Soil and growth studies in Douglas-fir <Pseudotsuga menziesii (Mirb.) Franco> stands near Molalla, Oregon

In 1957 three Douglas-fir stands (15-, 25-, and 40-year-old age classes) were selected on a tree farm in the Cascade foothills of northwestern Oregon to study factors affecting site productivity. Soil-moisture, soil-temperature and seasonal radial- growth pattern measurements were made with a Colman moisture meter and a dial gauge dendrometer on thinned and unthinned subareas. Soil temperatures were significantly increased by thinning in each stand. This effect decreased with soil depth and increased with increasing intensity of thinning. Root distribution studies eight years after thinning showed that a heavily thinned stand had relatively few roots in the lower profile between the remaining trees, in contrast to the complete distribution throughout the profile in the unthinned subareas. This would indicate less overall soil-moisture depletion with thinning since the available soil-moisture levels measured within the fully rooted zone were not significantly different for the various thinning treatments. As expected radial growth increased significantly with increasing intensity of thinning. The amount and proportion of late season growth tended to increase with increasing thinning intensity although the observed differences were not statistically significant. Late season growth, however, was significantly increased by increasing crown position (intermediate to dominant) regardless of thinning treatment. Irrigation of selected trees at a ridgetop area extended the spring maximum radial growth rate throughout most of the summer. On comparable non-irrigated trees growth retardation was detectable in the range of one-half to one atmospheres of soil-moisture tension. Growth cessation occurred when soil-moisture-tension values reached the two to five atmosphere range. Irrigation of trees in a young hillside bench stand slightly increased radial growth rate during the first two years, but decreased growth in relation to non-irrigated trees during the next two years apparently as a result of excess irrigation. Fertilization with high rates of nitrogen and phosphorus fertilizers increased the late season radial growth of irrigated and non-irrigated trees in both stands. The late season response is attributed to a dwindling supply of mineralized soil nitrogen in the late summer and early fall. Irrigation and fertilization combined gave approximately additive increases in radial growth which ranged from 35 to 60 percent. It is hypothesized that potential radial growth (except for short periods at the start and end of the growing season) is equivalent to the maximum spring rate. Departures from this maximum result from deficiencies in moisture, mineral nutrient elements, or heat. Thus the maximum growth rate is proposed as an important characteristic, reflecting the potential carbohydrate productive capacity of the crown when raw material levels exceed rate limiting supplies. The fertility status of each research area was assessed by various means including soil chemical and physical analyses, sampling and chemical analyses of forest floor and litter fall material and pot fertility tests. The relative ranking of these areas in terms of fertility was consistent with their site index or productivity rating. Monterey pine and Douglas-fir seedling test plants grown on the potted soils from the different horizons in each profile responded to nitrogen, phosphorus, and sulphur additions. In field tests nitrogen fertilization stimulated radial growth of Douglas-fir whereas phosphorus fertilizer at high application rates resulted in a growth depression.