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Abstract

Inverse-estimated net carbon exchange time series spanning two decades for six North
American regions are analyzed to examine long-term trends and relationships to
temperature and precipitation variations. Results reveal intensification of carbon uptake
in eastern boreal North America (0.1 PgC/decade) and the Midwest United States
(0.08 PgC/decade). Seasonal cross-correlation analysis shows a significant relationship
between net carbon exchange and temperature/precipitation anomalies during the western
United States growing season with warmer, dryer conditions leading reduced carbon uptake.
This relationship is consistent with “global change-type drought” dynamics which drive
increased vegetation mortality, increases in dry woody material, and increased wildfire
occurrence. This finding supports the contention that future climate change may increase
carbon loss in this region. Similarly, higher temperatures and reduced precipitation are
accompanied by decreased net carbon uptake in the Midwestern United States toward the
end of the growing season. Additionally, intensified net carbon uptake during the eastern
boreal North America growing season is led by increased precipitation anomalies in the
previous year, suggesting the influence of “climate memory” carried by regional
snowmelt water. The two regions of boreal North America exhibit opposing seasonal
carbon-temperature relationships with the eastern half experiencing a net carbon loss with
near coincident increases in temperature and the western half showing increased net carbon
uptake. The carbon response in the boreal west region lags the temperature anomalies by
roughly 6months. This opposing carbon-temperature relationship in boreal North America
may be a combination of different dominant vegetation types, the amount and timing of
snowfall, and temperature anomaly differences across boreal North America.