Desert annuals like grasses and wildflowers are adapted to the arid environment in many different ways. These include thick, waxy coverings on leaves and stems which reduce exposure and thus evaporative water loss; small leaves which reduce water loss while the plant transpires or "breathes" and receive less solar radiation; and deep taproots to reach further into the soil or shallow widespread roots that absorb surface water quickly.
Despite these adaptations, most desert wildflowers avoid drought and heat by surviving as seeds or bulbs stored in the soil, sometimes for decades. These seeds will only germinate after significant seasonal rainfall, so wildflower growth in Arches is highly variable year to year. April and May are generally the best months to see wildflowers, then again in early fall if there are a lot of summer monsoons. Some desert plants take advantage of the nights' cooler temperatures to flower. These evening-blooming plants include evening primrose, sacred datura, sand verbena and yucca.
The yucca and the yucca moth have a fascinating nighttime association. After mating, the female moth gathers pollen from one yucca flower, packs it into a ball, and then flies into the night, locating other yucca flowers primarily by "smelling" with her antenna. She visits several flowers, each time laying some eggs in the base of the pistil and packing some of the pollen from her pollen ball down the pistil for her young to feed on. Thus she fertilizes the yucca flowers. Yucca flowers are only pollinated by yucca moths, and yucca moth young only feed on yucca pollen.
Over 400 kinds of plants grow and flourish in Arches, despite extreme temperatures and low rainfall. Plants play an important role in the Arches National Park ecosystem. Plants have adaptations that put them into three different categories: drought escapers, drought resistors and drought evaders. Drought escapers are plants that take advantage of good growing conditions when they exist. For example, these plants will grow when there is enough water. Seeds of these plants may wait years until there is a rainstorm, and then grow. Many flowers and grasses are in this category. Drought resistors are those plants that have specialized parts that help them survive without much water. Some of these plants have small leaves to reduce water loss through the stomata. Others have spines or hairs on their leaves to minimize evaporation. Yucca plants have a long taproot that helps the plant find water below the reach of other plants. Other drought resistor plants include cacti, mosses and sagebrush.
Drought evaders are plants that live only where there is a lot of water. Springs, rivers and streams are examples of places where there is water in the desert. In Arches National Park, drought evader plants might grow near the Colorado River or in Courthouse Wash. Drought evader plants include cottonwood trees, willows, ferns and even poison ivy! All of these plants require a reliable source of water. The plants at Arches National Park would not be as healthy or numerous without the help of organisms like cyanobacteria. Cyanobacteria live on top of the soil. They have sticky sheaths, which bind to individual grains of sand and absorb water. They are invisible to us when they are young. When they are at least 50 years old, fungi, algae and mosses grow with the cyanobacteria to form a crust. The crust is called Cryptobiotic Soil Crust. The name comes from two Greek words, krypto which means hidden, and bitik (os) which means life.
As the crust builds up nutrients, plants begin to grow. There are various plant communities within the park, including pinyonand juniper woodlands, desert shrublands, grasslands, hanging gardens, and riparian corridors. One would not expect to find a water-loving fern living within a few feet of a cactus, but that is exactly what can happen at Arches National Park because of the diverse communities.
More than most plants, the cactus seems perfectly suited to life in an arid climate. The cactus, especially the saguaro, has become emblematic of the American southwest. Nine species of cactus are found in Arches, though the saguaro is not one of them. Cacti are plants that have succulent stems, pads or branches with scales and spines instead of leaves. Cactus pads are actually modified stems with a waxy coating. The prickly spines are modified leaves that break up the evaporative winds blowing across pad surfaces, and help shade the stem. Root systems are usually broad and shallow, and rainwater is soaked up quickly.
Small rain roots actually grow as soon as soil is moistened by rain, and later dry up. All plants photosynthesize, collecting carbon dioxide through holes in their leaves called "stomata" and converting it into sugar and oxygen. Cacti utilize CAM photosynthesis, a process unique to succulents. In CAM photosynthesis, stomata open only at night when the plant is relatively cool, so less moisture is lost through transpiration. However, photosynthesis also requires sunlight.
The CAM process includes a way of chemically storing the carbon dioxide until the sun comes out, when it can be used to complete the photosynthetic process. Stomata are like windows; they have to be open to let air and water in or out, but sunlight can come in even if they're closed. Despite their prickly armor, cacti are not immune to predators. Many rodents gnaw on cactus pads, and other mammals, including bears and humans, enjoy the sweet red fruit of the prickly pear.
Cheatgrass (Bromus tectorum) has drastically altered landscapes in Arches and throughout the American west. This annual grass arrived in shipments of European wheat during the late 1800s, and quickly established itself in many areas. Cheatgrass now covers over 100 million acres. Cheatgrass is well adapted to the high desert climate and can out-compete many native plants. This is partially because cheatgrass uses a growth strategy unlike any other in the high desert ecosystem. While most desert plants are dormant during winter, cheatgrass germinates in the fall and spends the winter building roots and storing energy. By early spring, cheatgrass is ready to begin its aboveground growth while other plants are just breaking dormancy. Since this strategy appears so effective, it is interesting that no native plants make use of it.
Scientists explain this in two ways. First, it is possible that climate change has created a new "niche" that no native plants are able to exploit. Secondly, it is also possible that extreme climatic events which native plants can survive might someday wipe out this relative newcomer. In addition to germinating earlier, cheatgrass also uses subsurface water more efficiently and colonizes disturbed areas more quickly. As a result, native grasslands are increasingly rare, especially where wildfires or livestock grazing have occurred. In areas where cheatgrass dominates, both biological diversity and soil health decline. Unfortunately, few animals are known to eat cheatgrass, and scientists have not found any means to control it.
Cryptobiotic soil crust is a living ground cover that forms the foundation of high desert plant life in Arches and the surrounding area. This knobby, black crust is dominated by cyanobacteria, but also includes lichens, mosses, green algae, micro fungi and bacteria.
Cyanobacteria, previously called blue-green algae, are one of the oldest known life forms. It is thought that these organisms were among the first land colonizers of the earth's early land masses, and played an integral role in the formation and stabilization of the earth's early soils. Extremely thick mats of these organisms converted the earth's original carbon dioxide-rich atmosphere into one rich in oxygen and capable of sustaining life.
When wet, Cyanobacteria move through the soil and bind rock or soil particles, forming an intricate web of fibers. In this way, loose soil particles are joined together, and an otherwise unstable surface becomes very resistant to both wind and water erosion. The soil-binding action is not dependent on the presence of living filaments. Layers of abandoned sheaths, built up over long periods of time, can still be found clinging tenaciously to soil particles, providing cohesion and stability in sandy soils at depths up to 10cm.
Nitrogen fixation is another significant capability of cyanobacteria. Vascular plants are unable to utilize nitrogen as it occurs in the atmosphere. Cyanobacteria are able to convert atmospheric nitrogen to a form plants can use. This is especially important in desert ecosystems, where nitrogen levels are low and often limiting to plant productivity.
Soil crusts have other functions as well, including an ability to intercept and store water, nutrients and organic matter that might otherwise be unavailable to plants.
Grasses grow throughout Arches. Individual grasses sprout almost anywhere there is soil. Grasslands form in areas where wind-blown sediment and erosion have created a layer of soil that is several feet thick. Small grasslands form in potholes that have filled with dirt. Most desert grasses can be fit into two groups: bunch and sod-forming. Bunch grasses are classic desert plants that occur in scattered clumps. This growth pattern reduces competition for limited soil nutrients and water. Indian ricegrass and needle-and-thread are bunch grasses. The relatively large ricegrass seeds are rich in protein and were an important source of food for Native Americans.
Needle-and-thread has a sharp seed attached to a wound "thread" that drives the seed into the ground as it unwinds. Both of these grasses are perennial, becoming dormant during droughts. Ricegrass plants have been known to live over 100 years. Sod forming grasses are what most people have in their yards. Galleta and blue grama are sod-forming perennials native to Arches, and usually grow together. Unlike most desert grasses, galleta can withstand heavy grazing and is important forage for bighorn sheep and mule deer. The seed head of blue grama looks like eyelashes. Cheatgrass is a sod-forming grass that was accidentally brought to the United States in the 1800s. This European annual is now established throughout the west and frequently takes over areas disturbed by fire or livestock grazing.
There is a great deal of exposed rock in Arches, and much of it is spotted with multicolored lichens. Lichens usually grow on north-facing surfaces since reduced solar radiation is an advantage for many organisms in the desert. Lichens also colonize healthy, mature cryptobiotic soil crust, and occasionally on live or dead plant material. Many species of lichen are found in Arches. A lichen is actually a simple community of at least two organisms, namely fungi with green algae or cyanobacteria, though sometimes with both. The lichen structure is more elaborate and durable than either fungi or algae alone.
Green algae and cyanobacteria manufacture food via photosynthesis, while fungi provide a buffer against weather and are capable of extracting nutrients from soil and rock. Lichens are well adapted to arid climates. They can carry on food production at any temperature above 32º Fahrenheit. Lichens can absorb more than their own weight of water, and can absorb temporary water like dew almost directly into their algal cells (the water does not need to go through roots and stems as it does in vascular plants).
Many plants benefit from the presence of lichens. The cyanobacterial component of lichens can transform atmospheric nitrogen (unusable to most organisms) into a form that is an essential nutrient for life. This is especially important in desert ecosystems, where lack of nitrogen is known to limit productivity.
Mosses and liverworts are some of the many organisms found in Arches that most people do not associate with deserts. Mosses can tolerate long periods of complete dehydration and occupy a variety of habitats in the park, including exposed rocks, cryptobiotic soil crusts, riparian areas and sometimes trees. They do best in shady canyons, north-facing slopes and at the bases of shrubs. Most liverworts must be near water to survive, and are very rare in the park.
Mosses and liverworts are small, primitive, non-vascular plants. They lack the conductive tissue most plants use to transport water and nutrients. Instead, moisture is absorbed directly into cells by osmosis. The most abundant mosses in Arches can remain dry for years, and will rehydrate in seconds after contact with water. Some species begin photosynthesizing less than one hour after being moistened. There is no complete inventory of mosses and liverworts in Arches.
At least 20 moss species are known to colonize cryptobiotic soil crusts, with Syntrichia caninervis being the most common. Grimmia orbicularis accounts for 80 percent of the moss found on rock surfaces. Like all photosynthetic organisms, mosses are primary producers that build biomass through photosynthesis. They enrich ecosystems with organic matter, forming the basis of the food chain. As a component of cryptobiotic soil crusts, mosses trap airborne soil particles, reduce erosion, retain water and may enhance water infiltration.
Studying the woody plants of Arches is made easy by the fact that, as a rule, they grow rather small and far apart. Limited by lack of water, shrubs and trees must disperse in order to survive. Once established, these desert plants are tenacious. Their roots will split rocks in search of nutrients, and many can live over 100 years. Shrubs and trees are distinguished by their height (a less reliable indicator in the desert) and the number of stems (shrubs have several). Common shrubs include Mormon tea, blackbrush, four-wing saltbush and cliffrose. Mormon tea contains a drug similar to ephedrine, which is used in nasal decongestants and as a stimulant. Blackbrush is important winter forage for desert bighorn sheep, despite its thorny nature. In Arches, tree diversity is greatest in riparian corridors where water is plentiful.
Netleaf hackberry, box elder, Russian olive, tamarisk and Fremont's cottonwood grow in these areas. Both Russian olive and tamarisk are non-native species that can supplant native trees and significantly alter stream environments. Mixed stands of pinyon pine and Utah juniper cover millions of acres in the southwest, including much of Arches. These trees grow closely associated and dominate the landscape in dry, rocky terrain at elevations between 4,500 and 6,500 feet. As elevation decreases, trees become more scattered and Juniper more common because it is more drought resistant than pinyon. Pinyon pines have crooked trunks, reddish bark and are very slow growing. Trees 4 to 6 inches in diameter and 10 feet tall may be 80 to 200 years old. Their root systems are extensive and often mirror the size of the above ground tree. Pinyons produce compact cones that contain tasty, protein-rich seeds called pinenuts. Pinenuts were a major source of food for Native Americans and are still popular today.
Animals like the bushy-tailed woodrat, the pinyon mouse and the pinyon jay also prize them. The Utah juniper is the classic desert tree. Its twisting, often-dead branches seem to epitomize the struggle of life with little water. When moisture is scarce, a juniper will actually stop the flow of fluids to some outer branches so that the tree has a better chance for survival. Scale-covered leaves and bluish, waxy-coated seeds help the tree conserve moisture.
Desert potholes provide homes to a fascinating array of small organisms and microorganisms. Pothole dwellers have unique adaptations which enable them to survive in this feast or famine environment. Potholes are very easily disturbed. Pothole organisms are sensitive to sudden water chemistry changes, temperature changes, sediment input, being stepped on, and being splashed out onto dry land. Human use of pothole water by swimming, bathing or drinking may change the salinity or pH of a pool drastically. More importantly, this change occurs suddenly, unlike the slow, natural changes to which organisms can adapt.
Hikers should therefore avoid using water in potholes as well as walking through dry ones. While these tiny ecosystems may seem unimportant, they can act as an indicator for the health of the larger ecosystems in which they occur. These pools do not have the ability to counteract acids, so the acid rain caused by industrial pollution may be lethal. Pothole health is monitored at various locations in order to track significant changes in our environment.
Datura (Datura wrightii) produces the largest flower in canyon country. Many visitors are surprised at the amount of vegetation in Arches. Plants are critical components of all ecosystems, and Arches is no exception. Plants capture particulate dust in the air, filter gaseous pollutants, convert carbon dioxide to oxygen, provide animal habitat and possess many raw materials useful to humans. Many adaptations enable plants to survive the extremes of temperature and aridity found in Arches. These adaptations are grouped in three categories: drought escaper's, drought resistors and drought evaders. Drought escaper's are plants that make use of favorable growing conditions when they exist. These plants are usually annuals that grow only when enough water is available. Seeds may lie dormant for years if conditions are not favorable. Most grasses are escaper's, as are wildflowers that bloom after seasonal rains during spring or late summer.
Drought resistors are typically perennials. Many have small, spiny leaves that reduce the impact of solar radiation, and some may drop their leaves if water is unavailable. Spines and hairy leaves act to reduce exposure to air currents and solar radiation, limiting the amount of water lost to evaporation. Cacti, yuccas and mosses are examples of drought resistors. Yuccas have extensive taproots 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, mosses green up immediately. Drought evaders, the final group, survive in riparian areas where water is plentiful. Monkey flower, columbine and maidenhair fern are found in well-shaded alcoves near seeps or dripping springs. Cottonwoods and willows require a lot of water, and only grow along river corridors and intermittent streams where their roots can reach the water table easily. Soil chemistry and depth are also important factors that influence where plants grow. Deep soils tend to be covered with grasses. Shrubs like blackbrush and purple sage favor shallow sandy soil, while greasewood and Mormon tea are signs of alkalinity. The dominant plant community in Arches, the pinyon-juniper woodland, colonizes rocky soils and fractured bedrock.
Drought escapers are plants that make use of favorable growing conditions when they exist. These plants are usually annuals that grow only when enough water is available. Seeds may lie dormant for years if conditions are not favorable. Most grasses are escapers, as are wildflowers that bloom after seasonal rains during spring or late summer.
Drought resistors are typically perennials. Many have small, spiny leaves that reduce the impact of solar radiation, and some may drop their leaves if water is unavailable. Spines and hairy leaves act to reduce exposure to air currents and solar radiation, limiting the amount of water lost to evaporation. Cacti, yuccas and mosses are examples of drought resistors. Yuccas have extensive taproots 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, mosses green up immediately.
Drought evaders, the final group, survive in riparian areas where water is plentiful. Monkey flower, columbine and maidenhair fern are found in well-shaded alcoves near seeps or dripping springs. Cottonwoods and willows require a lot of water, and only grow along river corridors and intermittent streams where their roots can reach the water table easily.
Soil chemistry and depth are also important factors that influence where plants grow. Deep soils tend to be covered with grasses. Shrubs like blackbrush and purple sage favor shallow sandy soil, while greasewood and Mormon tea are signs of alkalinity. The dominant plant community in Arches, the pinyon-juniper woodland, colonizes rocky soils and fractured bedrock.