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Fire is the major disturbance in North American boreal forests, and is thought to be the most important process that determines the carbon balance in North American boreal forests. This study conducted four years of tower flux measurements in a burned ecosystem from one to four years after a fire, and nine years of measurements in a young regeneration from five to 13 years after a fire in interior Alaska. The fire scar acted as a source of 248 g C m−2 yr-1 one year after the fire, and the annual CO2 emissions continuously decreased until seven years after the fire. At the final year of the study period, 13 years after the fire, the older forest became a CO2 sink. During the 13 years after the fires, the total post-fire emissions were 767 g C m−2 across both sites. Gross primary productivity (GPP) and ecosystem respiration (RE) recovered to those of mature black spruce forests 10 years after the fire. The successional recovery of GPP was mostly explained by the recovery of the leaf area index (LAI). Anomalous weather, such as a cold spring, hot summer, and high summer rainfall, increased the CO2 emissions rather than the uptake. In interior Alaska, the post-fire CO2 emissions (35–48 Tg C) were estimated to be approximately one third to fourth of the direct CO2 emissions (156 Tg C) by combustions from 1998 to 2017, which indicates that post-fire emissions are important to the regional CO2 balance. The forest successional trajectory at young age still contains large uncertainties due to lack of data, and thus adding new data improves our understanding of the post-fire CO2 balance.

In this study, we conducted an incubation experiment on a frozen soil core collected from a black spruce forest in Interior Alaska, in order to investigate potential changes in greenhouse gaseous (...

In the face of increasing temperature at high latitudes, ecosystem respiration (RE) is a key to determining sink and source dynamics of a boreal forest. In this paper, we analyzed four-year RE data obtained in an open black spruce forest—a typical boreal forest ecosystem with permafrost in Interior Alaska. RE measured as nighttime CO2 effluxes for both the ecosystem and the forest floor were clearly situated along the exponential temperature-dependent curve, except for the data obtained in extremely dry conditions in mid-summer. More than 93 % of RE data measured at nighttime with high vapor pressure deficit (VPD > 400 Pa) were lower than the values predicted from the temperature-dependent curve. Consequently, the year 2013 (with an unusually dry summer) had a 15 % lower amount of annual RE than that could be expected from the temperature-dependent curve without considering the effect of high VPD. The suppression of RE under dry conditions was also related to decreases in soil moisture and net ecosystem productivity. Finally, assuming daytime RE could be extrapolated from the temperature-dependent curves, annual daytime RE estimated with the effect of high VPD was decreased by up to 62% from RE estimated without the VPD effect. The down-regulation of RE presented in this study postulates a possible negative feedback for the carbon budget of boreal forests in response to climate warming.

Understanding how high-latitude terrestrial productivity and evapotranspiration change in association with rising atmospheric CO2 concentration ([CO2]), also known as ‘CO2 fertilization’, is important for predicting future climate change. To quantify the magnitude of this fertilization effect, we have developed a big-leaf model that couples photosynthesis and stomatal conductance processes. This model was inverted by inputting eddy covariance CO2 and H2O fluxes from four black spruce forests in Alaska to infer spatially representative ecophysiological parameters using a global optimization technique. Inferred seasonal variations in a maximum carboxylation rate at 25 °C per unit leaf area and stomatal conductance suggest greater photosynthetic capacity per unit leaf area during the mid-growing season, compared to spring and autumn. The interannual variability of parameters suggest that warm summers stimulate photosynthetic capacity and dry summers force stomatal regulation. Based on the model with optimized parameters, small but clear increases in gross primary productivity (GPP) and decreases in latent heat flux (LE) were estimated to be associated with rising [CO2] from 2002 to 2014 (p

An open black spruce forest, the most common ecosystem in interior Alaska, is characterized by patchy canopy gaps where the forest understory is exposed. This study measured CO2, sensible heat, and latent heat fluxes with eddy covariance (EC) in one of those large canopy gaps, and estimated understory fluxes in a black spruce forest in 2011–2014. Then understory fluxes and ecosystem fluxes were compared. The understory fluxes during the snow-free seasons were determined by two approaches. The first approach determined understory fluxes as the fluxes from the canopy gap, assuming that fluxes under the canopy crown also had the same magnitude as the canopy gap fluxes. The second approach determined the understory fluxes by scaling canopy gap fluxes with a canopy gap fraction, assuming that only canopy gaps, which mostly constitutes the forest floor, contribute to fluxes. The true understory fluxes would be in between these two estimates. Overall, the understory accounted for 53% (39–66%), 61% (45–77%), 63% (45–80%), 73% (56–90%), and 79% (59–98%) of the total net ecosystem productivity (NEP), gross primary productivity (GPP), ecosystem respiration (RE), sensible heat flux (H), and latent heat flux (LE), respectively. The ratio of understory NEP (NEPU) to the ecosystem NEP (NEPE) and similarly calculated LEU/LEE during the daytime increased with vapor pressure deficit (VPD) at low VPD conditions (∼2000 Pa) at half-hourly temporal scale. At high VPD conditions, however, NEPU/NEPE decreased with VPD, whereas LEU/LEE was maintained at the high level even at high VPD conditions. Despite large ranges of the estimates for the understory contributions, we conclude that the understory plays an important role in the carbon and energy balances of the black spruce ecosystem, and their contribution highly depends on the level of VPD.

Ecosystem-scale methane (CH 4 ) 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 CH 4 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 CH 4 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 CH 4 exchange for this ecosystem. Total snow-free season (day of year 134–280) CH 4 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 CH 4 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 CH 4 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 CH 4 emission. Different responses of CH 4 exchange to environmental conditions, depending on the position of a small catchment, should be considered when estimating the spatial variation in CH 4 exchange accurately in ecosystems over permafrost.

A fibre-optic DTS (distributed temperature sensing) system using Raman-scattering optical time domain reflectometry was deployed to monitor a boreal forest research site in the interior of Alaska. Surface temperatures range between −40 ∘C in winter and 30 ∘C in summer at this site. In parallel experiments, a fibre-optic cable sensor system (multi-mode, GI50/125, dual core; 3.4 mm), monitored at high resolution, (0.5 m intervals at every 30 min) ground surface temperatures across the landscape. In addition, a high-resolution vertical profile was acquired at one-metre height above the upper subsurface. The total cable ran 2.7 km with about 2.0 km monitoring a horizontal surface path. Sections of the cable sensor were deployed in vertical coil configurations (1.2 m high) to measure temperature profiles from the ground up at 5 mm intervals. Measurements were made continuously over a 2-year interval from October 2012 to October 2014. Vegetation at the site (Poker Flat Research Range) consists primarily of black spruce underlain by permafrost. Land cover types within the study area were classified into six descriptive categories: relict thermokarst lake, open moss, shrub, deciduous forest, sparse conifer forest, and dense conifer forest. The horizontal temperature data exhibited spatial and temporal changes within the observed diurnal and seasonal variations. Differences in snow pack evolution and insulation effects co-varied with the land cover types. The apparatus used to monitor vertical temperature profiles generated high-resolution (ca. 5 mm) data for air column, snow cover, and ground surface. This research also identified several technical challenges in deploying and maintaining a DTS system under subarctic environments.

We evaluate local differences in thermal regimes and turbulent heat fluxes across the heterogeneous canopy of a black spruce boreal forest on discontinuous permafrost in interior Alaska. The data were taken during an intensive observing period in the summer of 2013 from two micrometeorological towers 600 m apart in a central section of boreal forest, one in a denser canopy (DC) and the other in a sparser canopy, but under approximately similar atmospheric boundary layer (ABL) flow conditions. Results suggest that on average 34% of the half-hourly periods in a day are nonstationary, primarily during night and during ABL transitions. Also, thermal regimes differ between the two towers; specifically between midnight and 0500 Alaska Standard Time (AKST) it is about 3°C warmer at DC. On average, the sensible heat flux at DC was greater. For midday periods, the difference between those fluxes exceeded 30% of the measured flux and over 30 W m−2 in magnitude more than 60% of the time. These differences are due to higher mechanical mixing as a result of the increased density of roughness elements at DC. Finally, the vertical distribution of turbulent heat fluxes verifies a maximum atop the canopy crown (2.6 h) when compared with the subcanopy (0.6 h) and above canopy (5.1 h), where h is the mean canopy height. We argue that these spatial and vertical variations of sensible heat fluxes result from the complex scale aggregation of energy fluxes over a heterogeneous canopy.

High-latitude warming has had a discernible effect on the productivity of boreal forests. Here, we report a change in the growth responses of a major biome of boreal North America, black spruce, to...