One Best Hike: Grand Canyon. Elizabeth WenkЧитать онлайн книгу.

      Riparian: This community exists only in locations with permanent water, such as along the banks of the Colorado River and its side canyons. The availability of water means that large trees and dense thickets of shrubs and herbs are able to grow in these locations. The dominant species include Fremont cottonwood, coyote willow, seepwillow, saltcedar, honey mesquite, and catclaw acacia. The riparian vegetation along the Colorado River has changed significantly since the completion of the Glen Canyon Dam in 1964; water flows are now restricted and the river no longer floods as severely, a process that once scoured the vegetation along the banks and deposited vast quantities of sediment. Beginning in 1996 the Bureau of Reclamation has three times released higher flows from Glen Canyon Dam to replicate historic flooding. The largest flow occurred in March 2008, when they released twice the usual flow of water. Preliminary research indicates that this flood effectively mimicked natural floods, creating large sandbanks, reducing the establishment of nonnative plants, and creating backwater environments.

      These vegetation zones can be identified by just a few dominant shrubs and trees, but more than 900 species occur below the canyon rims and in spring a diligent botanist might locate several hundred species along this walk. Any attentive hiker will notice many tens of distinctive and, when flowering, colorful species. Such diversity exists because the landscape is complex, creating many unique combinations of physical, chemical, and biological conditions. Each species prefers a certain soil depth, soil made from a specific rock, a specific small-scale climate, and a specific topographic position. Your walk will take you past geographic features that include solid rock, washes, steep slopes, seeps, small dunes, river terraces, the Tonto Platform, stream banks, ridges, and valleys, each home to a different collection of species.

      DESERT ADAPTATIONS

      Colorful flowers attract your attention—and that of pollinators—but stare also at the leaves and stems of plants, for they you will remind you what life is like for plants in the region. For instance, shrubs and perennial herbs that grow in a desert environment must have traits that minimize water loss: Lack of leaves (e.g., cactus), drought-deciduous leaves (leaves that fall off in summer; e.g., blackbrush), small leaves (e.g., Mormon tea) and hairy leaves (e.g., brittlebush and big sagebrush) are all common desert adaptations. Most water loss occurs because plants need to cool their leaves by transpiring (evaporating) water, a process much like humans sweating. Minimizing leaf area and having leaf hairs to reflect light and keep leaves cooler are two very effective ways to preserve water.

      However, plants require leaves, or at least green surfaces; the green color indicates plant parts that can photosynthesize, or turn the sun’s energy into the sugars necessary for the plants to grow. In many species green stems compensate for limited leaf area. Drought-deciduous shrubs and many perennial herbs sprout leaves in spring when water is available and effectively hibernate during the hot summers. Other species are annuals. These plants grow for only a single season, germinating when soils are sufficiently moist and producing flowers and seeds in quick succession to avoid dry soils.

      Below are descriptions of 38 species along the Bright Angel and South Kaibab trails, including the most common species, as well as some showy, difficult-to-miss species that occur along a specific stretch of trail. They are organized by growth form (trees and larger shrubs, herbs and small shrubs, and cacti and agaves) and are approximately in order of their appearance from the canyon rim to bottom. A list of all species in the Grand Canyon is available on the Southwest Environmental Information Network: http://swbiodiversity.org/seinet/checklists/checklist.php?cl=94.

      Trees and Larger Shrubs