. Twenty-year growth of ponderosa pine saplings thinned to five spacings in central Oregon. Ponderosa pine Oregon; Forest thinning Oregon. Figure 3.—Plot thinned to spacing, with understory vegetation controlled. Some understanding of tree distribu- tion and crown closure for various spacings and vegetation treatment can be gained by comparing sketches in figure 2. Although trees left were sometimes not in the exact location for equal spacing, they were never closer than two-thirds the distance of treat- ment spacing. Crown closure 20 years after thinning varied from 86 percent where
. Twenty-year growth of ponderosa pine saplings thinned to five spacings in central Oregon. Ponderosa pine Oregon; Forest thinning Oregon. Figure 3.—Plot thinned to spacing, with understory vegetation controlled. Some understanding of tree distribu- tion and crown closure for various spacings and vegetation treatment can be gained by comparing sketches in figure 2. Although trees left were sometimes not in the exact location for equal spacing, they were never closer than two-thirds the distance of treat- ment spacing. Crown closure 20 years after thinning varied from 86 percent where trees were feet apart and vegetation was controlled to only 18 percent where trees were feet apart and vegetation was not controlled. Logging of overstory and thinning of saplings was started in the fall of 1957 and completed the fall of 1958. Thus, there was one growing season between thinning and initial measurement on some plots and up to two seasons on others. The one-year delay in com- pleting logging and thinning affected all spacings and did not inadvertently bias growth measurements. All recent logging and thinning slash was re- moved from the plots and burned. Diameters and heights of all trees were measured in the fall of 1959 and every four growing seasons for the next 20 years. Diameters were measured with a steel tape to the nearest one- tenth inch, and heights with a sec- tioned aluminum pole to the nearest one-tenth foot. Tree volume for this report was com- puted using an improved volume equa- tion for second-growth ponderosa pine recently developed by DeMars and The data base for this equation was obtained from north- central Washington and central and eastern Oregon. Therefore, volume esti- mates presented here will differ from those in previous publications on this study (Barrett 1970, 1973). Percent cover of understory vegetation on 15 plots was measured by system- atic sampling of 100 points per plot (Heady et al. 1959). 3 Donald J. DeM
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