Increased high‐latitude photosynthetic carbon gain offset by respiration carbon loss during an anomalous warm winter to spring transition

Abstract

Arctic and boreal ecosystems play an important role in the global carbon (C) budget, and whether they act as a future net C sink or source depends on climate and environmental change. Here, we used complementary in situ measurements, model simulations, and satellite observations to investigate the net carbon dioxide (CO2) seasonal cycle and its climatic and environmental controls across Alaska and northwestern Canada during the anomalously warm winter to spring conditions of 2015 and 2016 (relative to 2010–2014). In the warm spring, we found that photosynthesis was enhanced more than respiration, leading to greater CO2 uptake. However, photosynthetic enhancement from spring warming was partially offset by greater ecosystem respiration during the preceding anomalously warm winter, resulting in nearly neutral effects on the annual net CO2 balance. Eddy covariance CO2 flux measurements showed that air temperature has a primary influence on net CO2 exchange in winter and spring, while soil moisture has a primary control on net CO2 exchange in the fall. The net CO2 exchange was generally more moisture limited in the boreal region than in the Arctic tundra. Our analysis indicates complex seasonal interactions of underlying C cycle processes in response to changing climate and hydrology that may not manifest in changes in net annual CO2 exchange. Therefore, a better understanding of the seasonal response of C cycle processes may provide important insights for predicting future carbon–climate feedbacks and their consequences on atmospheric CO2 dynamics in the northern high latitudes.

Recent Amplified Global Gross Primary Productivity Due to Temperature Increase Is Offset by Reduced Productivity Due to Water Constraints

“…The rapidly changing arctic and boreal ecosystems are crucial components of the Earth system that store more than 30% of terrestrial carbon stocks (Apps et al, 1993; Pan et al, 2011). While boreal ecosystems have remained a persistent terrestrial carbon sink (Ciais et al, 2010), recent models and observations predict that increasing air temperatures will reduce the carbon uptake capacity of these biomes over the next century (Liu et al, 2019; Natali et al, 2019). Longer growing seasons and earlier observed photosynthesis from climate warming (Assmann et al, 2019; Box et al, 2019; Parazoo et al, 2018) lead to increased rate and duration of evapotranspiration which can deplete SM and plant available water in the late growing season (Buermann et al, 2013, 2018; Lian et al, 2019; Parida & Buermann, 2014; Yi et al, 2014; Zhang et al, 2020).…”

High Vapor Pressure Deficit Decreases the Productivity and Water Use Efficiency of Rain‐Induced Pulses in Semiarid Ecosystems

“…Pulses are transient periods characterized by high rates of gross ecosystem photosynthesis ( GEP ), evapotranspiration ( ET ), and ecosystem respiration ( R e ) that occur when rain events temporarily mitigate water stress effects on plant and microbial activity (Huxman, Snyder, et al, 2004; Noy‐Meir, 1973; Schwinning et al, 2004). With high rates of biogeochemical cycling, pulses have a disproportionately large impact on annual GEP (Kannenberg et al, 2020), which is a key driver of variation in the net ecosystem production ( NEP = GEP − R e ) and ecosystem‐scale water use efficiency ( WUE = GEP / ET ) of semiarid regions (Baldocchi et al, 2018; Biederman et al, 2016; Jia et al, 2016; Liu et al, 2019; Scott et al, 2015). Therefore, it is necessary to examine controls on ecosystem carbon and water fluxes during these important subannual periods (Barnes et al, 2016; Jenerette et al, 2012; Jung et al, 2017).…”

Multiple‐scale negative impacts of warming on ecosystem carbon use efficiency across the Tibetan Plateau grasslands

“…Findings from single‐scale studies have advantages and drawbacks. Experimental observations can deepen the mechanistic understanding of the carbon cycle and its drivers (Z. Liu, Kimball, et al., 2019). However, the availability of these types of observations is limited, especially for regions with harsh environmental conditions (Qiu, 2008; Y. Zhang, Zhu, et al., 2019).…”

“…However, the availability of these types of observations is limited, especially for regions with harsh environmental conditions (Qiu, 2008; Y. Zhang, Zhu, et al., 2019). It is challenging to estimate the carbon cycle and climate interactions in a spatially and temporally continuous manner based on experimental observations alone, leading to data insufficiencies in constraining and validating the carbon fluxes at large scales (Z. Liu, Kimball, et al., 2019). Remote sensing observations can provide spatially continuous data and can act as a favourable surrogate for estimating CUEe or CUEc at extended spatial scales.…”