Plants In Arches National Park
Photograph by brewbooksFlikr
Desert plants, since they are rooted in place, must cope with extremes in temperature, water availability and solar radiation physiologically rather than behaviorally. In fact, surface temperatures in direct sunlight are commonly 25 to 50 degrees F warmer than the air temperature six feet above. Most desert plant adaptations seem to be geared towards minimizing water loss: a difficult task since plants must "breathe" (collecting Carbon Dioxide from the air) in order to photosynthesize, losing body water to the atmosphere in the process.
Drought escapers are plants that make use of favorable growing conditions when they exist. These plants are usually annuals and complete their life cycles in a matter of days or weeks when water is plentiful enough for them to do so. Seeds may lie dormant for years if conditions are not favorable. Most grasses are "escapers," as are the spring wildflowers that sometimes bloom during April and May.
Drought resistors are typically perennials. Many perennials have small, spiny leaves which reduce the impact of solar radiation; others may drop their leaves when water is unavailable. Spines and hairs on leaves act as a buffer against warm air currents, limiting the amount of water lost to evaporation. Plants also use "solar tracking" to regulate their exposure to the sun. Cacti store water within their bodies and have extensive, shallow root systems that are able to soak up rainwater quickly. Yucca have extensive tap roots that are able to use water beyond the reach of other plants. Moss, a plant not commonly associated with deserts, thrives because it can tolerate complete dehydration: when rains finally return, the plant greens up almost immediately. Another extreme adaptation can be found in the utah juniper tree, one of the most common plants in the southwest. During a drought, junipers can self-prune, shutting off water flow to one or more their branches in order to conserve enough water for the rest of the tree to survive.
Drought evaders take advantage of wetter "micro climates" found in the desert. Monkey flower, columbine, easter flower, and ferns are found in well-shaded alcoves near seeps or dripping springs. Cottonwood, willow and cattail all require lots of water, and only grow in riparian areas where their roots can reach the water table easily.
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