# Carbon Dynamics of _Pinus palustris_ Ecosystems Following Drought
## 4 Discussion
### Disturbance and Recovery
#### 4.1.2 Drought  
Using measures of productivity as an indication of resiliency [67], recovery from drought was dependent on the temporal scale in which we assessed our data. On the 24-h period, the maximum diurnal uptake rates were slightly less from the seric site as compared to the mesic site for all drought and non-drought periods (Figures 1 and 6). We primarily attribute this result to the fact that the seric site had \(\sim\)40% less basal area. However, the diurnal patterns also differed, with a sharper decline in NEE rates during the afternoon at the seric sites, _i.e._, hysteresis (Figures 1 and 6), resulting in less net C uptake in the seric site when integrating under the daytime hours (Figure 6). We attribute this hysteresis in both sites to stomata closure taking place in response to increasing mid-day VPD (VPD threshold of 2.5 KPa detected by [PERSON] _et al._[19]) and differences in soil water availability, together suggesting these trees still have access to deep water. _P. Palustris_'s ability to change its Cuptake in response to drought (and its recovery) is through changes in needle photosynthetic capacity (2-3 years needle cohort) and whole tree conductance [68]. The reduction in NEE rates was more pronounced in the first year of the drought compared to the second year, even though the PDSI was the same or greater in the second year of drought (Table 1 and Figure 2). The similar pattern in NEE rates was observed in the recovery from drought, where the rates from the first year after drought were not as large as those from the second year. Interestingly, the spectral patterns of NEE differed from pre-drought to drought conditions, where the amount of variance changed temporally (_i.e._, spectral density) shifting from a 12-h to a 24-h spectral peak (Figure 5). The diurnal \(\mathrm{R_{eco}}\) spectral peak was not altered pre-, post- or during the drought, and was \(\sim\)12 h (Figure 5). This suggests that greatest amount of (diurnal) variance in \(\mathrm{R_{eco}}\) occurred every 12-h, as would be expected for nighttime respiration. However, the shift from 12-h peak spectral density in NEE suggests that the maximum (diurnal) variance changed from controls on nighttime and daytime processes, to the importance of day-to-day variability. Put another way, the day-to-day variance in NEE during drought periods-and the first year of recovery from drought- exceeded that of the variability observed in nighttime \(\mathrm{R_{eco}}\) and that observed in \(\mathrm{NEE_{day}}\). However, because the \(\mathrm{R_{eco}}\) spectra did not change we attribute the shift in NEE spectra to that of \(\mathrm{NEE_{day}}\) alone. It also appears (Figure 5) that during year 2 post-drought, the NEE spectra are returning towards the 12-h peak. Taken in concert, these changes in patterns also speak to the annual phenotypic plasticity of this ecosystem. There was a large range in annual productivity among the pre-, post and drought years. Even with prescribed burns and a two-year severe drought, these ecosystems were still a sink of C with mean annual NEE rates of 2.1 \(\pm\) 0.18 and 0.7 \(\pm\) 0.18 Mt C ha\({}^{-1}\cdot\)year\({}^{-1}\) (\(\mu\)\(\pm\) 1 SE) for mesic and xeric sites, respectively, and a total C accumulation of 14.6 and 5.2 Mt C ha\({}^{-1}\) over the seven years of this study for the mesic and xeric sites, respectively. While the magnitude of GEE, NEE, \(\mathrm{R_{eco}}\) was larger at the mesic site as compared to the xeric, there was no significant difference when taking into account their variance. Hence, we could not reject Hypothesis 1: The magnitude of physiological change during recovery from drought would be minimal at the xeric site because the plants' phenotypical response has evolved with water as a limited resource, as compared to the mesic site. However, we could accept Hypothesis 2: Following a drought, the mesic site would be a greater sink of carbon compared to the xeric site.