Chapter 4. Vegetation
fremontii, S. gooddingii, P. sericea, and B. salicifolia all develop adventitious
roots in response to inundation, although T. ramosissima does not (Vandersande
et al. 2001).
Drought stress is another factor that may strongly influence arid-region
riparian vegetation. A variety of factors influence the ability of plants to survive
drought. Not surprisingly, species differ somewhat in their tolerance of dry
conditions. On the Bill Williams River in Arizona, Populus and Salix saplings
appeared to be very sensitive to groundwater decline, while Tamarix saplings
were more tolerant of dry conditions (Shafroth et al. 2000). Similarly, in green-
house experiments Tamarix seedlings had higher survival than S. gooddingii
seedlings in response to water table decline treatments (Horton and Clark 2001).
Adult Tamarix on the Bill Williams and Hassayampa Rivers in Arizona was
more physiologically tolerant of increased depth to groundwater than were adult
P. fremontii and S. gooddingii (Horton et al. 2001). The latter two species experi-
enced some mortality when the depth to groundwater was greater than 2.53 m,
though the effect was most severe for S. gooddingii. Elevated salinity levels may
compound the effects of drought stress, leading to mortality in salt-intolerant
species (Vandersande et al. 2001). At elevated salinity levels in greenhouse
experiments, rooted cuttings of T. ramosissima and P. sericea had higher water
use efficiency than did P. fremontii, S. gooddingii, and B. salicifolia, although at
the control salinity level no inter-species differences existed (Vandersande et al.
2001). Other factors that influence the ability of individual plants to survive dry
conditions include plant age, rate and duration of groundwater decline, climate,
and soil texture and stratigraphy (Shafroth et al. 2000).
Differences in plant tolerances to drought may reflect differences in the
sources of water used for transpiration. For example, Salix gooddingii trees used
only groundwater at sites on the San Pedro River in southeastern Arizona, while
and P. velutina trees
were also able
to use precipitation-derived soil
water at some sites (Snyder and Williams 2000). Reliance on groundwater makes
Salix vulnerable to groundwater declines, whereas Populus and Prosopis may be
able to survive periods of groundwater decline by relying on precipitation.
Tamarix appears to possess significant physiological drought tolerance (Horton
et al. 2001), which may include the ability to use water from unsaturated flood-
plain soils (Busch et al. 1992).