This figure shows changes from the mid-20th century (black line) to the early 21st century (blue line) of estimated monthly mean gross primary productivity (GPP, e.g. total photosynthesis; panel A), total ecosystem respiration (TER, e.g., the sum of respiration of all plants, animals, and soil microbes; panel B), and net ecosystem exchange of carbon (NEE, e.g., the difference between GPP and TER, which equals the growth of plant biomass and soil carbon stocks. Black and blue shading show respective uncertainties. Increases in all three components of carbon cycling indicate that the northern forest contributes substantially to the increased seasonal cycle amplitude of CO2 measured globally, such as the Moana Loa observation record. The study also develops several additional lines of evidence to attribute these increased rates of carbon cycling primarily to increasing temperature. Credit: Liu et al., 2020, Figure S7, panels A and B
Source: AGU Advances

Atmospheric CO2 rises and falls with the northern hemisphere winter and summer, respectively, and the amplitude of that seasonality is increasing. Is the vast boreal forest contributing to this trend, and, if so, why? Most models assume that increased CO2 stimulates photosynthesis, increased temperature stimulates respiration, and the difference between the two determines net ecosystem exchange (NEE) of carbon. Inferring photosynthesis and NEE from satellite data, Liu et al. [2020] find that 75% of the spatial variations in photosynthesis across the region were explained by growing season mean temperature. Modeling this response indicated that increased temperatures from 1960 to 2010 coincided with a 20% increase in the seasonal amplitude of photosynthesis and NEE. These results are important because, while future CO2 fertilization may have diminishing effects, a positive response to temperature implies continued expansion of high-latitude forests and their productivity. More work is needed to investigate whether respiration could overtake photosynthesis responses to temperature and whether nitrogen availability could limit responses.

Citation: Liu, J., Wennberg, P., Parazoo, N., Yin, Y., & Frankenberg, C. [2020]. Observational constraints on the response of high-latitude northern forests to warming. AGU Advances, 1, e2020AV000228.

—Eric A. Davidson, Editor, AGU Advances

Text © 2020. The authors. CC BY-NC-ND 3.0
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