Climate Change, Carbon Sequestration, and Wildfire Management in Sierran Mixed Conifer Forests

2 I. Abstract We measured the impacts of prescribed fire and small clear-cut tree harvesting on carbon dynamics in a mixed-conifer forest in the central Sierra Nevada. Soil CO 2 efflux, above ground tree biomass, annual tree radial growth, and ecosystem carbon stored as litter, fine root and in the mineral soil were measured in four treatment sites: an un-manipulated control, a prescribed fire site, and two harvested sites, in one of which the soil was mechanically ripped to reduce soil compaction, a common practice done on industrial forest lands in the Sierra Nevada. Biomass and radial tee growth was also measured in a thinned site and a thinned and burned site. The biomass was determined one year before treatment, in 2001 one year after, in 2003, and finally seven year after treatment in 2009. Treatments affected pools and exchanges of carbon, reducing biomass stocks and the capacity of the forest to uptake carbon, but also to released C from the soil surface. Soil respiration was decreased by both fire and harvest (circa 20%), in part because the disturbances altered energy input and water availability. Prescribed fire and thinning reduced stand biomass, removing mostly small unselected (fire) or selected (thinning) trees. The thinning effects were concentrated in the first post-treatment year, whereas fire effects on tree radial growth showed responses between the first and the seventh years after fire. In the seven years post-disturbance interval, fire decreased tree density 30%, radial growth 12%, and stand radial growth 27%. Thinning reduced only 9% tree density, and increased tree radial growth 20% and stand radial growth 6%, compared to the control site. Effects of fire and thinning on biomass and biomass productivity were similar. After seven years, the biomass of both the fire and the thinned site reached pre-treatment levels. Excluding the first post-disturbance year, the thinned site stored more carbon than the control site, were radial growth has being declining over the same period. The effects of fire combined with thinning were higher than for the singe treatment, and growth, biomass and productivity were decreased between 30 and 40% compared to the control site. Forest ecosystems constitute a major reservoir of the global terrestrial carbon (Houghton et al. 1990; Tans et al. 1990) and have the potential to sequester anthropogenic carbon emissions. Therefore, understanding carbon cycling in forest ecosystems is critical for estimating the future global carbon budget. In addition most …