Zscheischler, J, Fatichi, S, Wolf, S, Blanken, P.D, Bohrer, G, Clark, K, Desai, A.R, Hollinger, D, Keenan, T, Novick, K.A
NWT Accession Number: NWT1903
Ecosystem models often perform poorly in reproducing interannual variability in carbon
and water ﬂuxes, resulting in considerable uncertainty when estimating the land-carbon sink. While many
aggregated variables (growing season length, seasonal precipitation, or temperature) have been suggested
as predictors for interannual variability in carbon ﬂuxes, their explanatory power is limited and uncertainties
remain as to their relative contributions. Recent results show that the annual count of hours where
evapotranspiration (ET) is larger than its 95th percentile is strongly correlated with the annual variability
of ET and gross primary production (GPP) in an ecosystem model. This suggests that the occurrence of
favorable conditions has a strong inﬂuence on the annual carbon budget. Here we analyzed data from
eight forest sites of the AmeriFlux network with at least 7 years of continuous measurements. We show that
for ET and the carbon ﬂuxes GPP, ecosystem respiration (RE), and net ecosystem production, counting the
“most active hours/days” (i.e., hours/days when the ﬂux exceeds a high percentile) correlates well with the
respective annual sums, with correlation coeﬃcients generally larger than 0.8. Phenological transitions have
much weaker explanatory power. By exploiting the relationship between most active hours and interannual
variability, we classify hours as most ac tive or less active and largely explain interannual variability in
ecosystem ﬂuxes, particularly for GPP and RE. Our results suggest that a better understanding and modeling
of the occurrence of large values in high-frequency ecosystem ﬂuxes will result in a better understanding of
interannual variability of these ﬂuxes.
Carbon Cycle, Water Cycle, Interannual Variability, Evapotranspiration, Gross Primary Production, Ecosystem Respiration
Zscheischler, J, Fatichi, S, Wolf, S, Blanken, P.D, Bohrer, G, Clark, K, Desai, A.R, Hollinger, D, Keenan, T, Novick, K.A, (2016) Short-term favorable weather conditions are an important control of interannual variability in carbon and water fluxes. Biogeosciences 121 (8) :2186-2198 , DOI: 10.1002/2016JG003503
This material is based upon work supported by the National Science Foundation under Cooperative Agreement #DEB-1637686. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necesarily reflect the views of the National Science Foundation.
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