Serpentine ecosystem responses to varying water availability and prescribed fire in the US Mid-Atlantic region

Grasslands growing atop serpentine bedrock are subject to edaphic stresses and, as a result, are low productivity ecosystems. Nutrient limitations are so severe in some serpentine grasslands that plant growth is unaffected by increased water availability. However, little is known of how serpentine grasslands in eastern North America function and respond to environmental changes, including variation in water availability. Serpentine barrens of the U.S. Mid-Atlantic region are globally rare ecosystems currently threatened by forest encroachment. Prescribed fire has been applied to maintain these ecosystems, although ecosystem responses to prescribed fire remain unquantified. Monthly measurements of CO2 exchange and leaf area index (LAI) were made at managed and unmanaged sites over two years. Environmental factors influencing CO2 exchange rates were monitored, and soil composition was also assessed. Unusually dry conditions in 2012 led to suppressed CO2 exchange rates and LAI across sites, while wet conditions in 2013 resulted in higher CO2 exchange rates and LAI. Seasonal maxima for net ecosystem production (NEP), ecosystem respiration (ER), and gross ecosystem production occurred in the late summer in 2012 and mid-summer in 2013. Over the study period, maximum NEP and ER were 13.68 and -9.40 lmol CO2 m(-2) s(-1), respectively, while LAI reached 2.18 m(2) m(-2). Across sites, carbon storage in the top 10 cm of soil was 33.1-41.3 Mg C ha(-1). Prescribed fire did not affect CO2 exchange, LAI, or soil carbon storage in the four to six years following a fire. In contrast to prior studies, Mid-Atlantic serpentine barrens responded positively to increased soil water availability and were more productive than expected. These ecosystems were functionally similar to tallgrass prairie, suggesting that fire management practices from prairies may be applicable in Mid-Atlantic serpentine barrens. Increased frequency of prescribed fire would likely reduce litter inputs to the soil and maintain the ecosystem's natural edaphic stessors, thereby combating forest encroachment.