Estimating historical snag density in dry forests east of the Cascade Range

Harrod, Richy J.; Gaines, William L.; Hartl, William E.; Camp, Ann. 1998. Estimating historical snag density in dry forests east of the the Cascade Range. Gen. Tech. Rep. PNW-GTR-428. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacfic Northwest Research Station. 16 p. Estimating snag densities in pre-European settlement landscapes (i.e., historical conditions) provides land managers with baseline information for comparing current snag densities. We propose a method for determining historical snag densities in the dry forests east of the Cascade Range. Basal area increase was calculated from tree ring measurements of old ponderosa pine (Pinus ponderosa Dougl. ex Laws.) trees. Historical stand structure was assumed to be open and parklike, with low densities favoring larger diameter trees, and it was considered relatively stable at the landscape level. Snag density (S) was calculated by holding forest stand structure relatively constant (basal area range 13.8 to 18.4 square meters per hectare [60 to 80 ft/acre] and diameter size class distributions with q-factors of 1.1 or 1.2), assuming snag recruitment could be no greater than annual basal area increase, and estimating that all snags fall by 45 years; S = (a)*(1 + Σri), where a is annual recruitment and Σri is a sum of the ratios of snags remaining from previous recruitment years (i). If eight representative snag sizes are selected, snag density in historical landscapes ranged from 14.5 to 34.6 snags per hectare (5.9 to 14.1 snags per acre).