192
while an intermediate isotopic composition would
suggests that both water sources are being used. A
more precise quantification of the relative or absolute utilization of the two water sources is more
difficult and requires additional measurements,
such as of water flow rate through the plant and the
exchangeable water volume. This is particularly
important when one of the two water sources becomes available as a pulse.
This can be shown in a simple dynamic mixing
model, where we consider a constant transpiration
flow (E) through a well-mixed constant volume
(V), fed in part (f) by a labeled source and in part
(1 - f) by an unlabeled source. The concentration
of label in the fixed volume depends not only on
the relative contribution, f, of the labeled source to
the uptake, but also on the flow-to-volume ratio,
EN. When this ratio is low, the concentration of
label lags behind the true fraction, f, of labeled
source in the total flow, E. Thus, the true utilization
fraction of a pulsed source is likely to be underestimated, unless the dynamics of xylem water composition, as affected by EN, are accounted for. Actual pulse label dynamics are often more
complicated, because water uptake rates and ratios
may not remain constant during or after a pulse.
Thus, anyone-time estimation of the fraction of
water from a pulsed source in the transpiration
stream would give incomplete information on overall pulse utilization. In this case, pulse utilization
can be quantified precisely only when xylem composition and transpiration rates are monitored during and after the pulse. Then, total pulse utilization
can be estimated by integrating, over time, the fraction of pulsed resource in the xylem and the whole
plant transpiration rate. Since such intensive data
collection is often not possible, methods have been
developed to estimate integrated pulse utilization
based on substantially less information.
In studies in which the utilization of single pulses
is the focus, simplified models of pulse utilization
dynamics have been used to fill the gaps left by
limited data collection. Schwinning and Ehleringer
(2000) suggested that relative pulse utilization by
plants with contrasting life history strategies can be
estimated within an accuracy of 10 to 20% based
on only four data points: the time and magnitude
of the maximal fraction of pulsed source water in
the xylem, and two estimates of the whole-plant
EN ratios during and after the pulse. Schwinning
James R. Ehleringer, John Roden, and Todd E. Dawson
and Ehleringer (2000) suggest that seasonally integrated pulse utilization can be estimated by randomly sampling xylem water composition and
whole-plant transpiration rates throughout the season, regardless of time since last pulse. This method
makes no assumption on pulse dynamics, but simply represents a statistical estimate of the seasonally
integrated pulse-fed transpiration rate, based on
randomly selected subsamples.
Water-Use Efficiency
If water sources are differentially available to
plants with either different rooting depths or requirements for water, this could influence their water use behavior and thereby the efficiency with
which they fix carbon. Differences in water use
behavior have been assessed with the use of carbon isotope analyses of plant leaves (Farquhar et
aI., 1989) or tree ring cellulose (see Livingston and
Spittlehouse, 1993). In plants with the C 3 photosynthetic pathway, variation in the leaf carbon isotope ratio (813C) can be correlated with the longterm efficiency with which the water resource is
used (see Ehleringer 1991). This is because, during
the photosynthetic process, C 3 leaves discriminate
differentially against l3C02 more than 12C02, particularly when the leaf stomata are fully opened
(Farquhar et al. 1989). Therefore, if plants within
an ecosystem are using different water sources and
experiencing different levels of water stress that
lead to differences among species or over time in
the water use behaviors, this could be "recorded"
in the leaf or wood carbon being synthesized at
that time. More conservative water use behavior
should therefore, for example, in shallowly rooted
species that experience more water stress, lead to
a different leaf or wood 8l3C than more deeply
rooted species that are using deeper, more stable
water sources. This sort of pattern has been documented in arid land systems by Flanagan et al.
(1992). Assessments of the variation in 8l3C have
also been used to estimate evapotranspiration components over simple plant canopies (see Bariac et
al. 1989; Ebdon et aI. 1998). This sort of approach
has the potential to help link water sources and
water use behavior to the total system water flux
in ecosystem-level investigations.
while an intermediate isotopic composition would
suggests that both water sources are being used. A
more precise quantification of the relative or absolute utilization of the two water sources is more
difficult and requires additional measurements,
such as of water flow rate through the plant and the
exchangeable water volume. This is particularly
important when one of the two water sources becomes available as a pulse.
This can be shown in a simple dynamic mixing
model, where we consider a constant transpiration
flow (E) through a well-mixed constant volume
(V), fed in part (f) by a labeled source and in part
(1 - f) by an unlabeled source. The concentration
of label in the fixed volume depends not only on
the relative contribution, f, of the labeled source to
the uptake, but also on the flow-to-volume ratio,
EN. When this ratio is low, the concentration of
label lags behind the true fraction, f, of labeled
source in the total flow, E. Thus, the true utilization
fraction of a pulsed source is likely to be underestimated, unless the dynamics of xylem water composition, as affected by EN, are accounted for. Actual pulse label dynamics are often more
complicated, because water uptake rates and ratios
may not remain constant during or after a pulse.
Thus, anyone-time estimation of the fraction of
water from a pulsed source in the transpiration
stream would give incomplete information on overall pulse utilization. In this case, pulse utilization
can be quantified precisely only when xylem composition and transpiration rates are monitored during and after the pulse. Then, total pulse utilization
can be estimated by integrating, over time, the fraction of pulsed resource in the xylem and the whole
plant transpiration rate. Since such intensive data
collection is often not possible, methods have been
developed to estimate integrated pulse utilization
based on substantially less information.
In studies in which the utilization of single pulses
is the focus, simplified models of pulse utilization
dynamics have been used to fill the gaps left by
limited data collection. Schwinning and Ehleringer
(2000) suggested that relative pulse utilization by
plants with contrasting life history strategies can be
estimated within an accuracy of 10 to 20% based
on only four data points: the time and magnitude
of the maximal fraction of pulsed source water in
the xylem, and two estimates of the whole-plant
EN ratios during and after the pulse. Schwinning
James R. Ehleringer, John Roden, and Todd E. Dawson
and Ehleringer (2000) suggest that seasonally integrated pulse utilization can be estimated by randomly sampling xylem water composition and
whole-plant transpiration rates throughout the season, regardless of time since last pulse. This method
makes no assumption on pulse dynamics, but simply represents a statistical estimate of the seasonally
integrated pulse-fed transpiration rate, based on
randomly selected subsamples.
Water-Use Efficiency
If water sources are differentially available to
plants with either different rooting depths or requirements for water, this could influence their water use behavior and thereby the efficiency with
which they fix carbon. Differences in water use
behavior have been assessed with the use of carbon isotope analyses of plant leaves (Farquhar et
aI., 1989) or tree ring cellulose (see Livingston and
Spittlehouse, 1993). In plants with the C 3 photosynthetic pathway, variation in the leaf carbon isotope ratio (813C) can be correlated with the longterm efficiency with which the water resource is
used (see Ehleringer 1991). This is because, during
the photosynthetic process, C 3 leaves discriminate
differentially against l3C02 more than 12C02, particularly when the leaf stomata are fully opened
(Farquhar et al. 1989). Therefore, if plants within
an ecosystem are using different water sources and
experiencing different levels of water stress that
lead to differences among species or over time in
the water use behaviors, this could be "recorded"
in the leaf or wood carbon being synthesized at
that time. More conservative water use behavior
should therefore, for example, in shallowly rooted
species that experience more water stress, lead to
a different leaf or wood 8l3C than more deeply
rooted species that are using deeper, more stable
water sources. This sort of pattern has been documented in arid land systems by Flanagan et al.
(1992). Assessments of the variation in 8l3C have
also been used to estimate evapotranspiration components over simple plant canopies (see Bariac et
al. 1989; Ebdon et aI. 1998). This sort of approach
has the potential to help link water sources and
water use behavior to the total system water flux
in ecosystem-level investigations.