Abstract
Ongoing spring warming allows the growing season to begin
earlier, enhancing carbon uptake in northern ecosystems1–3
.
Here we use 34 years of atmospheric CO2 concentration
measurements at Barrow, Alaska (BRW, 71◦ N) to show that
the interannual relationship between spring temperature and
carbon uptake has recently shifted. We use two indicators: the
spring zero-crossing date of atmospheric CO2 (SZC) and the
magnitude of CO2 drawdown between May and June (SCC). The
previously reported strong correlation between SZC, SCC and
spring land temperature (ST) was found in the first 17 years
of measurements, but disappeared in the last 17 years. As
a result, the sensitivity of both SZC and SCC to warming
decreased. Simulations with an atmospheric transport model4
coupled to a terrestrial ecosystem model5
suggest that the
weakened interannual correlation of SZC and SCC with ST in
the last 17 years is attributable to the declining temperature
response of spring net primary productivity (NPP) rather
than to changes in heterotrophic respiration or in atmospheric
transport patterns. Reduced chilling during dormancy and
emerging light limitation are possible mechanisms that may
have contributed to the loss of NPP response to ST. Our results
thus challenge the ‘warmer spring–bigger sink’ mechanism.
