Carbon stocks and fluxes in fire disturbed landscapes of Colorado, U.S.A.

In terrestrial ecosystems, ecological disturbances can strongly regulate material and energy flows. This often results from the reduction in biomass and associated ecological relationships and physiological processes. Researchers have noted an increase in the size and severity of disturbances, such as wildfire, in recent decades. While there is significant research examining post-disturbance carbon stocks and recovery, there is less known about the fate and quality of post-disturbance carbon pools. In an effort to understand the recovery and resilience of forest carbon stocks to severe wildfire we examined the carbon and black carbon (pyrogenic) stocks (e.g. above ground biomass, coarse woody debris, charcoal, soils) and export fluxes (stream export, soil leachate, soil respiration) within the burn scars and nearby reference sites of five 2002 Colorado fires. The fires encompass large precipitation and ecosystem gradients (relatively dry montane Ponderosa forests to relatively wet subalpine Lodgepole forests), allowing us to control for various state factors in our analyses. With the exception of the Hinman fire (subalpine, Lodgepole dominated), there is little forest regrowth more than a decade later, with only a few saplings found in burned plots; instead forbes and grasses dominate. Fire also reduces soil C stocks (by 16 to 68%) across all sites. In addition, with the shifts in vegetation we hypothesize that there will be corresponding changes in soil organic matter (SOM), altering the residence time of C in soil. Soil incubation experiments reveal that organic matter bioavailability is significantly greater in three of the burned sites, suggesting that the new sources of SOM are more bioavailable. Stable isotopic analyses of SOM and the evolved CO2 from the incubation studies will allow us to test this hypothesis. Fire also affects the amount and nature of dissolved and particulate organic matter (DOM and POM, respectively) leaving the watershed. For example, the dissolved organic matter exported from burned watersheds is less aromatic and has lower C:N than DOM exported from reference watersheds. In addition to measuring these fluxes, we assessed the fate of leached DOM via laboratory bioassays to determine the likelihood of additional CO2 losses to the atmosphere.