# Carbon Dynamics of _Pinus palustris_ Ecosystems Following Drought
## 3 Results
### Light and Temperature Responses Following Drought  
When adjusted for the seasonality of temperature, the apparent quantum efficiency, \(\alpha\), responded in opposite ways to drought (as measured by PDSI) when comparing both the xeric _versus_ mesic site, and the fire recovery _versus_ post-recovery period (Figure 3A). During the fire recovery period (_i.e._, observations within 30-day post-fire), drier conditions prevailed (and decreasing PDSI, moving right to left in Figure 3A), and were associated with less negative values of \(\alpha\) at the xeric site but more negative \(\alpha\) at the mesic site, resulting in higher capacity for C uptake (_reference_ Equation (3) and micrometeorological convention). Conversely, during the post-recovery period (_i.e._, observations outside of the 30-days post-fire), drier conditions were associated with moderately lower \(\alpha\) in the xeric  
Figure 1: NEE, GEE, and Reeco seasonal diurnal diel curves in the mesic site (panels (**A**), (**C**), and (**E**), respectively) _versus_ xeric site. (panels (**B**), (**D**), and (**F**), respectively).  
site, and the relationship between PDSI and \(\alpha\) closely resembled that of the mesic site during the fire recovery period, while at the mesic site, drier conditions were associated with slightly higher \(\alpha\).  
The maximum ecosystem CO\({}_{2}\) uptake rate, \(P_{max}\), responded similarly to changes in PDSI when adjusted for the seasonality of temperature (Figure 3B). \(P_{max}\) was higher under drier conditions (low values of PDSI) at both sites and with all fire conditions. However, the response of \(P_{max}\) to drier conditions was markedly stronger in the fire recovery period at the xeric site.  
There was very little change in the ecosystem respiration parameter, \(R_{d}\), associated with changes in PDSI during the fire recovery period (Figure 3C). During the fire recovery time, \(R_{d}\) responded to PDSI in opposite ways depending on site. \(R_{d}\) was lower during drier conditions at the xeric site, but higher during drier conditions at the mesic site.  
Figure 2: Monthly sums of Gross Ecosystem Exchange, Net Ecosystem Exchange and Ecosystem respiration for the mesic (**A**) and xeric sites (**B**), respectively; Palmer Drought Severity Index and summed monthly precipitation for the two stands (**C**). Shaded area indicates period of drought (PDSI \(<\)-3.0), and arrows indicate dates of prescribed fire occurrence.  
When adjusted for the seasonality of temperature, the response of \(R_{0}\), the base respiration rate when air temperature is 0 \({}^{\circ}\)C, to changes in PDSI was similar to that of \(R_{d}\) during the fire recovery period (Figure 4A), with slightly higher values of \(R_{0}\) during drier conditions. During the fire recovery period, \(R_{0}\) responded similarly to drier conditions at the mesic site, but was slightly lower in magnitude when compared to the post-fire recovery period. At the seric site, \(R_{0}\) had a much stronger response to PDSI during the fire recovery period; \(R_{0}\) was much lower during drier conditions.  
At the seric site, the empirical coefficient \(b\) also had a strong response to PDSI during the fire recovery period; \(b\) was much higher during drier conditions (Figure 4B). In contrast, \(b\) was slightly lower during drier conditions at the mesic site. During the post-recovery period, \(b\) was also lower with drier conditions.  
Figure 3: Least square mean estimated light response parameter values for fire recovery period and post-recovery by site: (**A**) Apparent quantum efficiency (\(a\)), (**B**) maximum ecosystem CO\({}_{2}\) uptake rate (\(P_{max}\)), and (**C**) ecosystem respiration (\(R_{d}\)).