Forest ecosystems in the Colorado Front Range have evolved to thrive in the unique climatic conditions of the region and natural disturbance regimes that existed prior to European settlement. Knowledge of how forests were structured in the past and the factors that affect their establishment and growth is essential to their management. Part of Notable Lectures & Performances series, Colorado College. Recorded January 25, 2007.
Given a trend of increasing wildfire frequency and intensity in the Western U.S., it is important to understand how the recovery and resilience of forest C stocks are impacted by severe fire. The purpose of this study was to look at how fire affected the size and relative bioavailability of soil C stocks, influencing rates of C mineralization. Study sites were in a montane Ponderosa Pine forest of Central Colorado and a subalpine Lodgepole Pine forest of Northern Colorado. I measured terrestrial C stocks and soil C bioavailability in plots burned in the 2002 Hayman and Hinman fires as well as undisturbed reference plots in each forest. Analyses included estimates of charcoal, aboveground biomass C, soil (top 10 cm) and water-extractable C and N. Soil incubation experiments were also conducted to measure the rate of microbial respiration per g soil C (a proxy for relative bioavailability). Total terrestrial C was ~53% lower in the montane ecosystem relative to the subalpine ecosystem, and >50% lower in burned plots relative to reference plots in both ecosystems. Soil C bioavailability did not vary with fire history in the subalpine forest, but was ~44% higher in burned plots in the montane forest. These results suggest that fire resulted in a significant reduction in terrestrial C storage. In addition, soil C bioavailability depended on plant regrowth. In the montane ecosystem, there were no tree seedlings in burned plots; as a result, C inputs to soil came primarily from grass and forb litter, which is more bioavailable than woody material. Comparatively rapid reestablishment of saplings in the subalpine forest meant that the quality of the soil C pool was similar in burned and undisturbed plots. Therefore, ecosystem response to wildfire differed with forest type, and lasting effects of wildfire on the quantity and bioavailability of soil C was determined by regrowth of trees.