• Energy Research

Terrestrial forest ecosystems are crucial to the global carbon cycle and climate system; however, these ecosystems have experienced significant warming rates in recent decades, whose impact remains uncertain. This study investigated radial tree growth using the tree-ring width index (RWI) for forest ecosystems throughout the Northern Hemisphere to determine tree growth responses to autumn climate change, a season which remains considerably understudied compared to spring and summer, using response function and random forest machine learning methods. Results showed that autumn climate conditions significantly impact the RWI throughout the Northern Hemisphere. Spatial variations in the RWI response were influenced by geography (latitude, longitude, and elevation), climatology, and biology (tree genera); however, geographical and/or climatological characteristics explained more of the response compared to biological characteristics. Higher autumn temperatures tended to negatively impact tree radial growth south of 40° N in regions of western Asia, southern Europe, United State of America and Mexico, which was similar to the summer temperature response found in previous studies, which was attributed to temperature-induced water stress.

Abstract Arctic and boreal permafrost ecosystems in Eastern Siberia, considered crucial to the climate system and global carbon cycle, are particularly vulnerable to climate change. This study investigates carbon dioxide (CO2) exchange fluxes over northeastern Siberia from 2013 to 2015 in a taiga–tundra boundary ecosystem for which such measurements are scarce. The growing season (May–September) net CO2 exchange flux (NEE) was −39.4 (−60.1 to −20.2) gCm−2, with ecosystem respiration (RE) = 306.2 (288.1–317.9) gCm−2 and gross primary production (GPP) = −345.5 (−372.5 to −317.7) gCm−2. Microclimatic factors determining these CO2 exchange fluxes change seasonally. These fluxes are significantly affected by the timing of the onset of C uptake, which is reflected by changes in the soil temperature in spring and early summer, following which fluxes respond well to the photosynthetic photon flux density, especially for NEE. These CO2 exchange fluxes at the northeastern Siberian taiga–tundra boundary ecosystem are significantly smaller than those previously reported at southern-taiga forest sites. Spring snow meltwater-rich soil moisture conditions render southern-taiga sites as stronger CO2 sinks in June than taiga–tundra boundary ecosystem, which may be largely responsible for the pronounced north–south gradient in growing season NEE.

AbstractThe FLUXNET2015 dataset provides ecosystem-scale data on CO2, water, and energy exchange between the biosphere and the atmosphere, and other meteorological and biological measurements, from 212 sites around the globe (over 1500 site-years, up to and including year 2014). These sites, independently managed and operated, voluntarily contributed their data to create global datasets. Data were quality controlled and processed using uniform methods, to improve consistency and intercomparability across sites. The dataset is already being used in a number of applications, including ecophysiology studies, remote sensing studies, and development of ecosystem and Earth system models. FLUXNET2015 includes derived-data products, such as gap-filled time series, ecosystem respiration and photosynthetic uptake estimates, estimation of uncertainties, and metadata about the measurements, presented for the first time in this paper. In addition, 206 of these sites are for the first time distributed under a Creative Commons (CC-BY 4.0) license. This paper details this enhanced dataset and the processing methods, now made available as open-source codes, making the dataset more accessible, transparent, and reproducible.

Abstract The tree-ring width index (RWI) and satellite-derived vegetation indices, such as the Normalized Difference Vegetation Index (NDVI), are used as long-term indicators of the past forest carbon uptake. However, fundamental questions remain with respect to what is represented by the RWI and NDVI at the ecosystem level. To address this question, we compared tree-ring parameters (RWI and the carbon isotope ratio: δ13C) and NDVI products with forest ecosystem CO2 fluxes estimated using the eddy covariance method, at a larch forest in eastern Siberia. The RWI and tree-ring δ13C correlated well with the ecosystem gross primary production (GPP), and their temporal stabilities were high during 2004–2014. However, the NDVI products did not show any temporally stable relationship with the GPP. This could be ascribed to significant changes in the understory vegetation in this forest, i.e., from dense cowberry to shrubs and moisture-tolerant grasses, because of an excessively moist environment during 2007–2008. Changes in the understory vegetation could be reflected by the NDVI products but not by the GPP. Our results indicate that it is more feasible to study forest carbon uptake using tree-ring parameters than using satellite-derived vegetation indices in such larch-dominated forest ecosystems in eastern Siberia.