Cyclic Occurrence of Fire and Its Role in Carbon Dynamics along an Edaphic Moisture Gradient in Longleaf Pine Ecosystems

<div><p>Fire regulates the structure and function of savanna ecosystems, yet we lack understanding of how cyclic fire affects savanna carbon dynamics. Furthermore, it is largely unknown how predicted changes in climate may impact the interaction between fire and carbon cycling in these ecosystems. This study utilizes a novel combination of prescribed fire, eddy covariance (EC) and statistical techniques to investigate carbon dynamics in frequently burned longleaf pine savannas along a gradient of soil moisture availability (mesic, intermediate and xeric). This research approach allowed us to investigate the complex interactions between carbon exchange and cyclic fire along the ecological amplitude of longleaf pine. Over three years of EC measurement of net ecosystem exchange (NEE) show that the mesic site was a net carbon sink (NEE = −2.48 tonnes C ha⁻¹), while intermediate and xeric sites were net carbon sources (NEE = 1.57 and 1.46 tonnes C ha⁻¹, respectively), but when carbon losses due to fuel consumption were taken into account, all three sites were carbon sources (10.78, 7.95 and 9.69 tonnes C ha⁻¹ at the mesic, intermediate and xeric sites, respectively). Nonetheless, rates of NEE returned to pre-fire levels 1–2 months following fire. Consumption of leaf area by prescribed fire was associated with reduction in NEE post-fire, and the system quickly recovered its carbon uptake capacity 30–60 days post fire. While losses due to fire affected carbon balances on short time scales (instantaneous to a few months), drought conditions over the final two years of the study were a more important driver of net carbon loss on yearly to multi-year time scales. However, longer-term observations over greater environmental variability and additional fire cycles would help to more precisely examine interactions between fire and climate and make future predictions about carbon dynamics in these systems.</p> </div

Interactive effect of Windspeed*Site and PAR*Site on NEE.

<p>Least square mean values of NEE at the xeric, intermediate and mesic sites over (a) a range of wind speeds, and (b) a range of PAR values. FCT 1 is the first 28 days following fire, FCT 2 and 3 are the next two 28 day periods, FCT 4 represents the next 140 days, and FCT 5 represents the next 224 days. To mitigate for autocorrelation PAR and NEE are 28 day means.</p

Type III tests of fixed effects for the models of WUE and ET.

<p>WUE did not significantly differ among the sites, and ET did not significantly differ over the fire cycle or among the sites. Tables include for each effect, the degrees of freedom in the numerator (Num. DF), degrees of freedom in the denominator (Den. DF), and the value of the F statistic (F value) and its corresponding P-value (Pr>F).</p

Type III tests of fixed effects for the models of NEE, R_eco and GEE.

<p>Tables include for each effect, the degrees of freedom in the numerator (Num. DF), degrees of freedom in the denominator (Den. DF), and the value of the F statistic (F value) and its corresponding P-value (Pr>F).</p

Representative pre- and post-fire temperature response curves.

<p>Representative nighttime temperature response curves pre- (left) and post-fire (right) in January 2009 (top) and March 2011 (bottom) at the xeric, intermediate and mesic sites (green, red and blue, respectively).</p

NEE, fuel consumption and net biome exchange (NBE) (tonnes ha⁻¹) over the study period.

<p>Total NEE values are the sums of annual NEE from the three years of the study. NBE is the sum of total NEE and fuel consumption during the two fires.</p

Monthly NEE, GEE and R_eco over the study period.

<p>Monthly NEE, GEE and R_eco over the study period.</p

Representative pre- and post-fire light response curves.

<p>Representative light response curves pre- (left) and post-fire (right) in January 2009 (top) and March 2011 (bottom) at the xeric, intermediate and mesic sites (green, red and blue, respectively).</p

Interactive effect of PAR*FCT on NEE and VPD*FCT on GEE.

<p>Least square mean values by fire cycle time (FCT) (a) over a range of PAR for NEE, and (b) over a range of VPD for GEE. FCT 1 is the first 28 days following fire, FCT 2 and 3 are the next two 28 day periods, FCT 4 represents the next 140 days, and FCT 5 represents the next 224 days. To mitigate for autocorrelation PAR and NEE are 28 day means.</p

Environmental variables over the study period.

<p>Environmental variables measured at xeric (closed triangles, dashed lines), intermediate (open circles, dotted lines) and mesic (closed circles, solid lines) sites at the Jones Center from October 2008 to October 2011. Monthly means were calculated for: (a) photosynthetically active radiation (PAR), (b) air temperature (T_air), (c) volumetric water content of the soil (VWC), (d) vapor pressure deficit (VPD), and (e) windspeed. Monthly sums were calculated for (f) precipitation.</p