Autumn warming reduces the CO₂ sink of a black spruce forest in interior Alaska based on a nine‐year eddy covariance measurement

Abstract

Nine years (2003-2011) of carbon dioxide (CO2) flux were measured at a black spruce forest in interior Alaska using the eddy covariance method. Seasonal and interannual variations in the gross primary productivity (GPP) and ecosystem respiration (RE) were associated primarily with air temperature: warmer conditions enhanced GPP and RE. Meanwhile, interannual variation in annual CO2 balance was controlled predominantly by RE, and not GPP. During these 9 years of measurement, the annual CO2 balance shifted from a CO2 sink to a CO2 source, with a 9-year average near zero. The increase in autumn RE was associated with autumn warming and was mostly attributed to a shift in the annual CO2 balance. The increase in autumn air temperature (0.22 °C yr(-1)) during the 9 years of study was 15 times greater than the long-term warming trend between 1905 and 2011 (0.015 °C yr(-1)) due to decadal climate oscillation. This result indicates that most of the shifts in observed CO2 fluxes were associated with decadal climate variability. Because the natural climate varies in a cycle of 10-30 years, a long-term study covering at least one full cycle of decadal climate oscillation is important to quantify the CO2 balance and its interaction with the climate.

Carbon dioxide sources from Alaska driven by increasing early winter respiration from Arctic tundra

“…Carbon uptake by boreal forests was smaller in 2013 than in other years, when the climate was warmer and drier. There was less CO 2 respiration during the early winter periods of 2014, possibly due to high June through August precipitation and cooler temperatures; long-term measurements of CO 2 fluxes have shown that water-saturated soils exhibit lower rates of soil respiration in boreal forests (33)(34)(35). The remaining 18% of Alaskan land surface (here called the "Mixed" area), which includes coastal plains, mountainous areas, and areas that cannot be classified as mostly forest or tundra, was a net source of carbon each year.…”

“…1). In tundra, uptake is delayed for some time after snow melts and ecosystem greenness increases, whereas evergreen trees may begin to take up CO 2 when air temperatures rise above 0°C in spring (33). Models driven by solar-induced chlorophyll fluorescence (SIF) appear to capture the delay between snowmelt and net CO 2 uptake in both ecosystems (SI Appendix, Growing Season Length), whereas models driven by vegetation indices (e.g., enhanced vegetation index) do not (23).…”

Interannual variability of ecosystem carbon exchange: From observation to prediction

“…Nevertheless, temperature anomalies tend to change ER more than GPP (Yvon‐Durocher, Jones, Trimmer, Woodward, & Montoya, ). For example, based on a 9‐year eddy covariance measurement record, years with autumnal warming reduced the annual CO 2 sink because of the stimulation of ER in a black spruce forest (Ueyama, Iwata, & Harazono, ). Moreover, nitrogen deposition tends to favour GPP more than ER at the ecosystem scale (Fernández‐Martínez et al, ).…”

Vegetation greenness and land carbon-flux anomalies associated with climate variations: a focus on the year 2015

“…Several studies reported an enhanced greening during spring and summer in the Northern Hemisphere (Myneni et al, 1997;Zhou et al, 2001), as driven by increasing spring and summer temperatures (Barichivich et al, 2013;Nemani et al, 2003), leading to enhanced land carbon uptake and a long-term increase in the seasonal amplitude of atmospheric CO 2 in northern latitudes (Forkel et al, 2016;Graven et al, 2013). However, for autumn, even though ending of the growing season has been delayed because of autumn warming (Barichivich et al, 2013), land carbon-uptake termination time is found to have advanced as well, due to enhanced autumn respiration , which ultimately reduced the annual net ecosystem carbon uptake (Hadden and Grelle, 2016;Ueyama et al, 2014). For TeNH, we also found a significant negative relationship between land carbon-uptake anomalies and temperature for Q3 using the CAMS inversion data, consistent with the enhanced respiration by autumn warming found in the aforementioned studies.…”