Coarse woody debris in Wyoming lodgepole pine forests

Downed logs, stumps, woody roots, and snags are present as coarse woody debris (CWD) in forest ecosystems. CWD provides forest floor structure, contributes to soil organic matter formation, and serves as important habitat for many organisms. Unlike natural fires, which create more CWD than they remove, clearcut timber harvesting may reduce CWD. This study investigates the effects of clearcutting and fires on CWD biomass and distribution in lodgepole pine forests. CWD biomass was measured in burned, clearcut, and unburned/uncut stands in the Medicine Bow National Forest (MBNF) and in Yellowstone National Park (YNP) in Wyoming. Maps of the spatial arrangement of all downed logs, stumps, live trees, and standing-dead trees were constructed for each stand. Estimates of the amount of CWD consumed or converted to charcoal by fire were made from a recently-burned stand in YNP. A simulation model, DEADWOOD, was created to simulate various clearcutting and fire regimes over a 1000-year period. Measurements following a single clearcut or fire revealed that downed CWD > 7.5 cm in diameter did not differ between the two, but fires created more potential CWD as standing-dead trees. Notably, only 7.6% of CWD was consumed during a single fire, and an additional 7.6% was converted to charcoal. After 1000 years of simulation, 100-year fire return intervals produced CWD that occupied more of the forest floor than did 200- or 300-year intervals. One-hundred percent occupancy of the forest floor by CWD occurred more quickly during the 100-year interval fires (1125 yr) than the 200- (1350 yr) or 300-year (1300 yr) interval fires. Simulations revealed that current harvest regimes require centuries longer for 100% of the forest floor to be occupied by wood (1800--3600 yr) than under fire regimes. Forest floor nitrogen storage was also sensitive to changes in the type and intensity of disturbances. CWD in the most highly-decomposed stages provided the largest storage of nitrogen (N), and amounts were higher during burn simulations than clearcuts, regardless of fire-return interval. Only when post-harvest CWD slash was set at twice the present amounts did clearcutting produce sufficient CWD to maintain forest floor nutrient levels similar to those produced by fire.