Effects of Fire Interval Restoration on Carbon and Nitrogen in Limestone- and Basalt-derivedsoils of the Mogollon Rim, Arizona

Prescribed fire was returned into over-stocked ponderosa pine stands on the Mogollon Rim of Arizona for the purpose of restoring fire into the ecosystem and removing fuel buildups. Prescribed fires have been ignited at intervals of 1, 2, 4, 6, 8, and 10 years to determine the best fire return interval for Southwest ponderosa pine ecosystems. Two sites were treated; one on volcanic-derived soils, and the other on sedimentary-derived soils near Flagstaff, Arizona, starting in 1976 and 1977, respectively. Samples from upper 5 cm of the A horizons were analyzed for total carbon and nitrogen using an elemental analyzer. Soil carbon and nitrogen levels were highly variable and exhibited an increasing, but inconsistent, concentration trend related to burn interval. High spatial variability measured within treatments is probably due to micro-site differences (location of samples in the open, under large old-growth trees, in small-diameter thickets, in pole-sized stands, next to downed logs, etc.). Stratification of samples by micro-site differences could possibly reduce the within plot variability but add considerable complexity to the sampling design. INTRODUCTION The pre-European settlement ponderosa pine forests of the Mogollon Rim consisted of open stands of uneven-aged trees with a significant grass-forb understory. Light surface-fires occurred on an average interval of 2 to 12 years in Arizona and New Mexico (Weaver 1951, Cooper 1960, Dietrich 1980). These fires consumed forest floor material, burned most of the young regeneration, and promoted growth of a dense, grassy understory. Catastrophic crown fires were rare due to lack of ladder fuels, and the clumpy, widely spaced ponderosa pine canopy (Dieterich 1980; Sackett 1980). Fine fuels reduction from heavy sheep and cattle grazing and then modern forest fire suppression resulted in the development of dense, overstocked stands. Forest floor fuel loads that were 0.4 4.5 Mg/ha prior to 1870 have since increased by one to nearly two orders of magnitude. Average loadings of naturally fallen fuels were 49 Mg/ha two decades ago with some stands accumulating up to 112 Mg/ha (Sackett 1979, Sackett et al. 1996). Annual accumulations since then have been in the range of 1.3 to 7.8 Mg/ha/yr. Tree densities that were once <130 stems/ha have increased dramatically, especially in dense thickets with