. Twenty-year growth of ponderosa pine saplings thinned to five spacings in central Oregon. Ponderosa pine Oregon; Forest thinning Oregon. S 1 2 3 4 5 * n * 1 2 3 4 5 a 3 u> - Z - (0 75 3 C C A3 X) O a. - Period 2 3 4 5 1 2 3 4 5 1 2 3 4 5 Understory vegetation left Understory vegetation controlled 1000 500 250 125 Trees per acre Spacing (feet) 62 Figure 8 —Periodic annual basal area increment during five 4-year measurement periods. 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 1959 Years since thinning 20 24 28 4 8 Understory ve
. Twenty-year growth of ponderosa pine saplings thinned to five spacings in central Oregon. Ponderosa pine Oregon; Forest thinning Oregon. S 1 2 3 4 5 * n * 1 2 3 4 5 a 3 u> - Z - (0 75 3 C C A3 X) O a. - Period 2 3 4 5 1 2 3 4 5 1 2 3 4 5 Understory vegetation left Understory vegetation controlled 1000 500 250 125 Trees per acre Spacing (feet) 62 Figure 8 —Periodic annual basal area increment during five 4-year measurement periods. 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 1959 Years since thinning 20 24 28 4 8 Understory vegetation left Observed Projected. 1963 1967 1971 1975 1979 1983 198719591963 1967 1971 1975 1979 1983 1987 Measurement years Figure 9.—Net basal area attained during 20 years after thinning and predicted 8 years in the future, with vegetation allowed to develop naturally and vegetation controlled. Basal Area Basal area was significantly and posi- tively correlated with spacing (table 1). During the first four periods basal area increment increased steadily in all treat- ments where understory vegetation was controlled, with one exception (fig. 8), but during the last period increment declined in all treatments. Where under- story vegetation was left to develop naturally periodic basal area increment increased at all spacings during the first four periods but declined in the fifth period at only the two narrowest spacings. Periodic annual increment ranged from a high of square feet per acre during the fourth period at the closest spacing to a low of square foot at the widest spacing in the first period (fig. 8). The greater increments in basal area occurred where the initial basal area was greater. Understory vegetation consistently reduced basal area incre- ment at all spacings. This effect was most pronounced at the wider spac- ings, where reductions of around 50 percent occurred during some periods. Basal area at the closest spacing has accumulated more than 110 s
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