Arsenic levels in urine of forest workers applying silvicides.

THINNING overdense stands to improve growth rate of trees is one way in which the nation's supply of merchantable timber can be increased significantly.' In the Pacific Northwest alone, several million acres of such dense precommercial stands would benefit from thinning. Two basic methods are used to thin dense forest stands—felling unwanted trees with saws or killing them with chemicals. Economics aside, thinning by felling concentrates slash on the ground and presents a fire hazard, unsightly appearance, and decreased access to forage for livestock and game animals. Chemically killed trees are left standing. The leaves or needles fall first, then the branches, and eventually the stems. Chemical thinning allows better use of for-