Consequences of wildfire on ecosystem CO₂ and water vapour fluxes in the Great Basin

Increased fire frequency in the Great Basin of North America's intermountain West has led to large-scale conversion of native sagebrush (Artemisia tridentata Nutt.) communities to postfire successional communities dominated by native and non-native annual species during the last century. The consequences of this conversion for basic ecosystem functions, however, are poorly understood. We measured net ecosystem CO₂ exchange (NEE) and evapotranspiration (ET) during the first two dry years after wildfire using a 4-m diameter (16.4 m³) translucent static chamber (dome), and found that both NEE and ET were higher in a postfire successional ecosystem (-0.9-2.6 μmol CO₂m^-2 s^-1 and 0.0-1.0 mmol H₂O m^-2 s^-2, respectively) than in an adjacent intact sagebrush ecosystem (-1.2-2.3 μmol CO₂m^-2 s^-1 and -0.1-0.8 mmol H₂O m^-2 s^-2, respectively) during relatively moist periods. Higher NEE in the postfire ecosystem appears to be due to lower rates of above-ground plant respiration while higher ET appears to be caused by higher surface soil temperatures and increased soil water recharge after rains. These patterns disappeared or were reversed, however, when the conditions were drier. Daily net ecosystem productivity (NEP; g C m^-2 d^-1), derived from multiple linear regressions of measured fluxes with continuously measured climate variables, was very small (close to zero) throughout most of the year. The wintertime was an exception in the intact sagebrush ecosystem with C losses exceeding C gains leading to negative NEP while C balance of the postfire ecosystem remained near zero. Taken together, our results indicate that wild-fire-induced conversion of native sagebrush steppe to ecosystems dominated by herbaceous annual species may have little effect on C balance during relatively dry years (except in winter months) but may stimulate water loss immediately following fires.