Volume growth and response to thinning and fertilizing of Douglas-fir stands in southwestern Oregon.

Abstract

From data for 114 thinning and fertilizing trials in forests of southwestern Oregon and northern California with 5 or more years of observation, we produced equations to estimate gross cubic volume growth of 10- to 70-year-old Douglas-fir stands. These equations use stand escriptors (breast-height age, site index, and relative density) and treatment descriptors to estimate annual gross volume growth during a 10-year period for untreated and treated (fertilized or thinned, or both) stands. These predictions (SWOR) were compared with other growth predictions including DFSIM, a simulation model based on a broader, regionwide data base. Our predictions consistently showed greater gross and net growth of untreated Douglas-fir in this subregion than does DFSIM and generally showed greater volume gains from nitrogen fertilization of unthinned stands, especially on poor quality sites and in young stands. SWOR forecasts reduced gross volume growth during the 10-year period after thinning and predicts faster recovery after early thinning on good sites than on poor. Our data indicated that nitrogen fertilization could ncrease wood production in about 70 percent of unthinned and thinned Douglas-fir forests in this subregion. Gains in gross growth in a 10-year period after fertilization of 20-year-old site 85 stands with 200 Ib of nitrogen per acre were estimated as 800 and 650 cubic feet per acre, respectively, for unthinned and thinned stands.

Citation

Miller, R.E.; Clendenen, G.W.; Bruce, D. 1987. Volume growth and response to thinning and fertilizing of Douglas-fir stands in southwestern Oregon. Gen. Tech. Rep. PNW-GTR-221. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 38 p