The Effect of a Prescribed Burn in Southwestern Ponderosa Pine on Organic Matter and Nutrients in Woody Debris and Forest Floor

Abstract After 100 years of fire exclusion, controlled burning in the fall was used as a first step in the reintroduction of periodic burning in a southwestern ponderosa pine ecosystem near Flagstaff, Arizona, U.S.A. Organic matter storage in woody debris was decreased 63 percent from 2,325 g/m² to 869 g/m², with a disproportionate (99 percent) decline in the large, rotten wood. Nutrient storage in the woody debris decreased by 80 percent for N, 62 percent for P, 70 percent for Ca, 71 percent for Mg, and 74 percent for K. Forest floor storages were less drastically affected, with organic matter content declining 37 percent from 3,170 g/m 2 to 1,990 g/m² immediately after burning. Nutrient content of the forest floor was not significantly affected by burning due, in part, to the transfer of nutrients from woody debris to the forest floor. By 7 months after burning, the forest floor had declined by an additional 440 g/m² of organic matter, most likely from microbial mineralization. Also during this period forest floor storages for all nutrients declined significantly, except K which was unchanged. Mg exhibited the greatest proportional decline (40 percent), followed by N, P, and Ca, all of which decreased by approximately 25 percent. Numerous potential benefits to productivity may be associated with this prescribed burn--reduced fire hazard, accelerated nutrient mobilization, and reduced forest floor interception of precipitation. Forest Sci. 30:183-192.