomly selected sample plots adjacent to perennial coastal steams. Two data analysis methods were used to examine the relationship between the sample variables and the experimental parameters "years since harvest", and "width of harvest". These methods included a correlation Fisher's r to z test, and comparison between sites with an unpaired two-tailed t-test. Comparisons between sites indicate that a variety of stand structural and compositional characteristics are significantly affected both directly and indirectly by timber harvest. Variables found to be associated with these experimental parameters included canopy cover, the ratio of hardwood to conifer dominance, stand density, quantity of large woody debris, and the frequency and dominance of a variety of species. Canopy cover was negatively correlated to "years since harvest." The highest level of canopy cover was found on the youngest sites and the lowest level on the old-growth sites. The hardwood to conifer dominance ratio and the basal area of Alnus rubra (red alder) were correlated negatively to both "years since harvest" and "buffer width" indicating that timber harvesting favors hardwood species. Understory species such as Oxalis oregana (redwood sorrel), Anthyrium filix-femina (lady fern), Vaccinium parviflorum (billberry) were found preferentially in older forests and sites with wider buffer zones, while species such as Hedera helix (English ivy), Pampas cortedaria (pampas grass), and Myosotis latifolia (forget me not) were found preferentially in younger forests and areas with smaller buffer zones. The results of this study indicate that the edge effects associated with riparian forests are similar to those experienced by their upland counterparts. Community composition and structure are directly and indirectly affected by timber harvest history.