Micrometeorological measurements of methane flux at a boreal forest in central Alaska

Methane (CH_4) flux at a black spruce forest in central Alaska was determined by applying the three types of modified gradient method. One type used the eddy diffusivity obtained by CO_2 flux and CO_2 gradient. Others established the flux gradient relationship assuming Monin-Obukhov similarity. The wind speed and temperature profile functions were corrected for the influence of the roughness sublayer, and then applied to the modified gradient methods. More than 70% of the data were rejected by the strict quality control and a continental climate, such as calm wind. Although the diurnal variations of CH_4 flux by the three methods were different, the seasonal variation showed similar tendency; a weak emission on snowpack, an obvious emission around spring thaw, and CH_4 uptake in the late growing season. As calculated CH_4 flux was in the same range as with previous studies conducted by the chamber measurement

Seasonal Variation in Fraction of Absorbed Photosynthetically Actic Radiation and Vegetation Properties in Burned Forests in Interior Alaska

Fraction of absorbed photosynthetically active radiation (FAPAR) is an important ecophysiological parameter for carbon and water exchange modeling. However, validation studies of FAPAR are scarce, especially for disturbance area. One study (Steinberg et al., 2006) revealed that the MODIS FAPAR product is overestimated for burned boreal forests. Wildfire is a major disturbance in boreal forest ecosystems, and it significantly influences carbon and water exchange processes. It is important to explicitly incorporate burned areas in estimating regional exchanges. This study aims to provide a validation data for FAPAR by collecting data regarding absorption of photosynthetically active radiation (PAR) in burned boreal forests. It also focuses on obtaining an empirical relationship to estimate seasonal and interannual variations in FAPAR from vegetation indices in the early stage of recovery after wildfire.This study was partly supported by Carbon Cycle Program of IARC/NSF and the IJIS (IARC/JAXA Information System)

Methane exchange in a poorly-drained black spruce forest over permafrost observed using the eddy covariance technique

Ecosystem-scale methane (CH4) exchange was observed in a poorly-drained black spruce forest over permafrost in interior Alaska during the snow-free seasons of 2011–2013, using the eddy covariance technique. The magnitude of average CH4 exchange differed depending on wind direction, reflecting spatial variation in soil moisture condition around the observation tower, due to elevation change within the small catchment. In the drier upper position, the seasonal variation in CH4 emission was explained by the variation in soil water content only. In the wetter bottom, however, in addition to soil temperature and soil water content, seasonal thaw depth of frozen soil was also an important variable explaining the seasonal variation in CH4 exchange for this ecosystem. Total snow-free season (day of year 134–280) CH4 exchanges were 12.0 ± 1.0, 19.6 ± 3.0, and 36.6 ± 4.4 mmol m−2 season−1 for the drier upper position, moderately wet area, and wetter bottom of the catchment, respectively. Observed total season CH4 emission was nearly one order smaller than those reported in other northern wetlands, due probably to the relatively low ground water level and low soil temperature. The interannual variation of total snow-free season CH4 emission in the wetter bottom of the catchment was influenced by the amount of rainfall and thaw depth. On the other hand, in the drier upper position the amount of rainfall did not strongly affect the total season CH4 emission. Different responses of CH4 exchange to environmental conditions, depending on the position of a small catchment, should be considered when estimating the spatial variation in CH4 exchange accurately in ecosystems over permafrost.ArticleAGRICULTURAL AND FOREST METEOROLOGY. 214(0):157-168 (2015)journal articl

Temporal and spatial differences of methane flux at arctic tundra in Alaska

High latitude ecosystems were thought to enhance CH_4 emission in relation to the current arctic warming. However, we have little information about this potential feedback mechanisms on climate change, thus, model parameterization is insufficient and the observational data are required. We observed CH_4 flux at several types of tundra in Alaska over the growing seasons since 1995. From these observed data, we examined current CH_4 emission and its controlling factors on Alaskan tundra. Then we discussed about spatial and temporal differences in CH_4 flux. Daily trend of half hourly CH_4 flux had little relation with soil temperature, but the seasonal trend of daily flux changed with soil or water temperature. Cumulative CH_4 fluxes during the growing seasons were 8.1gCH_4m^(-2) on wet sedge tundra at Happy Valley in 1995, 3.3gCH_4m^(-2) on non-acidic moist tundra in 1996, and 3.58-8.24gCH_4m^(-2) on wet sedge tundra at Barrow between 1999-2003. Non-acidic tundra had low CH_4 emission with low CO_2 accumulation. There was large spatial difference in CH_4 flux caused by tundra type, and the large temporal difference at the wet sedge tundra reflected yearly weather variability

A multi-scale comparison of modeled and observed seasonal methane emissions in northern wetlands

Wetlands are the largest global natural methane (CH4) source, and emissions between 50 and 70° N latitude contribute 10–30% to this source. Predictive capability of land models for northern wetland CH4 emissions is still low due to limited site measurements, strong spatial and temporal variability in emissions, and complex hydrological and biogeochemical dynamics. To explore this issue, we compare wetland CH4 emission predictions from the Community Land Model 4.5 (CLM4.5-BGC) with siteto regional-scale observations. A comparison of the CH4 fluxes with eddy flux data highlighted needed changes to the model’s estimate of aerenchyma area, which we implemented and tested. The model modification substantially reduced biases in CH4 emissions when compared with CarbonTracker CH4 predictions. CLM4.5 CH4 emission predictions agree well with growing season (May–September) CarbonTracker Alaskan regional-level CH4 predictions and sitelevel observations. However, CLM4.5 underestimated CH4 emissions in the cold season (October–April). The monthly atmospheric CH4 mole fraction enhancements due to wetland emissions are also assessed using the Weather Research and Forecasting-Stochastic Time-Inverted Lagrangian Transport (WRF-STILT) model coupled with daily emissions from CLM4.5 and compared with aircraft CH4 mole fraction measurements from the Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE) campaign. Both the tower and aircraft analyses confirm the underestimate of cold-season CH4 emissions by CLM4.5. The greatest uncertainties in predicting the seasonal CH4 cycle are from the wetland extent, coldseason CH4 production and CH4 transport processes. We recommend more cold-season experimental studies in highlatitude systems, which could improve the understanding and parameterization of ecosystem structure and function during this period. Predicted CH4 emissions remain uncertain, but we show here that benchmarking against observations across spatial scales can inform model structural and parameter improvements

Latitudinal gradient of spruce forest understory and tundra phenology in Alaska as observed from satellite and ground-based data

The latitudinal gradient of the start of the growing season (SOS) and the end of the growing season (EOS) were quantified in Alaska (61°N to 71°N) using satellite-based and ground-based datasets. The Alaskan evergreen needleleaf forests are sparse and the understory vegetation has a substantial impact on the satellite signal. We evaluated SOS and EOS of understory and tundra vegetation using time-lapse camera images. From the comparison of three SOS algorithms for determining SOS from two satellite datasets (SPOT-VEGETATION and Terra-MODIS), we found that the satellite-based SOS timing was consistent with the leaf emergence of the forest understory and tundra vegetation. The ensemble average of SOS over all satellite algorithms can be used as a measure of spring leaf emergence for understory and tundra vegetation. In contrast, the relationship between the ground-based and satellite-based EOSs was not as strong as that of SOS both for boreal forest and tundra sites because of the large biases between those two EOSs (19 to 26 days). The satellite-based EOS was more relevant to snowfall events than the senescence of understory or tundra. The plant canopy radiative transfer simulation suggested that 84–86% of the NDVI seasonal amplitude could be a reasonable threshold for the EOS determination. The latitudinal gradients of SOS and EOS evaluated by the satellite and ground data were consistent and the satellite-derived SOS and EOS were 3.5 to 5.7 days degree− 1 and − 2.3 to − 2.7 days degree− 1, which corresponded to the spring (May) temperature sensitivity of − 2.5 to − 3.9 days °C− 1 in SOS and the autumn (August and September) temperature sensitivity of 3.0 to 4.6 days °C− 1 in EOS. This demonstrates the possible impact of phenology in spruce forest understory and tundra ecosystems in response to climate change in the warming Artic and sub-Arctic regions

Methane exchange in a poorly-drained black spruce forest over permafrost observed using the eddy covariance technique

Ecosystem-scale methane (CH4) exchange was observed in a poorly-drained black spruce forest over permafrost in interior Alaska during the snow-free seasons of 2011–2013, using the eddy covariance technique. The magnitude of average CH4 exchange differed depending on wind direction, reflecting spatial variation in soil moisture condition around the observation tower, due to elevation change within the small catchment. In the drier upper position, the seasonal variation in CH4 emission was explained by the variation in soil water content only. In the wetter bottom, however, in addition to soil temperature and soil water content, seasonal thaw depth of frozen soil was also an important variable explaining the seasonal variation in CH4 exchange for this ecosystem. Total snow-free season (day of year 134–280) CH4 exchanges were 12.0 ± 1.0, 19.6 ± 3.0, and 36.6 ± 4.4 mmol m−2 season−1 for the drier upper position, moderately wet area, and wetter bottom of the catchment, respectively. Observed total season CH4 emission was nearly one order smaller than those reported in other northern wetlands, due probably to the relatively low ground water level and low soil temperature. The interannual variation of total snow-free season CH4 emission in the wetter bottom of the catchment was influenced by the amount of rainfall and thaw depth. On the other hand, in the drier upper position the amount of rainfall did not strongly affect the total season CH4 emission. Different responses of CH4 exchange to environmental conditions, depending on the position of a small catchment, should be considered when estimating the spatial variation in CH4 exchange accurately in ecosystems over permafrost.ArticleAGRICULTURAL AND FOREST METEOROLOGY. 214(0):157-168 (2015)journal articl