184
ural conditions. Craig and Gordon (1965), Allison
et al. (1983), and Flanagan et al. (1991b) provide
the quantitative models necessary to predict the isotopic composition of open waters, drying soils, and
leaf water, respectively, from environmental parameters. Under strict conditions, it is possible to extrapolate evaporatively enriched water from either
drying soils or leaves back to the isotopic composition of the original, unevaporated source waters
(see Moreira et al. 1997).
Methods for Water Sampling,
Extraction, and Analysis
Water Sample Collection and Storage
Representative sampling and prevention of evaporative enrichment are the primary concerns when
collecting samples for determining the stable isotopes in water. When collecting water samples from
an open water surface (e.g., lake), do not sample
from a still surface, which may be enriched relative
to deeper water. Stirring up the sample area can
help to ensure a representative aliquot. Collect
stream water from the site of interest, since evaporative enrichment will occur as water moves down
the length of a river. Sampling from well water is
often a good proxy for ground water, but do not
keep the first water coming out of the pipe since it
may be enriched relative to the source. Fill the vials
and seal the lid with parafilm. It is desirable to store
water samples at cold temperatures (5°C), but shortterm storage at room temperature is adequate if the
vials are well sealed.
Similar concerns apply to sampling leaves for
extraction of water (Le., sealed in a vial immediately). Remember when sampling for leaf water
that the degree of evaporative enrichment in leaf
water will be a function of local humidity and leaf
temperature (Flanagan et al. 1991a). Thus, it is essential that there be consistency in sampling leaves
that are experiencing the same microclimatological
patterns (e.g., sun versus shade leaves). The portions of the leaf that are subject to evaporative enrichment are those regions away from the major
veins (Yakir et al. 1989). Therefore, some investigators prefer to use a punch to cut out those interveinal regions for sampling. Samples should be
James R. Ehleringer, John Roden, and Todd E. Dawson
stored in a freezer as soon as possible after sampling and kept there until the water is extracted.
Since there is no fractionation during water uptake, sampling xylem waters becomes a convenient
and reliable approach for determining the current
water source used by a plant (Ehleringer and Dawson 1992). Stems or other suberized tissues are a
convenient means of sampling xylem water in order
to determine plant water sources from the soil. Emphasis should be placed on sampling organs with a
rapid water turnover. For example, sampling of
small suberized stems is better than sampling a tree
trunk, where external and internal portions may be
turning over water at different rates. Use only
suberized stem materials, as green stem tissues are
subject to evaporative enrichment (Dawson and Ehleringer 1993).
Collection of precipitation from open areas
should be done immediately after the storm to avoid
evaporation. On the other hand, within a forest,
some evaporative enrichment will occur as
throughfall makes its way into the soil, in which
case, it makes more sense to sample the throughfall
since this is what actually enters the soil (Dawson
1998). Soil samples must be collected across the
range of possible rooting depths since water in surface soils is exposed to evaporative enrichment.
Soil samples are usually stored in the same type of
glass vials used to store leaf or stem samples.
If precipitation input and transpiration activity
occur at roughly the same time, then it is likely that
the atmospheric water vapor will be in isotopic
equilibrium with that of the input precipitation.
However, these input and loss processes are often
temporally offset and may be offset by as much as
several months in regions where precipitation occurs primarily in the winter and growth occurs later
in the spring and summer. In most cases, it is essential that atmospheric water vapor be collected,
since its isotopic value has such a strong influence
in determining the isotopic composition of leaf water. Atmospheric vapor is collected by using a pump
to flow air through a condensing-loop trap. This
requires a very cold trap (dry ice in an ethanol
slush) to ensure complete vapor extraction, since
any water vapor that escapes would be isotopically
altered. For later data interpretation, it is best to also
measure air temperature and humidity at the time
of vapor collection.
ural conditions. Craig and Gordon (1965), Allison
et al. (1983), and Flanagan et al. (1991b) provide
the quantitative models necessary to predict the isotopic composition of open waters, drying soils, and
leaf water, respectively, from environmental parameters. Under strict conditions, it is possible to extrapolate evaporatively enriched water from either
drying soils or leaves back to the isotopic composition of the original, unevaporated source waters
(see Moreira et al. 1997).
Methods for Water Sampling,
Extraction, and Analysis
Water Sample Collection and Storage
Representative sampling and prevention of evaporative enrichment are the primary concerns when
collecting samples for determining the stable isotopes in water. When collecting water samples from
an open water surface (e.g., lake), do not sample
from a still surface, which may be enriched relative
to deeper water. Stirring up the sample area can
help to ensure a representative aliquot. Collect
stream water from the site of interest, since evaporative enrichment will occur as water moves down
the length of a river. Sampling from well water is
often a good proxy for ground water, but do not
keep the first water coming out of the pipe since it
may be enriched relative to the source. Fill the vials
and seal the lid with parafilm. It is desirable to store
water samples at cold temperatures (5°C), but shortterm storage at room temperature is adequate if the
vials are well sealed.
Similar concerns apply to sampling leaves for
extraction of water (Le., sealed in a vial immediately). Remember when sampling for leaf water
that the degree of evaporative enrichment in leaf
water will be a function of local humidity and leaf
temperature (Flanagan et al. 1991a). Thus, it is essential that there be consistency in sampling leaves
that are experiencing the same microclimatological
patterns (e.g., sun versus shade leaves). The portions of the leaf that are subject to evaporative enrichment are those regions away from the major
veins (Yakir et al. 1989). Therefore, some investigators prefer to use a punch to cut out those interveinal regions for sampling. Samples should be
James R. Ehleringer, John Roden, and Todd E. Dawson
stored in a freezer as soon as possible after sampling and kept there until the water is extracted.
Since there is no fractionation during water uptake, sampling xylem waters becomes a convenient
and reliable approach for determining the current
water source used by a plant (Ehleringer and Dawson 1992). Stems or other suberized tissues are a
convenient means of sampling xylem water in order
to determine plant water sources from the soil. Emphasis should be placed on sampling organs with a
rapid water turnover. For example, sampling of
small suberized stems is better than sampling a tree
trunk, where external and internal portions may be
turning over water at different rates. Use only
suberized stem materials, as green stem tissues are
subject to evaporative enrichment (Dawson and Ehleringer 1993).
Collection of precipitation from open areas
should be done immediately after the storm to avoid
evaporation. On the other hand, within a forest,
some evaporative enrichment will occur as
throughfall makes its way into the soil, in which
case, it makes more sense to sample the throughfall
since this is what actually enters the soil (Dawson
1998). Soil samples must be collected across the
range of possible rooting depths since water in surface soils is exposed to evaporative enrichment.
Soil samples are usually stored in the same type of
glass vials used to store leaf or stem samples.
If precipitation input and transpiration activity
occur at roughly the same time, then it is likely that
the atmospheric water vapor will be in isotopic
equilibrium with that of the input precipitation.
However, these input and loss processes are often
temporally offset and may be offset by as much as
several months in regions where precipitation occurs primarily in the winter and growth occurs later
in the spring and summer. In most cases, it is essential that atmospheric water vapor be collected,
since its isotopic value has such a strong influence
in determining the isotopic composition of leaf water. Atmospheric vapor is collected by using a pump
to flow air through a condensing-loop trap. This
requires a very cold trap (dry ice in an ethanol
slush) to ensure complete vapor extraction, since
any water vapor that escapes would be isotopically
altered. For later data interpretation, it is best to also
measure air temperature and humidity at the time
of vapor collection.