12. Assessing Ecosystem-Level Water Relations Through Stable Isotope Ratio Analyses
193
Short-Term, Regional
Process-Level Applications
Across Ecosystems
Recycling of Water Among
and Between Ecosystems
The terrestrial biosphere influences global cycles of
CO2 and water. Stable isotopes can be used to probe
these cycles since biological fractionation and subsequent exchange with the atmosphere alters ambient isotopic signatures of CO 2 and water vapor
(Hsieh et al., 1998). For example, Gat and Matsui
(1991) used the 8D and 8 18 0 of precipitation across
a longitudinal gradient to quantify the relative contribution of evaporation and transpiration to the recycling of water vapor in the Amazon basin. There
is a predictable isotopic depletion of water in precipitation as stonn fronts progress (the continental
effect) due to Rayleigh distillation effects, and deviations imply that a source of isotopically enriched
vapor must be coming from the biosphere. The
source can be further delineated since vapor from
evaporation processes (lakes, rainfall, stemflow,
soil water, etc.) should be isotopically depleted
relative to that from transpiration (Gat and Matsui
1991; Martinelli et al. 1996). Rather than sampling
precipitation, Moreira et al. (1997) used flasks located on towers and balloons to sample atmospheric vapor directly and, using simple isotope
mixing models, demonstrated that canopy transpiration can be a major source of water vapor recycling in Amazonia. These kinds of studies can also
elucidate the effects of land use changes on ecosystem water cycles.
8 18 0 of Atmospheric Carbon Dioxide
The biosphere can also have a significant influence
on the oxygen isotope ratio of atmospheric CO2
(Francey and Tans 1987; Farquhar et al. 1993; Flanagan et al. 1997). The equilibrium isotope effect
that occurs between oxygen in CO2 and oxygen in
water may be the primary factor influencing the
8 18 0 of respired CO 2 (Farquhar and Lloyd 1993).
However, discrimination against C 18 0 16 0 during
photosynthesis acts to enrich atmospheric CO 2 in
18 0, while respiration has the opposite effect by
releasing CO2 depleted in 18 0. Thus, ecosystem
respiration flux can be partitioned into soil and
plant components due to the differences in the 8 18 0
of medium water (Flanagan et al. 1997; Ehleringer
and Cook 1998; Flanagan et al. 1999). In addition,
global-scale partitioning of CO 2 fluxes can be studied since the 8 18 0 of water differs widely between
marine and terrestrial sources (even between different biomes and latitudes) helping to resolve conflicts over the relative role of the oceans in net CO 2
uptake (Farquhar et al. 1993).
Long-Term, Temporal Scaling
of Ecosystem Processes
Decadal-to-Century: Tree Rings
Tree rings have been frequently used as a record of
long-tenn climatic fluctuations. The hydrogen and
oxygen isotopic composition of tree ring cellulose
reflects, to a first approximation, precipitation inputs (Yapp and Epstein 1982; Lawrence and White
1984; White et al. 1994), and many studies have
used the variations in 8D and 8 18 0 in tree rings for
temperature reconstruction (Gray and Thompson
1976; Epstein and Yapp 1977). However, White et
al. (1985) observed that pine trees from wet and
dry microsites, that presumably receive similar precipitation inputs, exhibited differences in their cellulose 8D values, suggesting that water stress
and/or humidity differences may alter leaf water 8D
and thus tree ring cellulose 8D. There has been substantial controversy over exactly what environmental information is recorded in the 8D and 8 18 0 of
tree ring cellulose (e.g., DeNiro and Cooper 1989;
Edwards 1990). However, Roden et al. (1999) have
recently developed and tested a mechanistic model
for the interpretation of hydrogen and oxygen isotope ratios in cellulose and have concluded that
both water source and humidity infonnation are recorded in tree rings.
The 8l3C of tree ring cellulose has also been used
to study temperature and humidity signals (Loader
et al. 1995; Lipp et al. 1996), climate change (Leavitt and Long 1991), water use efficiency (Duquesnay et al. 1998), soil moisture availability (Saurer
et al. 1997), atmospheric 8l3C, and the global CO 2
cycle (Switsur and Waterhouse 1998) over centurylong time scales. These relationships are derived
from the Farquhar et al. (1982) model for carbon
193
Short-Term, Regional
Process-Level Applications
Across Ecosystems
Recycling of Water Among
and Between Ecosystems
The terrestrial biosphere influences global cycles of
CO2 and water. Stable isotopes can be used to probe
these cycles since biological fractionation and subsequent exchange with the atmosphere alters ambient isotopic signatures of CO 2 and water vapor
(Hsieh et al., 1998). For example, Gat and Matsui
(1991) used the 8D and 8 18 0 of precipitation across
a longitudinal gradient to quantify the relative contribution of evaporation and transpiration to the recycling of water vapor in the Amazon basin. There
is a predictable isotopic depletion of water in precipitation as stonn fronts progress (the continental
effect) due to Rayleigh distillation effects, and deviations imply that a source of isotopically enriched
vapor must be coming from the biosphere. The
source can be further delineated since vapor from
evaporation processes (lakes, rainfall, stemflow,
soil water, etc.) should be isotopically depleted
relative to that from transpiration (Gat and Matsui
1991; Martinelli et al. 1996). Rather than sampling
precipitation, Moreira et al. (1997) used flasks located on towers and balloons to sample atmospheric vapor directly and, using simple isotope
mixing models, demonstrated that canopy transpiration can be a major source of water vapor recycling in Amazonia. These kinds of studies can also
elucidate the effects of land use changes on ecosystem water cycles.
8 18 0 of Atmospheric Carbon Dioxide
The biosphere can also have a significant influence
on the oxygen isotope ratio of atmospheric CO2
(Francey and Tans 1987; Farquhar et al. 1993; Flanagan et al. 1997). The equilibrium isotope effect
that occurs between oxygen in CO2 and oxygen in
water may be the primary factor influencing the
8 18 0 of respired CO 2 (Farquhar and Lloyd 1993).
However, discrimination against C 18 0 16 0 during
photosynthesis acts to enrich atmospheric CO 2 in
18 0, while respiration has the opposite effect by
releasing CO2 depleted in 18 0. Thus, ecosystem
respiration flux can be partitioned into soil and
plant components due to the differences in the 8 18 0
of medium water (Flanagan et al. 1997; Ehleringer
and Cook 1998; Flanagan et al. 1999). In addition,
global-scale partitioning of CO 2 fluxes can be studied since the 8 18 0 of water differs widely between
marine and terrestrial sources (even between different biomes and latitudes) helping to resolve conflicts over the relative role of the oceans in net CO 2
uptake (Farquhar et al. 1993).
Long-Term, Temporal Scaling
of Ecosystem Processes
Decadal-to-Century: Tree Rings
Tree rings have been frequently used as a record of
long-tenn climatic fluctuations. The hydrogen and
oxygen isotopic composition of tree ring cellulose
reflects, to a first approximation, precipitation inputs (Yapp and Epstein 1982; Lawrence and White
1984; White et al. 1994), and many studies have
used the variations in 8D and 8 18 0 in tree rings for
temperature reconstruction (Gray and Thompson
1976; Epstein and Yapp 1977). However, White et
al. (1985) observed that pine trees from wet and
dry microsites, that presumably receive similar precipitation inputs, exhibited differences in their cellulose 8D values, suggesting that water stress
and/or humidity differences may alter leaf water 8D
and thus tree ring cellulose 8D. There has been substantial controversy over exactly what environmental information is recorded in the 8D and 8 18 0 of
tree ring cellulose (e.g., DeNiro and Cooper 1989;
Edwards 1990). However, Roden et al. (1999) have
recently developed and tested a mechanistic model
for the interpretation of hydrogen and oxygen isotope ratios in cellulose and have concluded that
both water source and humidity infonnation are recorded in tree rings.
The 8l3C of tree ring cellulose has also been used
to study temperature and humidity signals (Loader
et al. 1995; Lipp et al. 1996), climate change (Leavitt and Long 1991), water use efficiency (Duquesnay et al. 1998), soil moisture availability (Saurer
et al. 1997), atmospheric 8l3C, and the global CO 2
cycle (Switsur and Waterhouse 1998) over centurylong time scales. These relationships are derived
from the Farquhar et al. (1982) model for carbon