Read an Excerpt
Landscaping on the New FrontierWaterwise Design for the Intermountain West
By Susan E. Meyer Roger K. Kjelgren Darrel G. Morrison William A. Varga
UTAH STATE UNIVERSITY PRESSCopyright © 2009 Utah State University Press
All right reserved.
Chapter OneNative Landscapes of the Intermountain West
To design beautiful and functional native landscapes, the first step is to learn to look at landscapes in nature and to begin to understand why they look the way they do. Even intuitively obvious truths about intermountain landscapes need to be given some thought. For example, all westerners know that, to escape the heat of summer, a picnic in the mountains is generally a good approach. In the winter, we know that we can head for the desert to escape from the snow. Plants respond to these climate differences at least as much as people do. The native plant communities in high mountain valleys are completely different from the plant communities in the desert country, where people often go to seek winter sunshine.
As you drive up into the mountains from towns nestled in the valleys at their feet, first the low sagebrush steppe vegetation gives way to foothill communities characterized by small trees like gambel oak and bigtooth maple, or to a pygmy evergreen forest made up of juniper and pinyon pine. Further up, patches of quaking aspen and white fir or lodgepole pine start to appear, interspersed with meadow communities of grasses, low shrubs, and an abundance of wildflowers. If you are driving up a canyon with a year-round stream, you will see the difference right away between the streamside vegetation, which is very green and lush, and the hillsides above, which support shrubs and grasses found in much drier environments. Often, arriving in the aspen/white-fir or lodgepole pine zone is enough to relieve the heat of summer, but if the road continues to wind upward, it will pass through evergreen forests of sub-alpine species of spruce and fir, until at last it reaches timberline and breaks out into alpine tundra, the dwarf community that lives on the high, windswept ridges that are too harsh to support trees.
Each of these plant communities represents a response to a set of environmental conditions that define the habitat for the species that occur in that community. By understanding how plants interact with environmental conditions in nature, you will begin to see how you might create designed landscapes that capture the essence of these natural landscapes. You can use suites of species with complementary needs as well as complementary aesthetic features, and group them into patterns that reflect the natural patterns you have observed.
A hallmark of the Intermountain West is its great variability in terms of climate, topography, and geology. The basic theme is that of a generally semi-arid region with cold winters and dry summers, but there are a multitude of variations upon this theme. Majestic mountain ranges rise up like islands out of the Great Basin desert lowlands, while mighty rivers dissect immense canyons into the giant staircase of high mesas on the Colorado Plateau, and the massive spine of the Sierra-Cascade axis creates rain shadow effects far to the east. All this topographic diversity creates an incredible array of growing conditions for plants, and a corresponding diversity in plant communities.
As mentioned above, the most basic climatic trend in the region is related to elevation. Dry environments with hot summers in the desert valleys give way to successively cooler and moister conditions as you travel up into the mountains. Superimposed on this basic pattern are microclimate variations created by differences in slope and exposure. A close look at a west-facing range like the Wasatch Mountains will reveal the great importance of exposure in our region. At higher elevations, the north-facing exposure can be cloaked in white fir and aspen, while the south-facing exposure supports a mountain brush community usually found on warmer, drier sites.
Look lower down on the mountain, and you will see that the mountain brush community occupies the north-facing slopes, while the south-facing slopes support the characteristic low shrubs and bunchgrasses of the sagebrush steppe community. Each plant community occurs under a characteristic climatic regime, but that regime is the result of the interplay of a number of factors, such as latitude, elevation, exposure, and slope.
Light is another factor that has a major impact on plant communities. In fact, the differences caused by exposure are largely due to differences in the duration and intensity of sunlight. Northern exposures are shaded for longer periods than southern exposures, especially during spring and fall, at least in the Northern Hemisphere. Topographic relief can have an even more dramatic effect on light in places where vertical cliffs and deep canyons are part of the landscape. In a canyon bottom, the ground surface may be shaded for most of the day, and the plants that grow there, while enjoying moderated temperatures and better moisture, must be able to tolerate low light intensity. Rock outcrops in the desert can have a similar effect. Lastly, the plants themselves can change the light environment for associated species. For example, the shade under the closed canopy of an evergreen mountain forest is so dense that only a few species are shade tolerant enough to grow there, whereas the dappled shade created by an aspen forest supports a very diverse suite of understory species.
Over a region as vast as the Intermountain West, latitude has a major impact on climate, particularly temperature. At a given elevation in the southern part of the region, temperatures will generally be much warmer than those at that same elevation in the northern part. For example, St. George, Utah, and Boise, Idaho, have similar elevations, but St. George is in the Mojave Desert, which has relatively warm winters and very hot summers, while Boise, five hundred miles farther north, has much colder winters and cooler summers. The elevation at timberline is also much lower at more northerly latitudes than it is in the south, so plant communities are shifted downward in elevation as one goes farther north. This makes it hard to predict climate in a particular place just from a knowledge of its elevation, but the trend for cooler, moister environments at higher elevations can be seen throughout the region.
Another important effect of topographic relief is seen in the way that the water that falls as rain and snow in the mountains is redistributed. The fact that this water is channeled into streams and subsurface aquifers, where it can be captured and harvested by people, has made settlement of this generally semiarid region possible. And it is still this mountain water that provides for irrigation of farmland and urban landscapes in the desert valleys below, not to mention supplying potable water for direct human consumption. This natural redistribution of water also has important implications for native plant communities. On steeper slopes, less water is available to plants, because more water will run off before it has a chance to penetrate the soil. The soils also tend to be shallower on steep slopes, reducing water availability even more. For plants growing at the foot of the slopes, namely along the channels that collect water from adjacent slopes, water availability is greatly increased. Stream channels often support riparian (riverside) plant communities, made up of species that have much higher water needs than could be met by rainfall alone in a given location.
Geology also modifies the effect of climate on plants. First, geology on a grand scale determines the landforms that develop in a region-range after range of mountains separated by broad, sediment-filled valleys and salt playas in the Great Basin, vast lava plains punctuated by deep canyons on the Columbia Plateau and Snake River Plains, and giant flat-topped mesas of sandstone, shale, and clay on the Colorado Plateau. These landforms, in turn, affect large-scale weather patterns and create the effects of elevation and topography that we have already noted.
On a geological scale more immediately important to plants, different types of rock weather into soils that have distinct physical properties. These different soils interact with climate to produce characteristic growing environments for plants. Soils formed by the weathering of sandstone or granite tend to be coarse and sandy or gravelly, with relatively low water-holding capacity, while those weathered from limestones, shales, and basalts tend to be finer-textured, to contain more clay, and to have a relatively high water-holding capacity. In the mountains, loamy soils relatively high in clay definitely support more tree growth than sandy or gravelly soils. The explanation is that, once the root zone is filled with water, the rest of the water can drain out the bottom of the soil profile to join with subterranean groundwater. In a loamy soil, the root zone will contain more water when it is filled, making more water available to the trees that grow there, while in a sandy or gravelly soil, more water will be lost out the bottom. This is also true in the temperate environment of eastern North America, where sandy soils are observed to be more "droughty" than loamy soils.
Interestingly, however, in desert environments, the opposite phenomenon can be observed. Sandy soils definitely support more plant growth in deserts than loam or clay soils. The reason for this is that in dry environments, it rarely if ever rains enough to fill the soil profile to the point where water drains below the root zone. A much more important source of water loss from soil in deserts is through evaporation from the surface. And it turns out that sandy or gravelly soils lose much less water through surface evaporation than loam or clay soils, because once a coarse soil dries out on the surface, the below-ground reservoir of water is protected from further evaporation by the layer of dry surface material. In effect, the layer of dry sand acts as a mulch. Sandy soils have little ability to wick water to the surface, whereas clay soils continually wick water from below to replace that lost by evaporation, so that clay soils dry out evenly, and water stored deep below the surface is quickly lost. This means that more subsurface water will be available to plants over a longer period in sandy soil, a difference that is of considerable importance to desert plants.
Soil interacts with climate and topography in another essential way, and this involves feedback from the resulting plant community. These interactions influence fertility and organic matter (humus) content and indirectly affect the complex of microorganisms that occur in the soil and within the root zones of plants. It is easy to observe that dry desert environments support plant communities made up of mostly small plants at wide spacing, while wetter mountain environments support larger plants, such as trees, that grow in close proximity, often with a dense understory, as well. These contrasting scenarios represent differences in productivity, which in turn usually result in differences in the total mass of plant material per unit area that an environment can generate and sustain. Plant productivity generally increases as water availability increases and decreases at cooler temperatures.
The most productive plant communities in our region are the streamside and wetland communities at lower elevations, where warm temperatures and a longer growing season are combined with high water availability. Traditional home landscapes are usually managed to maintain high productivity, much like a wetland, and that is why they require a constant investment of resources such as water and fertilizer. This is in contrast to most native plant communities, which are made up of plants that are generally very frugal in their use of resources. For native plants, artificially rich topsoil and perpetual watering are definitely not necessary.
There is a straightforward relationship between plant productivity and the fertility and organic matter content of the soil, with more productive streamside, wetland, and mountain plant communities having a soil organic matter content substantially higher than that found in the soils of desert communities. But organic matter is a double-edged sword. For plants adapted to rich soils, life is good in such a soil. But disease organisms lurk in organic matter. Many desert plants, never having encountered such conditions, generally have little resistance to these diseases. Rich soil does not enhance their performance, and it may even threaten their lives.
This observation points to a more general paradigm: plants that come from different environments have different needs. Each combination of climate, topography, and soil creates a particular environment or habitat where a particular suite of native plants can be found growing together as a community. By matching plants to habitats that meet their requirements, and by grouping plants with similar requirements together, you can create designed landscapes made up of groups of plants that will thrive in each other's company.
With the help of this book, you can learn to recognize native plant communities without any special training in botany or ecology. You can learn to look at plant communities with the eyes of someone who wants to know what it is about being in a certain kind of place, surrounded by a certain community of plants, that feels so welcoming. If you have any doubt that the plant community itself is a major element of the magic of a wild place you love, no matter how spectacular the scenery, try imagining that place without any plants.
Fortunately, it is much more realistic to re-create the ambience of such a place in a designed landscape by using elements on the scale of plants and rocks than it is to attempt to re-create the scenery itself. If you observe closely, you will see how the feeling created by a place is evoked on many scales. These range from the grand scale of cliffs and rivers, which is frankly beyond reach in a designed landscape, to the miniature scale of the arrangement of pebbles, fallen leaves, grass clumps, and sand in a canyon bottom. And there are many scales in between. One of the most powerful ways to distill the essence of a place is to choose your scale carefully. You can also echo patterns observed on a grand scale, re-creating them on the more achievable scale of a designed landscape.
In order to assess what it is you like about a particular plant community and how you could go about capturing its essential features in a designed landscape, you will need a basic understanding of native plant community structure. In this book, we divide intermountain plant communities into four principal groups, based on climate and, more specifically, on average annual precipitation.
Desert plant communities occur where the average yearly precipitation (including both rainfall and the water contained in snowfall) is roughly ten inches or less. Semi-desert communities are found where precipitation averages more than ten inches but less than fifteen inches a year. Foothill communities occur where average annual precipitation is between fifteen and twenty inches, and mountain communities occur where annual precipitation averages more than twenty inches.
In the semi-arid climate that prevails in our region, average precipitation is somewhat of an abstraction, because there is tremendous variation in total precipitation from year to year. Being optimists, people tend to interpret above-average years as "normal" and label below-average years as drought years. But in fact, in most of the Intermountain West, more than half the years are below average. This is because wet years tend to be very wet, and to pull the average upward.
The average precipitation boundaries described above should not be interpreted as absolute, because many other factors, particularly evaporation rates, act to either accentuate or minimize the effects of too much or too little rainfall. Similarly, the boundaries between different plant communities are usually not sharp. Different communities often interfinger, weave, or blend, especially in situations where complex topography or geology creates a range of different microhabitats within an area. Another variable to consider is the time of year the precipitation is received. In the northern part of our region, summer rainfall is not common, even in the mountains, and rarely occurs at all in the valleys. As you go further south, the influence of subtropical summer monsoons becomes more pronounced, so that in southeastern Utah, for example, up to half the annual precipitation falls in the summer, even in the valleys.
Excerpted from Landscaping on the New Frontier by Susan E. Meyer Roger K. Kjelgren Darrel G. Morrison William A. Varga Copyright © 2009 by Utah State University Press . Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.