Search

Seasonal phenological dynamics of vegetation hold important clues on ecosystem performance towards management goals, such as carbon uptake, and thus should be considered in projections of their targeted services. However, in wetlands spatio-temporal heterogeneity due to mixing of open water, soil, green and dead vegetation makes it difficult to generalize ecosystem functioning across different regions. Remote sensing observations can provide spatially-explicit, cost-effective phenology indicators; however, little is known about their capacity to indicate the links between wetland ecosystem structure and function. Here we assessed this potential by comparing one-year Enhanced Vegetation Index (EVI) from satellite products at high (5m; RapidEye) and low (30m; Landsat) spatial resolutions with eddy covariance time series of net carbon exchange, field digital camera (phenocam) greenness and water temperature among three floristically similar restored wetlands in California, USA. Phenological timing differed by wetland site: depending on satellite, the range in site-median start of greening was up to 28 days, end of greening – up to 73 days, start of senescence – up to 79 days, and end of senescence – up to 10 days. Key transition dates from satellite inputs agreed with seasonal changes in net carbon exchange, phenocam greenness and water temperatures, suggesting that phenological contrasts could result in part from site differences in vegetation configuration and litter affecting the exposure of canopy, soil and water to sunlight and thus sub-canopy microclimate and ecosystem functioning. Yet, the agreement between satellite inputs was non-systematic, with the greatest disparities at the more heterogeneous, less vegetated site. Phenological model fitting uncertainty increased with greater spatial resolution, highlighting the tradeoff between the accuracy of representing vegetation and the complexity of local seasonal variation. These findings highlight the sensitivity of satellite-derived phenology to structural and functional heterogeneity of ecosystems and call for more rigorous spatially-explicit analyses to inform assessments of restoration and management outcomes.

Wetlands can influence global climate via greenhouse gas (GHG) exchange of carbon dioxide (CO2 ), methane (CH4 ), and nitrous oxide (N2 O). Few studies have quantified the full GHG budget of wetlands due to the high spatial and temporal variability of fluxes. We report annual open-water diffusion and ebullition fluxes of CO2 , CH4 , and N2 O from a restored emergent marsh ecosystem. We combined these data with concurrent eddy-covariance measurements of whole-ecosystem CO2 and CH4 exchange to estimate GHG fluxes and associated radiative forcing effects for the whole wetland, and separately for open-water and vegetated cover types. Annual open-water CO2 , CH4 , and N2 O emissions were 915 ± 95 g C-CO2  m-2  yr-1 , 2.9 ± 0.5 g C-CH4  m-2  yr-1 , and 62 ± 17 mg N-N2 O m-2  yr-1 , respectively. Diffusion dominated open-water GHG transport, accounting for >99% of CO2 and N2 O emissions, and ~71% of CH4 emissions. Seasonality was minor for CO2 emissions, whereas CH4 and N2 O fluxes displayed strong and asynchronous seasonal dynamics. Notably, the overall radiative forcing of open-water fluxes (3.5 ± 0.3 kg CO2 -eq m-2  yr-1 ) exceeded that of vegetated zones (1.4 ± 0.4 kg CO2 -eq m-2  yr-1 ) due to high ecosystem respiration. After scaling results to the entire wetland using object-based cover classification of remote sensing imagery, net uptake of CO2 (-1.4 ± 0.6 kt CO2 -eq yr-1 ) did not offset CH4 emission (3.7 ± 0.03 kt CO2 -eq yr-1 ), producing an overall positive radiative forcing effect of 2.4 ± 0.3 kt CO2 -eq yr-1 . These results demonstrate clear effects of seasonality, spatial structure, and transport pathway on the magnitude and composition of wetland GHG emissions, and the efficacy of multiscale flux measurement to overcome challenges of wetland heterogeneity.

Atmospheric methane (CH4) concentrations have shown a puzzling resumption in growth since 2007 following a period of stabilization from 2000 to 2006. Multiple hypotheses have been proposed to explain the temporal variations in CH4 growth, and attribute the rise of atmospheric CH4 either to increases in emissions from fossil fuel activities, agriculture and natural wetlands, or to a decrease in the atmospheric chemical sink. Here, we use a comprehensive ensemble of CH4 source estimates and isotopic δ13C-CH4 source signature data to show that the resumption of CH4 growth is most likely due to increased anthropogenic emissions. Our emission scenarios that have the fewest biases with respect to isotopic composition suggest that the agriculture, landfill and waste sectors were responsible for 53 ± 13% of the renewed growth over the period 2007–2017 compared to 2000–2006; industrial fossil fuel sources explained an additional 34 ± 24%, and wetland sources contributed the least at 13 ± 9%. The hypothesis that a large increase in emissions from natural wetlands drove the decrease in atmospheric δ13C-CH4 values cannot be reconciled with current process-based wetland CH4 models. This finding suggests the need for increased wetland measurements to better understand the contemporary and future role of wetlands in the rise of atmospheric methane and climate feedback. Our findings highlight the predominant role of anthropogenic activities in driving the growth of atmospheric CH4 concentrations. [ABSTRACT FROM AUTHOR]

Agricultural drainage of organic soils has resulted in vast soil subsidence and contributed to increased atmospheric carbon dioxide (CO2) concentrations. The Sacramento-San Joaquin Delta in California was drained over a century ago for agriculture and human settlement and has since experienced subsidence rates that are among the highest in the world. It is recognized that drained agriculture in the Delta is unsustainable in the long-term, and to help reverse subsidence and capture carbon (C) there is an interest in restoring drained agricultural land-use types to flooded conditions. However, flooding may increase methane (CH4) emissions. We conducted a full year of simultaneous eddy covariance measurements at two conventional drained agricultural peatlands (a pasture and a corn field) and three flooded land-use types (a rice paddy and two restored wetlands) to assess the impact of drained to flooded land-use change on CO2 and CH4 fluxes in the Delta. We found that the drained sites were net C and greenhouse gas (GHG) sources, releasing up to 341 g C m−2 yr−1 as CO2 and 11.4 g C m−2 yr−1 as CH4. Conversely, the restored wetlands were net sinks of atmospheric CO2, sequestering up to 397 g C m−2 yr−1. However, they were large sources of CH4, with emissions ranging from 39 to 53 g C m−2 yr−1. In terms of the full GHG budget, the restored wetlands could be either GHG sources or sinks. Although the rice paddy was a small atmospheric CO2 sink, when considering harvest and CH4 emissions, it acted as both a C and GHG source. Annual photosynthesis was similar between sites, but flooding at the restored sites inhibited ecosystem respiration, making them net CO2 sinks. This study suggests that converting drained agricultural peat soils to flooded land-use types can help reduce or reverse soil subsidence and reduce GHG emissions. [ABSTRACT FROM AUTHOR]

Water is a limited and valuable resource in California. A large proportion of the fresh water for southern California is supplied by the Sacramento and San Joaquin rivers. With recent efforts to restore large areas of land in the Sacramento–San Joaquin Delta region from farmland to managed wetlands, it is important to investigate the effect of wetland restoration on the local water cycle. We measured evapotranspiration using the eddy covariance method over six different sites in the Sacramento–San Joaquin Delta from 2013 to 2016. The three restored wetlands sites differed in time since restoration and wetland structure, i.e. the area of open water compared to closed vegetation. The three agricultural sites were a flooded rice field, an alfalfa field, and a grazed cattle pasture. In most years, annual evapotranspiration was significantly lower at the drained agricultural sites, with 652 ± 131 mm (2 year average ± standard deviation, SD) for the pasture and 901 ± 24 mm (3 year average ± SD) for the alfalfa field, compared to the two open water wetlands, with 1091 ± 144 mm (4 year average ± SD) for the Mayberry wetland and 1140 ± 67 mm (3 year average ± SD) at the East End wetland. Annual evapotranspiration at the flooded rice site (975 ± 50 mm, 4 year average ± SD) or the closed vegetation wetland (West Pond wetland, 996 ± 63 mm, 4 year average ± SD) was not significantly different from the drained alfalfa site across individual years. Our analysis showed that the structural difference between the wetlands, specifically the fraction of open water compared to closed vegetation, has a large impact on evapotranspiration dynamics. Through analysis of normalized equilibrium evaporation and nighttime evapotranspiration measurements we deduced that evapotranspiration at the wetland with a low fraction of open water surfaces was almost entirely dominated by plant transpiration with very little contribution from evaporation, despite the fact that the site was flooded and water was readily available. Both evaporation and transpiration contributed substantially to evapotranspiration at the two wetlands with larger fraction of open water surfaces. At the closed canopy site evaporation from subcanopy water seemed to be inhibited through two mechanisms: first, the closed canopy prevented heating of the water column and led to significantly cooler water temperatures, which reduced surface vapor pressure. Second, the closed canopy decoupled the water surface from the atmosphere and inhibited turbulent transport of water vapor away from the water surface. Our results provide valuable insights into the water use in California wetlands and can inform decisions on how to maximize water conservation during wetland restoration. [ABSTRACT FROM AUTHOR]

• On average, the bog site was near carbon neutral 8–13 years after rewetting. • The site alternated between being an annual C sink and source over the years. • Climatic variability and functional change drive interannual variability in c fluxes. • Excluding CH 4 would result in an overestimation of the net C sink. Peatland rewetting, a management effort to restore water levels in previously drained peatlands, is important for re-establishing the role of these peatlands as carbon (C) sinks. Since rewetted peatlands have a highly variable response to interannual variations in climatic conditions and functional changes, long term studies of C fluxes in these ecosystems are needed. Here, we evaluated the impact of climate variability and functional change on the interannual variability of CO 2 and CH 4 fluxes at Burns Bog, a rewetted temperate bog on the Pacific Coast in Canada, based on five years of eddy covariance measurements. We found that the site alternated between being an annual-scale net CO 2 sink or source, ranging from -32.6 ± 21.5 (±95% CI) to 11.9 ± 15.1 g CO 2 C m-2 yr-1, respectively, while consistently being a CH 4 source, ranging from 11.6 ± 0.7 to 18.0 ± 1.6 g CH 4 C m-2 yr-1. Over the five-year period, mean annual CH 4 emissions (13.7 ± 2.5 g CH 4 C m-2 yr-1; ±SD across years) entirely offset the CO 2 sink (-12.3 ± 20.4 g CO 2 C m-2 yr-1), resulting in the site being near-carbon neutral over this period (1.3 ± 23.9 g C m-2 yr-1). This finding indicates that excluding CH 4 fluxes from the net C balance results in an overestimation of the net C uptake at this site. Annual CO 2 emissions from the bog were greatest in the year with a dry and warm summer, emphasizing the importance of temperature and water table depth at the bog. Regardless of the greenhouse gas (GHG) metrics (i.e., global warming potential or sustained global warming potential) used in calculating the annual CO 2 -eq balance, the site consistently had a positive GHG balance across the study period. Despite mainly acting as a GHG source, the rewetted site will likely have a cooling effect on the climate system over long timescales compared to drained bogs that are large CO 2 sources. [ABSTRACT FROM AUTHOR]