Economic and ecological effects of diameter caps: A Markov decision model for Douglas-fir/western hemlock forests

Abstract

We evaluated some of the effects of not harvesting trees of 41 cm (16 in.) dbh or larger with uneven-aged management of Douglas-fir/western hemlock forests. The opportunity cost of diameter caps was measured by the difference in maximum expected net present value (NPV) or equivalent annual timber income, with or without diameter caps. For the two policies, Markov decision models were used to determine the best decision rules and their effects on the NPV, the forest area with late-seral structure, and the forest area with northern spotted owl nesting habitat structure. The opportunity cost of diameter caps was computed for 64 initial stand states defined by basal area of small, medium, or large trees of shade-intolerant species (mostly Douglas-fir) or shade-tolerant species (mostly western hemlock). The opportunity cost fell in four distinct quartiles. The 16 stand states with high basal area in large shade-intolerant and large shade-tolerant trees had the highest opportunity cost ($798-$816 ha-1year-1 or 58-76%, expected over an infinite horizon), whereas the 16 stand states with low basal area in large shade-intolerant and large shade-tolerant trees had the lowest ($59-$89 ha-1year-1 or 11-13%). The diameter caps policy increased considerably the expected area of forest with late-seral structure, over an infinite time horizon, and the expected area with spotted owl nesting habitat structure, compared with their current levels in the study region. Copyright © 2008 by the Society of American Foresters.