Introduction

This is one of a series of reports on a cooperative levels-of-growing-stock study on Douglas-fir in the Pacific Northwest designed to examine the effect of different levels of growing stock on wood production, tree size, and growth-growing stock ratios.

Report No. 1 presented the study plan, including analysis of data and description of installations. Report No. 2 covered the Hoskins study for the calibration period (1964-66) and the first treatment period (1967-70). This report covers the second (1971-73) and third (1974-75) treatment periods and includes summary data from the cali

bration and first treatment periods (see fig. 1 for pictorial comparison of plot 8 and plot 19 for 1963, 1966, and 1976).

The Hoskins study area is located. approximately 22 miles west of Corvallis near Hoskins, Oregon, on land owned by Starker Forests. The area is immediately east of the summit of the Coast Ranges (fig. 2) on a southern aspect with slopes from 15 to 55 percent. At the time the study was established (1963), the stand was 14 years of age at breast height (total age, 20 years) and contained an average of 1,727 trees per acre (fig. 3). The study area is site II. The stand is of natural origin following wildfires.

Figure 3.--Plot 22 (control) in 1963. Crop trees have a white circumferential band painted at the point of measurement. Noncrop trees have a different color band for each treatment and identifying numbers of corresponding color. This facilitates work in the plots and identifies treatments for observers.

Methods1/

During the summer of 1963, 27 plots, 1/5-acre in size, were established (fig. 4). Initial density was controlled by a calibration thinning to a prescribed level of basal area. The d.b.h. of each tree has been measured to the nearest 0.1 inch at the end of each growing season. Total height of selected trees was measured at the end of the 1963, 1966, 1970, 1973, and 1975 growing seasons (table 1). Thinnings were made when the height of the crop trees increased by 10 feet. Thus, thinnings were made when the height of the crop trees increased by 10, 20, 30, 40 feet, etc. Height growth (13.1 feet) during the first treatment period, however,

1The treatment schedule is found on the inside front cover. See Reports 1 and 2 for additional details. Other LOGS reports are listed on the back of the title page.

exceeded this because the plots were not thinned for 4 years. This irregularity in relation of height growth to thinning interval was corrected in the third treatment period which reduced the period to two growing seasons (1974-75).

Results and Discussion SECOND TREATMENT PERIOD

The second treatment period was three growing seasons (1971-73). The stand table at the beginning of the second treatment period is presented in table 9 and at the end of the period in table 11. Data for the trees removed during the second thinning are presented in table 10. Only one tree per acre per year died on treatments 2 and 5 compared with 62 trees per acre per year on the control plots (table 6).

All Live Trees

The basic data for all live trees (crop2 and noncrop) for each treatment by growing season are shown in table 2. The table shows the wide range in basic data among the various treatments and the control. For instance, at the end of the second treatment period, the total number of trees per acre varied from 118 to 1,087 and the quadratic mean d.b.h. from 6.6 to 11.8 inches. Basal area per acre varied from 85.1 to 256.3 square feet and total volume per acre from 2,303 to 6,955 cubic feet. The spread in quadratic mean diameter between treatments and control continues to widen. Most of this increase is due to growth because the quadratic mean diameter before and after each thinning are nearly the same. Total volume in cubic feet varies according to stand density in either number of trees or basal area--the higher the density, the greater the volume.

3

The gross periodic annual increment" (PAI) and cumulative volume yield (see appendix 2 for calculation procedures) for all trees are given in table 4. The cumulative volume yield data do not include the volume removed during the calibration thinning. The volume removed, however, was estimated to be 1,238 cubic feet per acre--the difference between the average volume of the eight treatments and the control (table 2). The diameter PAI varied from 0.42 inch to 0.62 inch on treatment plots compared with 0.15 inch for the control plots. The basal area and cubic-foot volume increment among the eight treatments was largest for treatment 7 and smallest for treatment 1. The gross basal area increment of the control plots was greater than that of any of the treatments for the 1971 growing season. For the 1973 growing

season, the gross basal area increment for treatment 7 was greater than that for the control plots; yet the total basal area per acre of the live trees was 148.7 and 247.4 square feet for treatment 7 and the control plots at the beginning of the 1973 growing season. The gross PAI for the second treatment period varied from 293 cubic feet per acre for treatment 1 to 583 cubic feet for the control plots. The basic stand data by treatment and plot for all live trees are shown in table 7.

Crop Trees

The stand data by treatment for the crop trees are given in table 3. At the end of the second treatment period (1973), the cubic-foot volume of the crop trees in the treatment plots was 1.6 to 1.9 times that of the crop trees in the control plots.

The crop tree increment data are shown in table 5. The basal area increment of the crop trees for the second treatment period for the thinned plots was 2.2 to 3.5 times greater than the basal area increment of the crop trees on the control plots. Of greater significance is that the PAI in cubic feet for the crop trees in the thinned plots is from 2.1 to 2.8 times that of crop trees in the controls.

The basic data for the crop trees by treatment and plot for the second treatment period are found in table 8.

THIRD TREATMENT PERIOD

The third treatment period was two growing seasons (1974-75) (see 'Methods"). Data for the trees removed during the third treatment period are presented in table 10. The stand table at the beginning of the third treatment period is given in table 13 and at the end of the period in table 14. Only one tree per acre per year died on treatments 5, 7, and 8 compared with 74 trees per acre per year on the control plots (table 6).