Abstract
Wetlands and their ability to mitigate climate change motivates restorative and protective action; however, scientific understanding of land-atmosphere interactions is restricted by our limited continuous observations of gaseous fluxes. Many wetlands are small in spatial scale and embedded in forested landscapes. Yet, little is known about how the relative sheltering of forests affects net carbon (C) and energy balance. Here, we analyze coterminous USGS and Ameriflux eddy covariance flux tower observations over three years in two shrub fens in Northern Wisconsin, one more sheltered (US-ALQ) than the other (US-Los). Unsurprisingly, the open site showed higher overall wind speeds. This should have implications for atmospheric fluxes in wetlands as wind-forced processes are essential in promoting gas exchange over water. While both sites had similar half-hourly net ecosystem exchange of CO2 (NEE) during daytime, there were significant differences in nighttime NEE, as well as in net radiation partitioning in early spring and late summer. Sensible heat (H) fluxes were smaller at the sheltered fen except for the months of July-September. In contrast, latent heat (LE) fluxes were higher in every month except July. Additionally, sheltered fen ecosystem respiration had a weaker linear correlation with air temperature (R: 0.08 versus 0.57 for the open fen). Our work suggests that canopy sheltering does not cause significant differences in half-hourly NEE during the day, but rather the largest differences such as lower CO2 emissions occur at nighttime due to higher variance at very low wind speeds. Sheltering also influenced direction of air flow, mean wind speeds in day versus night, energy balance, and sensible and latent heat fluxes. We discuss implications of these findings for wetland restoration.
Highlights
Wetland comparison studies often seek to understand gaseous fluxes according to wetland classification type, i.e., marsh, forested wetland, shrub etc. (Bernal and Mitsch, 2012; Turetsky et al, 2014; Coffer and Hestir, 2019)
Co-located flux towers in a sheltered and open fen with overlapping observations would allow us to test the role that sheltering has on carbon (C) uptake and energy balance as indicated by Bowen ratio [sensible heat flux (H)/latent heat flux (LE)]
The valley consists of low-lying sedge and provides an open area for high wind flow to develop, it is not wide enough for the wind to overcome the sheltering effect
Summary
Wetland comparison studies often seek to understand gaseous fluxes according to wetland classification type, i.e., marsh, forested wetland, shrub etc. (Bernal and Mitsch, 2012; Turetsky et al, 2014; Coffer and Hestir, 2019). Wetland comparison studies often seek to understand gaseous fluxes according to wetland classification type, i.e., marsh, forested wetland, shrub etc. Studies that focus solely on the effects of sheltering of wetlands through forests are limited. Sheltering is important in many wetlands which are small in size and surrounded by forest. Co-located flux towers in a sheltered and open fen with overlapping observations would allow us to test the role that sheltering has on carbon (C) uptake and energy balance as indicated by Bowen ratio [sensible heat flux (H)/latent heat flux (LE)]. Two eddy covariance flux tower sites, Lost Creek (US-Los) and Allequash Creek (US-ALQ), are used here to yield comparisons of gas exchange over wetlands in Northern Wisconsin and provide an opportunity for a more in-depth look than laboratory studies or large-scale syntheses
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