Red Butte Plants

Phenology. Plant activity is governed by two parameters: temperature and soil moisture availability. Cold winter temperatures limit growth activity between November and March (Caldwell, 1985; Comstock and Ehleringer, 1992). While a limited number of species, such as the early spring ephemeral Ranunculus testiculatus (bur buttercup), may begin activity during warm periods in February, most annuals do not begin growth until the warm periods between snow storms in early March. At lower elevations, a number of herbaceous perennials such as Balsamorhiza macrophylla (cutleaf balsamroot) may begin to leaf out during March, but most woody perennials do not leaf out until mid to late April. The annuals and most herbaceous species at lower elevations have completed growth and reproduction by mid-June and then remain dormant until the following autumn or spring (Smedley et al., 1991). In contrast, woody species at lower elevations remain active from April through October, although the vast majority of the growth will occur during the spring (Donovan and Ehleringer, 1991) . At higher elevations, vegetative and reproductive growth are delayed until late May or June by cold temperatures. Plants at the higher elevations will remain active throughout the summer, even though there may be little summer precipitation (Dina, 1970; Dina and Klikoff, 1973).

Adaptation. In the non-forested 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 are plants within the riparian communities along the canyon bottom. As such, many of the recent ecological studies within the Red Butte Canyon RNA has 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 developed in perennials occupying slope sites during late summer, whereas water potentials of adjacent riparian tree species were 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 are thought to occur in only the most drought-adapted aridland species. These late-summer water potential values on slope species were sufficiently low to close stomata and reduce photosynthesis to near zero values. Photosynthetic rates of riparian species decreased by 50-80% from non-stress values, but riparian trees were able to maintain positive net photosynthetic rates throughout the summer. More recently, Dawson and Ehleringer (1992) 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 were able to maintain photosynthetic rates throughout the year, albeit that photosynthetic rates were 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.

Several interesting studies have been conducted in Red Butte Canyon that shed light onto the nature of a plant's ability to tolerate water stress and persist through time. Treshow and Harper (1974) examined longevity of herbaceous perennials in grass-, mountain brush-, aspen- and conifer communities throughout the canyon. They observed that life expectancies of dominant herbaceous perennial species, such as Astragalus utahensis (Utah milkvetch), Balsamorhiza macrophylla (cutleaf balsamroot), Hedysarum boreale (northern sweetvetch), and Wyethia amplexicaulis (mulesears) were relatively short (3-20 years), when compared to the longer-lived (> 65 years) grass species, such as Agropyron spicatum (bluebunch wheatgrass) and Stipa comata (needle-and-thread). The ability to persist through successive drought years may have been one of the reasons that dicotyledonous species had shorter life expectancies than monocotyledonous species. Related to this, Smedley et al. (1991) examined the water-use efficiency of these and other herbaceous grassland species. Water-use efficiency, the ratio of photosynthesis to transpiration, serves as a measure of how much photosynthetic carbon gain occurs per unit water loss from the leaf. Dicot herbaceous perennials had consistently lower water-use efficiencies than their monocot counterparts (Fig. 12). The differences in intrinsic water-use efficiency within this life form may be a major contributing factor to the shorter life expectancy in dicot herbaceous species. Consistent with this pattern, Smedley et al. (1991) observed that water-use efficiency of annual species was significantly lower than that of perennial species in grasslands along the lower portions of the canyon. They also observed that perennials which persisted longer into the summer drought period had higher water-use efficiencies than those species that became dormant in late spring. During 1988-1990, precipitation was unusually low. The effects of the three-year drought are now seen in the Gambel oak and bigtooth maple at their lower distribution limits, especially on shallow soils, where stem dieback has become prevalent.

Ehleringer (1988) examined leaf-level adaptations of plants along the entire elevational transect within Red Butte Canyon. This study focused on determining patterns of leaf angle and leaf absorptance variation among species within communities exposed to different degrees of drought stress. Increased leaf angle and decreased leaf absorptance reduce the solar energy incident on leaves and are viewed as mechanisms for both reducing leaf energy loads (reducing leaf temperature) and increasing water-use efficiency. Along a transect from grassland through coniferous forest, very few plant species exhibited any significant changes in leaf absorptance. However, leaf angles among species became progressively steeper in drier habitats. This pattern is consistent with the notion that as plants are exposed to progressively drier environments, the general adaptive response of species within the community is to increase leaf angle, thereby reducing incident solar radiation levels.

In the grasslands on the lower portions of Red Butte Canyon, there is a most unusual plant species, Cymopterus longipes (long-stalk spring-parsley). Sometimes known as the "elevator plant", C. longipes is a prostrate herbaceous perennial with an elongating pseudoscape (a scape is a leafless flowering stalk arising from ground level; the pseudoscape is an elongation of the leaf-bearing stem in the region between the roots and existing leaves). Other Cymopterus species also have a pseudoscape, but in none of the other species is it as well developed as in C. longipes. In spring, solar heating of the ground surface increases soil and leaf temperatures and can result in a moderately warm leaf temperature (30-35°C). These temperatures are substantially higher than the optimum photosynthetic temperature for the elevator plant and result in both a decreased photosynthetic rate and a decreased water-use efficiency (Werk et al., 1986). To increase both the rate of photosynthetic carbon gain and to increase water-use efficiency, the pseudoscape elongates as spring temperatures progressively increase (Fig. 13). The result is that what was once a prostrate canopy is elevated above the warm soil surface and now exposed to cooler air temperatures above the ground surface. Werk et al. (1986) showed that the rate at which the pseudoscape elongates is dependent on the rate of soil-surface heating. Plants from protected or north-facing sites elongate less that those from exposed, southerly sites.

Donovan and Ehleringer (1991) examined relationships between water-use and the likelihood of establishment by common shrub and tree species in the lower portions of Red Butte Canyon. They observed that photosynthesis was greater in seedlings than adults throughout most of the growing season, but that water stress and water-use efficiency were lower in seedlings. Seedling mortality in several of the species was associated with higher water-use efficiencies, suggesting that mortality selection occurs with greater frequency in seedlings that are conservative in their water use before they have established sufficiently deep roots to survive the long summer drought period.

Few studies have addressed ecophysiological aspects of riparian ecosystems in the Intermountain West. This is somewhat surprising since riparian ecosystems are most often among the first to be damaged by human-related activities, from outdoor recreation to water impoundment to grazing. Red Butte Canyon, as one of the few remaining riparian systems in the Intermountain West not severely impacted by human activities, is ideal for studies of the adaptations of riparian plants and for comparative studies of species sensitivities to human-related activities.

In a recent study, Dawson and Ehleringer (1992) examined water sources used by riparian plants species. In their study, plants were segregated according to microhabitat and size: streamside versus non-streamside and juvenile versus adults (based on diameter at breast height). Their results were rather startling and suggest that a new perspective is necessary when evaluating riparian communities, their establishment potentials, and their sensitivity to disturbance. Dawson and Ehleringer (1991) used hydrogen isotope analyses of stem waters to determine the extent to which different categories of riparian trees utilized stream water, recent precipitation, or groundwater. Hydrogen isotopes are not fractionated by roots during water uptake; therefore, the hydrogen isotope ratios of stem water will reflect the water sources currently used by that plant. Rain, ground waters, and stream waters differ in their hydrogen isotope ratios, providing a signal difference that could be detected by stem-water analyses. Dawson and Ehleringer (1991) observed that among mature tree species, none were directly using stream water (Fig. 14). All were using waters from a much greater depth, which had a hydrogen isotope ratio more negative than either stream water or precipitation. Young streamside trees utilized stream water, but only when small. Young trees at non-streamside locations utilized precipitation, not having access to either stream water or deeper groundwater. One possible reason why streamside trees may not depend on stream water is that this surface water source may occasionally dry up during extreme drought years and become unavailable to these trees; another is because stream channels occasionally change their course and dependence on surface moisture would then result in increased drought stress and likely increased mortality rates. The long-term stream discharge rates suggest that stream water may be less dependable than deeper groundwater sources (Fig. 6).

Many plants do not contain both male and female reproductive structures in their flowers, but are present as either male or female plants (dioecy). Freeman et al. (1976, 1980) noted that dioecy was a common feature of plants in the Intermountain West. Furthermore, they observed that the two sexes were usually not randomly distributed across the landscape. Rather there was a spatial segregation of the two sexes such that females tended to predominate in less stressful microsites (wetter, shadier, etc.), whereas males occurred with greater frequencies on more stressful sites (drier, sunnier, saltier, etc.). In Red Butte Canyon, Freeman et al. (1976) investigated spatial distributions of Acer negundo (boxelder, a riparian tree) and Thalictrum fendleri (Fendler meadowrue, a perennial herb ). In both species, there was a strong spatial segregation of the two sexes.

Dawson and Ehleringer (1992) have followed up on the initial observations of spatial segregation in Acer negundo (boxelder), seeking to determine if intrinsic physiological differences among the sexes may contribute to plant mortality in different microsites. They observed that female trees had significantly lower water-use efficiencies than male trees on both streamside (where female predominate) and non-streamside locations (where males predominate). Male trees exhibited a higher water-use efficiency in dry sites than in streamside locations, but female trees exhibited no such response across microhabitats. The lack of a change in water-use efficiency by female trees on dry, non-streamside locations may contribute to an increased mortality rate, which then ultimately results in a male-biased sex ratio at these sites.