Berkelhammer, M., Noone, D.C., Wong, T.E., Burns, S.P., Knowles, J.F., Kaushik, A., Blanken, P.D., Williams, M.W.
Global Biogeochemical Cycles
NWT Accession Number: NWT1905
The transpiration (T) fraction of total terrestrial evapotranspiration (ET), T/ET, can vary across
ecosystems between 20–95% with a global average of ∼60%. The wide range may either reﬂect true
heterogeneity between ecosystems and/or uncertainties in the techniques used to derive this property.
Here we compared independent approaches to estimate T/ET at two needleleaf forested sites with a factor
of 3 diﬀerence in leaf area index (LAI). The ﬁrst method utilized water vapor isotope proﬁles and the second
derived transpiration through its functional relationship with gross primary production. We found strong
agreement between T/ET values from these two independent approaches although we noted a discrepancy
at low vapor pressure deﬁcits (VPD). We hypothesize that this divergence arises because stomatal
conductance is independent of humidity at low VPD. Overall, we document signiﬁcant synoptic-scale T/ET
variability but minimal growing season-scale variability. This result indicates a high sensitivity of T/ET to
passing weather but convergence toward a stable mean state, which is set by LAI. While changes in T/ET
could emerge from a myriad of processes, including aboveground (LAI) or belowground (rooting depth)
changes, there was only minimal interannual variability and no secular trend in our analysis of T/ET from the
15 year eddy covariance time series at Niwot Ridge. If the lack of trend observed here is apparent elsewhere,
it suggests that the processes controlling the T and E ﬂuxes are coupled in a way to maintain a stable ratio.
Ecosystem Fluxes, Transpiration Fraction, Evapotransporation, Water Cycle, Gross Primary Production, Leaf Area Index
Berkelhammer, M., Noone, D.C., Wong, T.E., Burns, S.P., Knowles, J.F., Kaushik, A., Blanken, P.D., Williams, M.W., (2016) Convergent approaches to determine an ecosystem's transpiration fraction. Global Biogeochemical Cycles 30 (6) :933-951 , DOI: 10.1002/2016GB005392
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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