Greening Hiatus in Eurasian Boreal Forests Since 1997 Caused by a Wetting and Cooling Summer Climate

Abstract

The Eurasian boreal forest ecosystem is a strong sink in the global carbon cycle. Satellite observations show significant change in the ecosystem in recent decades, specifically an increase in vegetation productivity since 1982 and a hiatus after 1997. Previous studies attributed this enhanced vegetation growth (also known as greening) to air temperature increases and a longer growing season, and the recent greening hiatus as a result of a warmer and drier climate. However, using satellite data, we found observational evidence that increases in summer peak growth dominated the overall greening trend and that a wetting and cooling climate during the peak growing season was the primary cause of the hiatus. Plain Language Summary Increases in vegetation growth or productivity is referred to as "greening." Scientists have found that the vegetation growth or productivity in Eurasian boreal forests increased from 1982. However, in 1997 that greening stagnated. Previous studies suggested that increases in air temperature prolonged the growing season, which may have been the driving force of the greening. Additionally, previous studies pointed to enhanced drought stress by warmer and drier summers as the reason for the greening hiatus. However, in revisiting this hypothesis we found evidence of an alternative explanation. First, we found that increases in summer peak growth, rather than a longer growing season, dominated the overall greening trends. Second, we found that wetting and cooling summer climates instigated the hiatus. Our findings suggest a more complex mechanism behind the shift in climate and vegetation growth in Eurasian boreal forests around 1997. This mechanism deserves further study.

Is the grass always greener? Land surface phenology reveals differences in peak and season‐long vegetation productivity responses to climate and management

“…By including relatively simple explanatory variables, we were able to identify changes through time and across communities and to identify divergent influences of temperature on peak versus season‐long productivity. Therefore, the definition of greening and implications of both measures should be clearly understood and incorporated in phenological studies (Gao et al., 2020 ). Furthermore, illustrating the consequences of greening and/or browning under contemporary or future scenarios should include assessment of their patterns across varied plant communities and assess the trade‐offs/exchanges of higher spring/peak productivity for lower season‐long production (Butterfield et al., 2020 ; Hu et al., 2010 ; Wang et al., 2011 ).…”

“…Overall, both precipitation and temperature were needed to fully explain phenological measures (e.g., Gao et al., 2020 ). Spatial and ecological differences in limiting factors—such as temperature restrictions on the length of the season in water‐sufficient areas versus water‐limited systems with ample growing days—lead to spatial variation (Chen et al., 2019 ; Cowles et al., 2018 ; Piao et al., 2019 ; Reed et al., 2019 ; Yang et al., 2021 ).…”

“…In addition, there are important differences in the implications of climate change for peak productivity versus season‐long productivity. Unfortunately, only peak or season‐long productivity is typically investigated, which can obscure comparisons of the mechanisms underlying different aspects of phenological changes (Gao et al., 2020 ). Precipitation and temperature have been identified as important drivers of seasonal variation in productivity; however, these relationships can vary across ecosystems and vegetation types (Fu et al., 2017 ; Maurer et al., 2020 ; Reed et al., 2019 ; Yang et al., 2021 ).…”

Antecedent climatic conditions spanning several years influence multiple land-surface phenology events in semi-arid environments

“…The factors that most strongly govern phenological aspects vary across space, and among different vegetation communities (Xia et al, 2015;Fu et al, 2017;Maurer et al, 2020;Wood et al, 2021). Furthermore, climatic controls on phenology are complex, may interact, and can be non-linear (Ponce-Campos et al, 2013;Knapp et al, 2015;Al-Yaari et al, 2020;Gao et al, 2020;Maurer et al, 2020), and ecosystems may respond to far more than just current conditions as they develop.…”

Comment on esd-2021-85

“…Anthropogenic greenhouse gas (GHG) emissions have led the global mean temperature to increase by approximately 1.1 degree Celsius since the industrial revolution (IPCC, 2013(IPCC, , 2018IPCC AR6 WGI, 2021). Changes in climate have caused impacts on natural ecosystems and human societies, such as mass ice sheet melt (Jevrejeva et al, 2016), devastating heat waves (Dosio et al, 2018), and increase in extreme climate events (Kirchmeier-Young and Zhang, 2020), exposing natural and human systems to uncertainties and the risks of unsustainable development (Gao et al, 2019(Gao et al, , 2020. Despite the scientific consensus on climate change, emission-reduction efforts have made slow or little progress with global GHG emissions continuing to rise (IPCC AR6 WGI, 2021).…”