Carbon dioxide exchange in the serpentine barrens of southeastern Pennsylvania: examining the role of fire management and environmental drivers

Background/Question/Methods

The serpentine barrens of southeastern Pennsylvania are of high conservation priority, given that they are decreasing in area due to forest encroachment and are home to many threatened and endangered species.  Fire has been used as a tool to maintain these ecosystems, particularly in regard to species composition and vegetation structure.  However, little is known about how fire management affects ecosystem function.  To address this knowledge gap, rates of carbon dioxide (CO₂) exchange were compared across burned and unburned serpentine barren ecosystems, and the role of various environmental drivers in determining these exchange rates was also evaluated.  Measurements of CO₂ exchange as net ecosystem production (NEP), ecosystem respiration (ER), and gross ecosystem production (GEP) were made monthly over the course of the growing season with a custom made, flow-through chamber.  Monitored environmental factors included air and soil temperature, soil moisture, photosynthetically active radiation (PAR), and vapor pressure deficit (D).  Rates of NEP, ER, and GEP were compared across burned and unburned study sites using linear mixed-effects models.  The influence of environmental drivers on CO₂ exchange rates was evaluated with stepwise multiple regression analyses.

Results/Conclusions

Rates of NEP, ER, and GEP did not differ significantly between burned and unburned study sites (p > 0.05).  Maximum rates of NEP and ER were reached in mid-summer and averaged 6.94 ± 0.84 and -6.00 ± 0.32 mmol CO₂ m^-2 s^-1, respectively.  Data were pooled across study sites to examine environmental drivers of CO₂ exchange.  Soil temperature, D, and PAR best predicted both NEP and GEP.  These relationships were significant (p < 0.01), and these models explained 24% and 42% of the variation in NEP and GEP, respectively.  Soil temperature alone best predicted ER.  This relationship generated a significant model (p < 0.0001) explaining 50% of the variation in ER.  The relatively poor explanatory power of these models indicates that additional research into the drivers of CO₂ exchange is merited.  Leaf area index and soil carbon content will be examined in the future.  Data presented here show that fire management in Pennsylvania’s serpentine barrens did not alter rates of CO₂ exchange relative to unmanaged sites.  These findings indicate that current fire management strategies are effectively maintaining ecosystem function, in terms of CO₂ exchange rates, while at the same time protecting the unique flora and structural attributes of these ecosystems.