Variation in Gas Exchange Characteristics Among Desert Plants
385
used by plants can be distinguished by examining the hydrogen isotope ratio
(3D) of water in xylem sap of stems and quantitative estimates of the uptake
of different moisture sources can be determined (White et al. 1985).
In desert ecosystems, Ehleringer and Cook (1991) used 3D observations
of xylem sap to analyze the extent to which Sonoran Desert perennials near
Needles, California, utilized summer precipitation. They observed that in
moderately dry summers, long-lived species (includes both trees such as
Acacia greggii, Carcidium fioridum, and Chilopsis linearis as well as shrubs
such as Ephedra viridis) did not utilize summer rain, but instead relied on
moisture from deeper soil layers. In contrast, shorter-lived perennials (such
as Ambrosia dumosa and Encelia farinosa) used moisture from those summer
rains whenever it was available. It was only during years of above-average
summer rains, when moisture penetrated to greater depths, that long-lived
species increasingly utilized summer rains.
Winter rains come as slow-moving frontal systems, and if they are of
sufficient magnitude, saturate the soil profile. In contrast, summer rains
occur as shorter, more intense convectional storms, often saturating only the
upper soil layers. As shown earlier, the frequency distributions of summer
and winter storms are not different at sites receiving the same average
amount of precipitation. Thus, it would appear that because of the short
intense duration of summer rains, the depth of penetration for storms of
equivalent rainfall would be less. For plant water use in the desert, the
situation is further compounded by the greater co values for plants growing
in summer-wet habitats. Thus, we might expect that Ll values to be lower for
plants using summer moisture.
Parts of southern Utah receive 30% or more of the annual precipitation
during the summer months. Ehleringer et al. (1991a) studied desert species
near Wahweap (Utah) over a 2-year period and observed that following
the onset of summer rains (3D value of approximately -25%0), annuals,
herbaceous perennials and CAM perennials used water from the upper
soil layers wetted by summer rains (Fig. 18.13). A fraction of the woody
perennials had 3D values intermediate between the summer rains and the
deeper soil layers (which were approximately -80%0), implying that both
water sources were being utilized in equal proportion. On the other hand,
the 3D values in a second group of woody perennials did not use any
moisture from the summer rains. Similar apparent niche separations for
summer what use by perennial shrubs have been described by Donovan and
Ehleringer (1992) in the Great Basin and Valentini et al. (1992) in the
macchia of Italy.
Taken together, these data suggest that some perennial species may have
rooting patterns allowing them to utilize both summer and winter rains,
whereas other perennial species have effective rooting patterns restricted to
deeper soil layers and thus the plants are unable to utilize summer rains.
Functionally dimorphic root systems for water uptake are a possibility, but
since the carbon cost of root turnover is high, this should only occur if the
385
used by plants can be distinguished by examining the hydrogen isotope ratio
(3D) of water in xylem sap of stems and quantitative estimates of the uptake
of different moisture sources can be determined (White et al. 1985).
In desert ecosystems, Ehleringer and Cook (1991) used 3D observations
of xylem sap to analyze the extent to which Sonoran Desert perennials near
Needles, California, utilized summer precipitation. They observed that in
moderately dry summers, long-lived species (includes both trees such as
Acacia greggii, Carcidium fioridum, and Chilopsis linearis as well as shrubs
such as Ephedra viridis) did not utilize summer rain, but instead relied on
moisture from deeper soil layers. In contrast, shorter-lived perennials (such
as Ambrosia dumosa and Encelia farinosa) used moisture from those summer
rains whenever it was available. It was only during years of above-average
summer rains, when moisture penetrated to greater depths, that long-lived
species increasingly utilized summer rains.
Winter rains come as slow-moving frontal systems, and if they are of
sufficient magnitude, saturate the soil profile. In contrast, summer rains
occur as shorter, more intense convectional storms, often saturating only the
upper soil layers. As shown earlier, the frequency distributions of summer
and winter storms are not different at sites receiving the same average
amount of precipitation. Thus, it would appear that because of the short
intense duration of summer rains, the depth of penetration for storms of
equivalent rainfall would be less. For plant water use in the desert, the
situation is further compounded by the greater co values for plants growing
in summer-wet habitats. Thus, we might expect that Ll values to be lower for
plants using summer moisture.
Parts of southern Utah receive 30% or more of the annual precipitation
during the summer months. Ehleringer et al. (1991a) studied desert species
near Wahweap (Utah) over a 2-year period and observed that following
the onset of summer rains (3D value of approximately -25%0), annuals,
herbaceous perennials and CAM perennials used water from the upper
soil layers wetted by summer rains (Fig. 18.13). A fraction of the woody
perennials had 3D values intermediate between the summer rains and the
deeper soil layers (which were approximately -80%0), implying that both
water sources were being utilized in equal proportion. On the other hand,
the 3D values in a second group of woody perennials did not use any
moisture from the summer rains. Similar apparent niche separations for
summer what use by perennial shrubs have been described by Donovan and
Ehleringer (1992) in the Great Basin and Valentini et al. (1992) in the
macchia of Italy.
Taken together, these data suggest that some perennial species may have
rooting patterns allowing them to utilize both summer and winter rains,
whereas other perennial species have effective rooting patterns restricted to
deeper soil layers and thus the plants are unable to utilize summer rains.
Functionally dimorphic root systems for water uptake are a possibility, but
since the carbon cost of root turnover is high, this should only occur if the