Carbon dioxide exchange in Mid-Atlantic serpentine barrens managed with prescribed fire

Background/Question/Methods

Serpentine barrens in the Mid-Atlantic region of the United States are home to rare plant and animal species, as well as endemic and disjunct plant species, but are threatened by forest encroachment in the absence of disturbance.  To maintain desired species and vegetation structure in these globally rare ecosystems, some barrens are managed with prescribed fire.  However, interactions between fire management and ecosystem function have not been assessed.  In an effort to better understand the effect of prescribed fire on carbon cycling in Mid-Atlantic serpentine barrens, rates of CO₂ exchange, as net ecosystem production (NEP), ecosystem respiration (ER), and gross ecosystem production (GEP) were measured monthly at each of six field sites in southeastern Pennsylvania.  Half the study sites were managed with prescribed fire in the preceding 4-5 years, while half were unmanaged.  The role of environmental variation in determining CO₂ exchange rates was also assessed.

Results/Conclusions

Rates of CO₂ exchange were not significantly different between burned and unburned study sites in 2012 (p > 0.05).  The highest CO₂ exchange rates were observed in June, August, and September, reaching maximum NEP and ER rates of 12.01 and -6.88 mmol CO₂ m^-2 s^-1, respectively.  There was a substantial decline in flux rates during July, a pattern corresponding with unusually dry conditions.  Despite no difference in flux rates, soil organic matter content was significantly higher in the top 5 cm of soil in burned study sites (p < 0.001).  Because there was no concurrent increase in ER at burned sites, this suggests that ER was dominated by autotrophic respiration.  Light availability, soil moisture, leaf area index, vapor pressure deficit, and air temperature explained 52% of the variation in NEP and 59% in GEP.  Soil moisture, leaf area index, and air temperature accounted for 40% of the variation in ER.  Rates of CO₂ exchange as metrics of ecosystem function indicated that managed serpentine barrens recover following management with prescribed fire.  However, processes governing soil carbon storage must vary between burned and unburned sites, a finding will be further investigated.