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Woody Plants of Utah
A Field Guide with Identification Keys to Native and Naturalized Trees, Shrubs, Cacti, and Vines
By Renée Van Buren Janet G. Cooper Leila M. Shultz Kimball T. Harper
Utah State University Press
Copyright © 2011 Utah State University Press
All right reserved.
Chapter One Introduction to the Woody Flora
A book on the native woody plants of Utah could vary in length, depending on how one defines the term "woody." For the purpose of this book, we define woody plants as those retaining woody tissue aboveground season after season. This definition includes taxa described as trees, vines, shrubs, cacti, subshrubs, and some suffrutescent species (decisions on which taxa to include become difficult—no doubt others might have different interpretations).
The anatomy of the stem usually determines whether a plant is woody or not. Woody plants have stems that persist aboveground through seasons unfavorable for growth, due to drought or low temperatures. Such plants produce secondary tissue (secondary xylem, phloem, and bark) from lateral meristems that result in an increase in girth. The ensuing sturdiness allows plants to grow taller and compete more favorably in the race to reach light. This growth in girth, in addition to apical growth resulting in increased height, permits these plants to become large and long lived. Arguably, the largest, oldest, and tallest organisms today are woody plants (giant sequoia, bristlecone pine, and coastal redwood).
This woody habit may be considered advantageous in various ways. Clearly, the perennial habit reduces problems associated with plant establishment, since conditions suitable for seedling growth may not be present every year. Woodiness is also likely to reduce plant palatability to herbivores and discourage herbivory (McKell 1975; Young, Eckert, and Evans 1979). Woodiness may also enhance a plant's ability to root deeply enough to ensure the acquisition of adequate water for survival (Thatcher and Hart 1974).
A relatively large number of woody plants have their origin within Utah's state boundaries. Many of these woody endemics are species in the families Asteraceae, Polygonaceae, and Cactaceae. As indicated in table 1, the family Asteraceae (sunflower family) is represented in Utah by 82 woody species (110 taxa), over twice the number of the next largest family, Cactaceae (35 species, 40 taxa). Francis (2004) reports that there are 5281 shrubby species and about 1300 tree species that are native or naturalized in the United States and its territories. Francis also reports that families with the largest number of woody species include Asteraceae, Rosacea, Fabaceae, Cactaceae, and Ericaceae, in descending order.
A book on the trees and shrubs native to Utah may appear to the casual observer as one needing very few pages to offer a full description of all the species occurring naturally in the state, especially in the case of trees. Utah is not a center for tree diversity. In temperate climates that are seasonal, due to periods of drought or adverse temperatures (such as those that occur in Utah), shrubby species are many times more numerous than tree species. The moisture required to support the tree habit is simply absent in the current and recent past climate of the state, except in moderately high elevations on mountain ranges and along perennial streams. However, globally, trees are extremely important, as they encompass 27% of the earth's surface that is not covered by water (FAO World Resources 2000–2001). Tree species contribute perhaps 25% of the known 350,000–450,000 vascular plants of the world (Scotland and Wortley 2004). Tree species usually require more moderate and better-watered environments than shrubby species; thus trees are more restricted in their ecological limits and distribution. Shrubs, rather than trees, extend into environments more subject to drought. Woody plants are less common in environments where the rooting zone is poorly aerated, due to flooding or clayey soils having only slow rates of gas exchange.
There are several definitions available for distinguishing trees from shrubs, such as Donoghue (2005), who defines trees as "tall plants, with a thickened single trunk, branching well above ground level." This definition, however, requires some accommodation for plants that are growing in environments where elevation, moisture, light, or herbivores may cause abnormal growth. One example of the impact environment can have on the growth habit of a species is the Krumholtz effect that occurs at timberline, where mature Engelmann's spruce are reduced to a shrubby thicket, compared with their height of up to 40 meters on more favorable sites.
For the purpose of this book, trees are generally defined as usually single trunked and more than 4 meters tall at maturity. However, many species we categorize as trees because of their height may have more than one trunk at ground level. In addition, some trees may be less than 4 meters tall but have a single trunk. Our ability to distinguish trees from shrubs becomes even more difficult when we are in short-statured community types, such as desert environments, where moderately tall shrubs seem tree-like relative to other vegetation.
Woody plant species provide many benefits to humans, such as beverages, medicines, lumber for infrastructure and shelter, fuel, oils, industrial chemicals, spices, dyes, and hundreds of different fruits, seeds, and some vegetative parts used as food. The 23,000 tons of newsprint used daily in the United States originates from woody plant products (Uno et al. 2001). Cultivars of woody plants line our city streets, yards, and parks as ornamentals.
Ecological services that woody plants provide to ecosystems include the production of oxygen and the uptake of carbon dioxide in vast forests, both boreal and tropical. Woody plants stabilize soils, thus reducing erosion and flooding, and provide food, habitat, and cover for wildlife. Since woody plants are often long lived, they are used to define particular plant communities, such as pinyon-juniper or sagebrush, or conifer forests.
Some plants form important symbiotic relationships with various bacteria, resulting in nitrogen fixation, which is significant, since nitrogen is often a limiting factor in plant growth. The most common microbial symbiont is Rhizobium, a bacterium that invades the root tips of plants (both herbaceous and woody) of the family Fabaceae (Acacia, Parryella, Prosopis, Psorothamnus, and Robinia). Another bacterial symbiont, Frankia, invades the roots of plants in the families Betulaceae (Alnus incana), Elaeagnaceae (Elaeagnus commutata), Rhamnaceae (Ceanothus spp.), and Rosaceae (Coleogyne, Cercocarpus spp., and Purshia spp.). Trees and shrubs (and most herbaceous plants) also form mutualistic relationships with fungal partners, or mycorrhizae, which aid the plant in nutrient and water uptake from the soil. Mycorrhizae are the link from the plant to the soil and help maintain soil structure, aid nutrient recycling, and increase the bacteria's ability to form nodules for nitrogen fixation. These important functions are especially critical to plants that occupy arid environments, such as those that occur in the Intermountain West.
Some woody species display poisonous qualities to domestic animals or to man, and they can cause reactions ranging from mild discomfort to possible death. Examples of poisonous woody plants include such species as poison ivy (Toxicodendron rydbergii), snakeweed (Gutierrezia sarothrae), scrub oak (Quercus gambelii), horsebrush (Tetradymia canescens), greasewood (Sarcobatus vermiculatus), skunkbush (Rhus aromatica), and European bittersweet (Solanum dulcamara). Seasonal discomfort due to allergies caused by juniper, sagebrush, rabbitbrush, and many other woody species also has negative impacts on us.
Approximately 75% of Utah's woody species are pollinated by animals, including birds, insects, bats, and rodents. The remaining species, notably Artemisia in the Asteraceae and Atriplex in the Amaranthaceae, are wind pollinated.
We should comment briefly on a group of subshrubs that are parasitic on various trees in the state and play an important role in the ecology of our wildlands. In Utah, parasitic mistletoe belonging to the family Viscaceae are represented by two genera containing eight species. The genera are separated by morphological characteristics of the flower and fruits and by their host plants. The genus Arceuthobium has six species that occur in Utah and grow primarily on conifers (Abies, Picea, Pseudotsuga, and Pinus). The genus Phoradendron is represented by two species: one grows primarily on juniper, and the other on Acacia and sometimes Larrea. Although tempting, plant hosts alone cannot be used to correctly identify the parasitic species of mistletoe, as more than one species may grow on a single host species; we refer you to Welsh et al. (2008), Cronquist et al. (3A, 1997), or another source for specific identification. These taxa are not included in the keys in Chapter Three or in the descriptions in Chapter Five, but instead are summarized in table 2.
Utah is rich in endemism relative to other states. The number of endemics in our region increases if one ignores state boundaries and observes more natural boundaries created by topography, such as those proposed by Welsh and Atwood (2009). These authors suggest that 11 geoendemic areas can be identified in Utah. They discuss why the areas occur and list the plants that inhabit them. Most of Utah's endemic flora is herbaceous; however, at least 75 of the species we consider woody in our book are included in Welsh and Atwood (2009). Woody endemics are especially common in the families Polygonaceae and Asteraceae, two plant families well represented generally in the state (Stein, Kutner, and Adams 2000).
Although Utah has many endemic woody species with a limited range and distribution, there are other shrubs and trees that occur in nearly every county or region of the state. The most prevalent woody species statewide are listed in table 3. Most of them have very broad habitat requirements and occur in a variety of plant communities.
Chapter Two Major Utah Plant Communities
Professionals and interested naturalists are usually introduced to the plant life of a region via a list of species known to occur in the area. Such species lists are technically known as floras. Floras are of unquestionable value to those interested in regional ecosystems, but they are of limited help to laymen seeking to understand the holistic aspects of landscapes readily discernable in an area.
Observers will easily recognize different aspects of the plant cover or vegetation in a locale. Forests are readily distinguishable from adjacent areas that support only herbaceous species or are dominated by shrubby plants that never grow taller than waist height. Other areas may appear to be barren rock, but upon closer study are seen to support a few distinctive, diminutive plant species that root and thrive in crevices in the stone. The viewer sees obvious variations in the vegetation in such overviews but can probably not identify any particular species. Such gross but distinctive aspects of regional plant life have come to be referred to as vegetation types, or plant communities.
Plant communities are characterized by the gross structure, or life form, of the prominent species in a particular habitat. Interestingly, species adapted to particular kinds of habitats often have similar life forms and longevity patterns. Large, long-lived, tree-like species require a perennial supply of water and mineral elements. At sites where the supply of such essentials is only seasonal or available from limited storage areas (such as shallow, porous rock; small crevices in rock; or accumulations of water- or wind-deposited sands or gravels), the adapted plants are either small or possess life spans that match those periods in which requirements essential for life are present in quantities adequate to support life.
Such plant-environment relationships result in vegetation characteristics that differ remarkably over an area in which the availability of life's essentials vary widely through time or space. Consequently, the gross vegetation aspects change across any landscape.
In this book we have recognized 20 vegetation complexes, or plant communities, which are listed in table 4. We realize that other distinctive vegetation associations occur in Utah, but they usually dominate limited areas. Examples of such distinctive plant communities that we have not recognized here include hanging gardens, crevice plant assemblages, rock faces colonized by lichens and small-bodied algal taxa, and colonizers of often relocated deposits of sand or gravel.
In table 4, we have shown the range in elevation and precipitation that is possible for each of the 20 community types considered. The elevation ranges listed for major species in each community have been primarily extracted from data in A Utah Flora (Welsh et al. 2008). A plant community is not equally well developed throughout its elevation range, as the development of each community complex will be modified by local topography, geology, and edaphic conditions. Precipitation variation within each plant community has been estimated from the relationship between precipitation and elevation along an elevation gradient observed in Washington and Iron counties in southwestern Utah. All data were extracted from files maintained by the Western Regional Climatic Center, Desert Research Institute, in Reno, Nevada.
Of the various plant communities recognized in table 4, eight are dominated by tree-sized plants, seven others are dominated by shrubby species that rarely grow over 6 or 7 meters tall, while five are dominated by herbaceous species. Tree-dominated communities include the pinyon-juniper communities of eastern and western Utah; the lower and higher elevation riparian vegetation types; and the forest communities dominated by Douglas fir, Engelmann's spruce and subalpine fir, ponderosa pine, and aspen-mixed conifers. Shrub-dominated communities include salt desert shrub, xeric sagebrush, big sagebrush-grass, the Colorado Plateau shrub complex, the Mojave Desert complex, and the mountainbrush communities of northern and southern Utah. Herb-dominated vegetation communities include the halophyte subshrub complex, marshlands, tall forb assemblages of higher elevations, sedge meadows dispersed throughout the forest and alpine communities above timberline. One might identify subdivisions within any major community type, such as wet meadows and dry meadows within the alpine community, or chaparral within the southern mountainbrush community. In addition, rare communities that do occur in Utah and harbor many endemic species include hanging gardens, rock crevices, sand dunes, bristlecone pine forests, Chinle soil outcrops, gypsum soil outcrops, pygmy sagebrush, Beaver Dam limestones, Arapian shale, Mancos mat saltbush, and others.
Communities dominated by herbaceous species
This community is characterized by saline soils with standing water (at least seasonally) or a water table within rooting zones where the soils are often saturated. It is a mosaic of sparsely vegetated and barren playa flats. The dominant vegetation includes saltgrass, Utah samphire, annual samphire, and pickleweed, a woody shrub.
Soils of marshlands are nonsaline and have either standing water or water that occurs within the rooting zone. The dominant species in this community are taller than halophyte vegetation types (over 0.5 m) and are usually monocotyledons (grasses and grass-like species), with a variety of herbs such as cattail, rice cutgrass, western eupatorium, dogbane, and reed grass, a large, introduced grass species that is increasingly aggressive and troublesome.
This community occurs at high elevations (usually above timberline) in mountains where exposure to wind, ice, light, and blown snow occur regularly. The soils are shallow and the growing seasons are very short. Approximately 16 woody species occupy this community type in Utah. Many of them are willows, such as arctic willow, snow willow, plane-leaf willow, and Cascades willow. Alpine communities can be subdivided into wet meadows and dry meadows. Wet meadow vegetation includes sedges, grasses (tufted hairgrass, Idaho fescue, sheep fescue, alpine timothy, spike trisetum), and forbs (moss campion, cushion paronychia, Rydberg's sandwort, dwarf clover, and American bistort). Suffrutescent woody species—such as whortleberry and bog laurel—may be present, along with low-growing willows. Dry meadows include herbaceous species such as yarrow, dandelion, Richard's geranium, and Penstemon species, but these sites have few woody plants.
Sedge meadow complex
The soils are well drained but often seasonally saturated at middle to high elevations. This community is dominated by sedges and other grass-like species. Approximately 14 woody species are found in this community, including willows, river birch, whortleberries, and other ericaceous species.
Excerpted from Woody Plants of Utah by Renée Van Buren Janet G. Cooper Leila M. Shultz Kimball T. Harper Copyright © 2011 by Utah State University Press. Excerpted by permission of Utah State University Press. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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