. Twenty-year growth of ponderosa pine saplings thinned to five spacings in central Oregon. Ponderosa pine Oregon; Forest thinning Oregon. Understory vegetation has had a pro- nounced deterrent effect on diameter growth especially at the wider spacings (fig. 6). This was most evident during the first three measurement periods and appears to have diminished in the last two. Severe drought during the last 4-year period may have influenced these relationships. Also, during this same period lack of snow cover and below freezing temperatures defoliated much of the ceanothus and some manzanita. Alth
. Twenty-year growth of ponderosa pine saplings thinned to five spacings in central Oregon. Ponderosa pine Oregon; Forest thinning Oregon. Understory vegetation has had a pro- nounced deterrent effect on diameter growth especially at the wider spacings (fig. 6). This was most evident during the first three measurement periods and appears to have diminished in the last two. Severe drought during the last 4-year period may have influenced these relationships. Also, during this same period lack of snow cover and below freezing temperatures defoliated much of the ceanothus and some manzanita. Although diameter growth at wider spacings increased during the first three periods, it has diminished since that time. This has been most evident and consistent during the last two periods of observation. A similar trend was observed in the Methow Valley study (Barrett 1981). This is probably due to increasing age and stand density (basal area). Twenty years after thinning the effect of understory vegetation on average diameter is most evident at the wider spacings. For example, where 62 trees per acre were left, average diameter was inches where the understory vegetation was controlled, compared to inches where vegetation was allowed to develop naturally (fig. 7). Where 125 trees per acre were left, the comparison is to inches. If diameters are projected to 28 years after thinning, when trees may be merchantable, diameters are estimated to be:4 4 Projections of tree diameter and basal area beyond 20 years after thinning were made by equations fitted to the data and by examining trends of diameter and basal area growth. Unforeseen climatic changes during the next decade could influence these estimates. Spacing Feet Trees per acre 1000 500 250 125 62 Vegetation left Vegetation controlled Estimated (inches) If we look at these results in terms of management goals and existing stock level guide
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