Aboveground and Belowground Carbon Pools After Fire in Mountain Big Sagebrush Steppe

Abstract

Postfire succession in mountain big sagebrush (Artemisia tridentata Nutt. subsp. vaseyana [Rydb.] Beetle) ecosystems results in a gradual shift from herbaceous dominance to dominance by shrubs. Determining the quality, quantity, and distribution of carbon (C) in rangelands at all stages of succession provides critical baseline data for improving predictions about how C cycling will change at all stages of succession under altered climate conditions. This study quantified the mass and distribution of above- and belowground (to 1.8-m depth) biomass at four successional stages (2, 6, 20, and 39 yr since fire) in Wyoming to estimate rates of C pool accumulation and to quantify changes in ecosystem carbon to nitrogen (C:N) ratios of the pools during recovery after fire. We hypothesized that biomass C pools would increase over time after fire and that C:N ratios would vary more between pools than during succession. Aboveground and live coarse roots (CR) biomass increased to 310 and 17 g C m-2, but live fine roots (FR) mass was static at about 225 g C m-2. Fine litter (< 1-cm diameter) accounted for about 70% of ecosystem C accumulation rate, suggesting that sagebrush leaves decompose slowly and contribute to a substantial soil organic carbon (SOC) pool that did not change during the 40 yr studied. Total ecosystem C (not including SOC) increased 16 g m-2 yr-1 over 39 yr to a maximum of 1 100 g m-2; the fastest accumulation occurred during the first 20 yr. C:N ratios ranged from 11 for forb leaves to 110 for large sagebrush wood and from 85 for live CR to 12 for bulk soil and were constant across growth stages. These systems may be resilient to grazing after fire because of vigorous regrowth of persistent bunchgrasses and stable pools of live FR and SOC.