Patterns of survival, damage, and growth for western white pine in a 16-year-old spacing trial in Western Washington

Abstract

Information about effects of planted spacing on growth and development of western white pine is scant because white pine blister rust disease has severely limited the survival of trees in young plantations. In the early 1980s, we established a western white pine spacing trial in the southern Cascades of Washington, using seedlings from a seedlot comprised primarily of open-pollinated seed from trees screened and certified as resistant to white pine blister rust. Initial square spacings ranged from 2 to 6 m; treatment plots were 0.4 ha in size and were replicated three times (six times for the 3 m spacing). Permanent measurement plots were installed at age 8, and the lower portion of all stems was pruned in the loth growing season to reduce blister rust infections. The plots were remeasured at ages 11 and 16. Sixteen years after planting, survival averaged 80%; most mortality was associated with blister rust, but the rate of new infections and mortality diminished substantially between ages 11 and 16; 71% of the planted trees were free of blister rust at age 16. Trees averaged 8.4 m tall (ranging from 7.6 m in the 2 m spacing to 8.8 m in the 4 and 5 m spacings) and 12.7 cm dbh (10.3 in the 2 m spacing to 14.0 cm in the 5 m spacing). Periodic annual growth from 11 to 16 yr in the three wider spacings averaged 0.7 m in height and 1.0 cm in diameter. Antler rubbing by elk caused substantial damage to stems, but wounds on most trees were overgrown in 2 to 4 yr. Rub damage did not appear directly related to spacing but rather to stem diameter, with damage limited primarily to stems >6 and <15 cm. Early growth rates in this trial were much greater than those attained in older, natural stands (as inferred from site index curves and yield tables) and in progeny tests and other young silvicultural trials planted elsewhere. We believe rust-resistant stock of western white pine merits greater consideration for planting in the Douglas-fir region.