Adaptation


In the nonforested portions of the Intermountain West, plant growth is largely restricted to spring and early summer periods by cold temperatures during winter and limited water availability during the summer (Caldwell 1985, Dobrowolski, Caldwell, and Richards 1990, Comstock and Ehleringer 1992). A number of recent reviews have addressed adaptation characteristics of plants growing in these environments (Caldwell 1985, DeLucia and Schlesinger 1990, Smith and Knapp 1990, Smith and Nowak 1990). For the most part, plants within Red Butte Canyon are exposed to a hot, dry environment, with little relief from developing water stress during the summer months. The only clear exception to this pattern is the series of plants within the riparian communities along the canyon bottom. To gain a better understanding of this occurrence, many of the recent ecological researchers within the Red Butte Canyon RNA have focused on mechanisms by which plant species have adapted to limited water availability.

Among the first ecophysiological studies was that by Dina (1970), who examined water stress levels of the dominant tree species in the lower portions of the canyon: Acer grandidentatum (bigtooth maple), Acer negundo (boxelder), Artemisia tridentata (big sagebrush), Purshia tridentata (bitterbrush), and Quercus gambelii (Gambel oak). Dina (1970) observed that midday leaf water potentials of -30 to -65 bars develop in perennials occupying slopes ites during late summer, whereas water potentials of adjacent riparian tree species are maintained between -20 and -30 bars during the same periods. Water potentials in the range of -10 to -15 bars cause many crop species to wilt and close their stomata, reducing transpirational water loss. Tolerance of water stress levels as low as -40 to -60 bars is thought to occur in only the most drought-adapted aridland species. These late-summer water potential values on slope species are sufficiently low to close stomata and reduce photosynthesis to near zero values. In Dina's (1970) study photosynthetic rates of riparian species decreased by 50-80% from nonstress valules, but riparian trees were able to maintain positive net photosynthetic rates throughout the summer. More recently, Dawson and Ehleringer (1993) and Donovan and Ehleringer (1991) conducted related studies and again observed that photosynthetic carbon gain of slope species is largely limited to spring and early summer, whereas riparian species are able to maintain photosynthetic rates throughout the year, albeit that photosynthetic rates are lower in summer than in spring.

Two common responses to limited water availability are avoidance and tolerance. Avoidance of water stress is accomplished by completion of growth and reproductive activities before the onset of the summer drought, whereas tolerance is associated with the evolution of features that allow plants to persist through the drought period.

More on Adaptation

Plant Longevity
Leaf-level Adaptations
Water-use and Water Stress
Spatial Segregation