Pre-commercial thinning effects on growth, yield and mortality in even-aged paper birch stands in British Columbia

The aim of this study was to quantify 5-year growth, yield and mortality responses of 9- to 13-year-old naturally regenerated, even-aged paper birch (Betula papyrifera Marsh.) stands to pre-commercial thinning in interior British Columbia. The study included four residual densities (9902-21,807 stems ha(-1) (unthinned control), 3000, 1000 and 400 stems ha(-1)) and four sites with 3-fold within-site replication in a randomised block design. The largest, straightest, undamaged trees were selected to leave during thinning. Thinning reduced stand basal area from 5.90 m(2) ha(-1) in the control to 2.50, 1.53 and 0.85 m(2) ha(-1) in the three thinning treatments, representing 42, 26 and 15% of control basal area, respectively. After 5 years, total stand volume per plot remained lower in the three thinning treatments than the control (50.20, 30.07, 18.99 and 11.86 m(3) in the control, 3000, 1000 and 400 stems ha(-1) treatments), whereas mean stand diameter, diameter increment, height, and height increment were increased by thinning, and top height (tallest 100 trees ha(-1)) was unaffected. When a select group of crop trees (largest 250 trees ha(-1)) in the thinning treatments was compared with the equivalent group in the control, there was a significant increase in mean diameter, diameter increment, basal area, basal area increment, and volume increment. Mean height, height increment, top height, and total volume were unaffected by thinning. Crop tree diameter increment was the greatest following thinning to 400 stems ha(-1) for all diameter classes. Thinning to 1000 stems ha(-1) resulted in lower diameter increment than thinning to 400 stems hat but tended to have higher volume increment. Dominant trees responded similarly to subdominant trees at 400 stems ha(-1), but showed the greatest response at 3000 stems ha(-1). Results suggest that pre-commercial thinning of 9-13-year-old stands to 1000 stems ha(-1) would improve growth of individual trees without seriously under-utilising site resources. (C) 2003 Elsevier B.V. All rights reserved.