Recovery Plan for the Northern Spotted Owl - DRAFT

A. Douglas-fir and Western Hemlock Stands(Oliver et al .1991)The following examples from Douglas-fir and western hemlock stands 30 to 70years old (Oliver et al. 1991) show trends of stand development under differentdensities. Diagrams are drawn to scale using information from stand simulators.Stands are projected with no treatment (Figures G.8, G.9, G.10). Inaddition, the 30-year-old stand (Figure G.8) is projected with two thinningregimes (low and moderate density); the 70-year-old stand is projected with athinning regime (moderate density) and a treatment to produce a multiplecanopystand (Figure G.9). In these stands, the second story would result fromrelease of conifers and hardwoods, or planting and seeding of conifers followingthinning. Development of a young plantation is shown in Figure G.10. Here,too, thinnings stimulate large tree and early understory development. Thesesimulations indicate that there are several advantages of manipulating overstorydensity to produce owl habitat:a) The size of the average tree, and the size of the largest 5 percent of thetrees in the stands, are increased. For example, the 30-year-old stand(stand B, Figure G.8) under a low density thinning regime is simulated atage 90 (year 2050) to have an average diameter of 36 inches and thelargest 5 percent of the trees have diameters of about 44 inches; whileStand A, with no activity, has trees of 18-inch average diameter and for thelargest 5 percent of the trees have 27-inch diameters.b) In stands which have had trees thinned, the canopy is "deeper," and treeshave larger crowns. For example, stands managed under the low-densitythinning regime, at age 90 years (year 2050), have crowns estimated fromabout 60 to 140 feet above ground (i.e., more than 60 percent of the lengthof the tree will have live crown on it); while on the untreated stand crownswill be about 110 to 140 feet above ground (see Figure G.8).c) Reducing overstory density will enhance development of an understory andencourage the development of multilayered stands typical of those used byowls (Figure G. 1). Without thinning, stands may be either too dense fortree and shrub seedlings to become established, or those that are presentwill grow very slowly.These stands likely will not produce large snags or logs on the forest floornaturally. Mortality due to crowding will kill trees less than 11 inches indiameter. However, stands at low density will have trees 25 inches diameterbreast height (dbh) and larger, some of which will be killed by pathogens andinsects, or which can be girdled or topped to make snags.Oliver et al. (1991) predicted that the risk of wind damage to unmanagedstands is greatest when they are about 30 to 70 years old (see Ruth and Harris1979). After about 70 years, the larger trees in the stand are likely to becomestable.The maximum possible costs and maximum possible returns for these scenarioshave been estimated (Table G.1).B. Douglas-fir from Oregon Coast Range (Birch 1991)Example 1: A 60-year-old Douglas-fir and western hemlock stand, on aproductive site (site index 130 feet at 50 years) with 280 trees per acre. Thegoal of the simulation was to produce a multilayered stand structure (similarto stand A in Figure G. 1 1) as quickly as possible.* At 60 years the stand was thinned leaving a) the largest 10 trees per acre(26 to 30 inches in diameter), b) trees in the smaller sizes (48 trees per503