A practical modification of horizontal line sampling for snag and cavity tree inventory

Snags and cavity trees are important structural features in forests, but they are often sparsely distributed, making efficient inventories problematic. We present a straightforward modification of horizontal line sampling designed to facilitate inventory of these features while remaining compatible with commonly employed sampling methods for the living overstory. The method is simpler in its implementation than traditional horizontal line sampling. We develop unbiased estimators and present methods for dealing with special cases, including boundary overlap. A field test of the method shows it to have time efficiency comparable with or better than ordinary prism cruising, and it requires far fewer sample locations to achieve similar confidence limits. The method may also be useful for inventorying other rare or unusual trees.