 |
| Titel |
Understanding the Spatiotemporal Variability of Inherent Water Use Efficiency |
| VerfasserIn |
Sven Boese, Martin Jung, Nuno Carvalhais, Markus Reichstein |
| Konferenz |
EGU General Assembly 2015
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250107969
|
| Publikation (Nr.) |
 EGU/EGU2015-7697.pdf |
|
|
|
|
|
| Zusammenfassung |
| The global carbon and water cycles are coupled via plant physiology. However, the resulting
spatial and temporal covariability of both fluxes on a global scale lacks sufficient
understanding. This is required to estimate the impact of atmospheric drought on
photosynthesis in water-limited ecosystems.
Water use efficiency (WUE) is an essential ecosystem diagnostic defined as
the ratio between gross primary productivity (GPP) and transpiration (T). WUE
is known to vary with vapour-pressure deficit (VPD) and therefore also in time.
The inherent water use efficiency (iWUE) accounts for the VPD effect on WUE
and aims at representing a largely time-invariant ecosystem property. However,
different ways of describing the functional response of iWUE to VPD are found in the
literature.
One established iWUE definition was proposed by Beer et al. (2009) and takes the form
of
iWUE = GPP-/
VPD- .
T
(1)
A similar definition can be derived from stomatal conductance theories such as Katul et
al. (2010) and takes the form of
/ ––
GPP-/
-VPD–
iWUE = T .
(2)
Here, we use eddy covariance measurements from the FLUXNET database to evaluate
both approaches for a globally representative set of biomes including tropical, temperate and
semi-arid ecosystems. Testing both definitions in a model–data fusion setup indicated that (2)
is more consistent with FLUXNET observations than (1). However, there still remains
considerable temporal variability of iWUE which is linked to seasonal changes in
VPD.
To explore up to which extent the temporal variability of iWUE may be related to the
prescribed functional responses to VPD, we treated the exponent of VPD as a global
parameter, here termed γ. When γ = 1 the functional response is equivalent to (1), while
when γ = 0.5 it corresponds to formulation of model (2)). The global estimate was found to
be significantly lower than 0.5, which would have been expected from stomatal conductance
theory at leaf level.
We assessed whether adding γ as site-specific parameter could be justified. The additional
model complexity was warranted by an increased goodness-of-fit as quantified by the
Akaike information criterion. However, temporal variations in iWUE persist. The
structural adequacy of the models was assessed via the correlation structure of the
residuals.
Ultimately, changing γ in the definition impacts the between-site variability of iWUE.
The iWUE estimates with γ = 1.0 were only weakly correlated with those with γ = 0.5. This
has crucial implications for spatial analyses on the drought response of water-limited
ecosystems.
We discuss uncertainties involved in the analysis and highlight possible mechanisms
responsible for the remaining temporal variability of iWUE. The consequences of differing
iWUE definitions for the analysis of global carbon and water cycles are explored. |
|
|
|
|
|
|