Coastal forested wetlands in the southeastern US provide important ecosystem services, including the conservation of biodiversity, supplying clean, abundant water resources, and the cycling and storage of carbon, which feeds back to stabilize regional climate (Noormets et al., 2010; Sun et al. 2010; Aguilos et al. 2020). However, large areas of these wetlands have been converted to other land uses, and all are increasingly threatened by sea level rise and a changing climate. Since 2005, with funding from USDA Forest Service, US DOE-AmeriFlux, and USDA NIFA, we have maintained a cluster of eddy covariance flux towers to provide continuous long-term monitoring of managed and natural forested wetlands in eastern North Carolina to assess the impacts of changing land use, altered hydrology, extreme weather events, and a warming climate.
The flux tower sites include two post-harvest and newly-planted loblolly pine (Pinus taeda) plantations (YP2–7, 2–7 yrs old; YP2–8, 2–8 yrs old), a mature rotation-age loblolly pine plantation (MP, 15–28 yrs old), and a natural bottomland hardwood forest (BHF, > 100 yrs old) along the lower coastal plain of North Carolina. The sites are coded as US-NC3, US-NC1, US-NC2, and US-NC4, respectively, of the Ameriflux network. Along with monitoring C cycling, we quantify the differences in inter-annual and seasonal water balance, and trends in evapotranspiration (ET) using the eddy covariance method, assessing key climatic and biological drivers of ET.
In a recent analysis of 37 site-years of data, we found that the total annual ET in plantation forests was more than half of the annual precipitation. Drained pine plantation forests, with high leaf area and productivity, had comparable ET compared to undisturbed forested wetland, which had lower productivity and higher inundation. Annual variability in precipitation explained most of the variation in drainage in both intensively managed pine plantations and natural bottomland hardwood forest. Compared to undisturbed natural forest, the ditching used with intensive pine management enhanced drainage at young plantations, while it was lower at the mature pine site. The mature plantation had higher annual ET (933 ± 63 mm) than the younger plantations (776 ± 74 mm for YP2–7 and 638 ± 190 mm for YP2–8). Climatic controls, especially net radiation and albedo, and higher leaf area, contributed to higher ET rates at the MP site, consistent with previous studies (Sun et al., 2010; Domec et al., 2012). The ET at the mature plantation site was also higher than that of undisturbed wetland forest (743 ± 172 mm), and here we attribute that to the greater productivity and climatic effects.
Chronosequence analysis of the pine sites showed that ET increased with stand age up to 10 years, then gradually stabilized for the remainder of the rotation of 28 – 30 years. Newly established plantations tended to have low ET due to low leaf area and developing root systems. However, ET increases over time were commensurate with increases in productivity (Aguilos et al., 2020). The ET-GPP coupling was moderate at the young pine plantations (R2 = 0.56 at YP2–7 and R2 = 0.73 at YP2–8). As the stands age (>15 years old), ET gradually stabilized as it approached that of the rotation age stand (~28 – 30 years old), the same period over which GPP also stabilized. The ET-GPP relationship at the MP site was very strong (R2 = 0.86). This positive ET-stand age relationship has implications for forest management in that the establishment of plantations of different age classes across the landscape will result in reduced variation in water table levels, thus reducing the effects of extreme water anomalies. However, the ET-stand age relationship at BHF was insignificant despite a high ET-GPP correlation (R2 = 0.74), suggesting greater climatic control of ET at the natural forest rather than biological constraints in this mature forest, that does not undergo large changes in LAI from year to year.
Our long-term monitoring revealed variation in the sensitivity of intensively managed pine to drought by age class. The young plantation, YP2–8, was very sensitive to decreased water availability, decreasing ET by 30 – 43 % during the extreme 2007 – 2008 drought, whereas reductions in ET at MP were only 8 – 11 %. Typically, the high water table of these lower coastal plain forests provides abundant soil water to meet atmospheric evaporative demand, but this was compromised by the severe drought. However, hydraulic redistribution (HR) by deep roots in the older stand may have played a crucial role in replenishing soil water to the upper soil layers (Domec et al., 2010), thus sustaining tree transpiration and ET. It has been shown that a reduction in tree transpiration occurs with a decrease in relative extractable soil water (REW) below the threshold of 0.4 (Granier et al., 1999). Prolonged soil water deficits in our study, with 177 – 188 days falling below 0.4 in 2007, which got worse in 2008 (200 – 258 days), indicate that trees in the younger stands (without HR), likely were under constant water stress during these periods. Drought-induced reduction in transpiration of young plantation forests, therefore, likely was due to a combination of low biomass/LAI, shallow root systems, low stem capacitance, and reduced plant hydraulic and stomatal conductances (Domec et al., 2009; 2012a).
Responses of C cycling to the severe drought of 2007-2008, were at times, counterintuitive, however. Interestingly, there was a 6 – 9 % enhancement in GPP at the MP site and a 20 – 53 % increase in GPP at YP2–8 during the drought (Aguilos et al., 2020) that were not accompanied by a similar rise in ET, suggesting increased water use efficiency (Domec et al., 2015). This result does not support our hypothesis that ET and GPP will remain tightly, positively coupled despite extreme climatic conditions. Although we found that ET and GPP respond to the same primary climatic drivers, the magnitude of response differs for the two processes during anomalous soil water years. This asynchrony of the response represents a decoupling of carbon and water cycling under extreme conditions. GPP is usually affected less during severe water depletion, since stomatal closure typically induces a stronger down-regulation of transpiration than photosynthesis, and transpiration is linearly related to stomatal conductance. In contrast, photosynthesis may be limited by a variety of other factors and does not respond linearly to instantaneous changes in stomatal conductance. Differences in sensitivity of ET and GPP to drought illustrate the challenges that must be overcome for ecosystem models to accurately simulate these processes.
The North Carolina cluster of flux towers represents the longest direct measurement of ET and forest water balance of lower coastal plain forested wetlands in the southeastern US, advancing our understanding of the hydrologic responses to land-use change (drained vs. natural hydrology), inter-annual variation in climate, climate extremes (drought, flooding) and long-term climate warming. Mature plantation forests that have higher biomass and productivity had similar or higher ET compared to undisturbed natural forested wetlands. This has implications for landscape level water management in the coastal regions. Forest land managers can sustain ecological functioning to extremes in water availability (drought or flooding) by taking advantage of the relatively large ET of pine plantations to moderate watershed yield (more from young plantations, less from mature), and water quality concerns from forest cutting and regeneration. The ditching practices of the past in coastal forested wetlands indeed increases runoff, thereby clearing the land of excess water during storms, but they also increase the risk of saltwater intrusion from storms and sea-level rise. Maintaining tree stocks may help mitigate the hydrological and climatic regulation functions (i.e., ET capacity) from disturbances such as land-use change (i.e., ditching). Our data suggest that these drained wetlands are resilient to extreme episodic droughts due to the shallow water table. Therefore, maintaining the water table through controlled drainage that alters ditch water levels based on-site hydrologic conditions can become increasingly important to mitigate drought effects on forest productivity under a changing climate. However, in the long-term these wetlands are vulnerable to climate change and sea-level rise, which likely alters the hydrology of the entire lower coastal plain physiographic region, with implications for forest productivity, mortality and carbon storage. Watershed management in coastal plain regions, therefore, should consider the potential combined effects of climate change, sea-level rise, and land-use dynamics on water balance.
(Excerpt from the NC sore flux site publication: https://www.sciencedirect.com/science/article/pii/S0168192321000642)
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