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THE TREE AND ITS GROWTH

Basic Statement:

Trees are distinguished from other plants by their large size and long lifespan. The uniqueness of trees as living organisms is, thus, based on growth characteristics. An understanding of how trees grow is essential for understanding the nature of their environmental interactions, the origin of different tree forms, and the structural characteristics of wood.

Key ideas:

The aboveground parts of a tree grow by (1) the addition of layers of wood on existing stems and branches, and (2) the extension of new branches from buds. Competition and dominance relationships among the buds determine the basic shape of trees. Tree shapes, however, can be modified by the action of environmental factors (e.g., light, wind, insects) on bud development. One reason for the remarkable resilience and longevity of trees is that even when tree crowns are severely damaged, some buds escape, initiate growth and eventually restore much of the crown structure.

Important terms:

apical meristem	terminal bud	xylem	determinant growth
phloem	long shoot	cork cambium	indeterminant growth
vascular cambium	short shoot	lateral bud

References:

1. Wilson, B. F.. 1971. The Growing Tree. University of Massachusetts Press, Amherst MA.
2. Zimmerman, M., and C. Brown. 1974. Trees: Structure and Function. Springer, New York.

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INHERITANCE AND NATURAL VARIATION

 

Basic statement:

Each individual is unique, yet it shares many characteristics (e.g., flower morphology, fruit structure, phyllotaxy) with all other members of its species. Both uniqueness and commonality reside in DNA, the molecule of genetic endowment that is passed on from generation to generation. Nevertheless, the variation among trees is striking and occurs at all levels of genetic relatedness. Variations in the characteristics of trees affects other plants and animals (e.g., shade tolerant shrubs, climbing lianas, nesting preferences of birds, feeding behavior of insects), and humans uses of trees take advantage of this variation in many ways (e.g., crown sizes and shapes in landscaping, wood texture and hardness in art).

Key ideas:

Trees, as all organisms, may be viewed as systems of environmental tracking, making use of relevant ancestral experience. This experience is recorded in the DNA molecule that resides in every living cell of their body. The information encoded in DNA is not only archival but serves as "software" for the tree's growth and development. Thus, it sets certain constraints to forms and structures while allowing for some variation in response to the specific environment the tree experiences. DNA is replicated faithfully within an individual (barring mutation) but undergoes recombination as it is passed on through sex cells to the next generation, thereby providing for varied progenies.

Variation among trees, as in all organisms, is due primarily to the interaction of the genotype with the environment. Within the lifespan of an individual, the expression of the genotype is modified by the environment. Over longer time periods, the interaction is more complex and the genetic make-up in successive generations may change in response to the environment. Differences among species or among individuals and populations of the same species result from both long- and short-term interactions. The importance of the genetic and environmental components of variation can be determined experimentally: typically by growing the progeny of different individuals in common environments, or by growing asexually propagated (cloned) material of the same individual in different environments.

Important terms:

DNA , genetic code	genotype
mitosis	phenotype
meiosis	common garden
gene expression	provenance test

References:

1. Alberts, B., D. Bray, J. Lewis, M. Raff, K. Roberts, and J. Watson. 1989. Molecular Biology of the Cell. 2nd Ed. Garland Publishing, New York.
2. Antonovics, J., and A. D. Bradshaw. 1970. Evolution in closely adjacent plant populations. VIII. Clinal patterns at a mine boundary. Heredity 25:349-362.
3. Dawkins, R. D. 1987. The Blind Watchmaker. W. W. Norton and Company, New York. 332 pp.
4. Futyma, D. J. 1986. Evolutionary Biology, 2nd Ed. Sinauer Associates Inc., Sunderland MA.
5. Gonick, L., and M. Wheelis. 1983. The Cartoon Guide to Genetics. Barnes and Noble, New York. pp. 106-163.
6. Lerner, I. M., and W. J. Libby. 1976. Heredity, Evolution, and Society. 2nd Ed. Freeman & Co., San Francisco. Chapter 6.
7. Mettler, L. E., and T. G. Gregg. 1969. Population Genetics and Evolution. Prentice Hall Foundation of Modern Genetics Series.
8. Mitton, J. B., Y. B. Linhart, J. L. Hamrick, and J. S. Beckman. 1977. Observations on the genetic structure and mating system of ponderosa pine in the Colorado Front Range. Theor. Appl. Genet. 51:5-13.

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EVOLUTIONARY MECHANISMS

 

Basic statement:

Organisms do not remain constant over generations. New species arise from old ones, while others decline. These genetic changes are the result of organic evolution.

Key ideas:

Organic evolution results from the interaction of several processes: 1) mutation, 2) gene flow between populations, 3) random events, 4) mating preferences, and 5) natural selection--differential survival/reproduction in response to the environment. Natural selection is thought to be the driving force of evolution. It maintains genotypes that are successful in a given environment. If the environment remains stable for many generations, natural selection results in adaptation (having a repertoire of solutions for conditions that can be anticipated, based on past experience). Since many environments are constantly changing, species do not remain stable but undergo change as a function of evolutionary mechanisms. Species that inhabit spatially heterogeneous environments often show pronounced polymorphisms (many forms).

Important terms:

• mutations
• migration
• genetic drift
• natural selection

References:

1. Antonovics, J., and A. D. Bradshaw. 1970. Evolution in closely adjacent plant populations. VIII. Clinal patterns at a mine boundary. Heredity 25:349-362.
2. Dawkins, R. D. 1987. The Blind Watchmaker. W. W. Norton and Company, New York. 332 pp.
3. Futyma, D. J. 1986. Evolutionary Biology, 2nd Ed. Sinauer Associates Inc., Sunderland MA.
4. Mettler, L. E., and T. G. Gregg. 1969. Population Genetics and Evolution. Prentice Hall Foundation of Modern Genetics Series.
5. Mitton, J. B., Y. B. Linhart, J. L. Hamrick, and J. S. Beckman. 1977. Observations on the genetic structure and mating system of ponderosa pine in the Colorado Front Range. Theor. Appl. Genet. 51:5-13.

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SPECIES CONCEPTS, CLASSIFICATION AND NOMENCLATURE

Basic statement:

The species is one of the most important units of biological organization. However, species are not totally autonomous nor sufficiently constant to be described by the characteristics of a typical individual.

Key ideas:

Species are the smallest groups that are structurally distinct and usually reproductively isolated from others. They are groups of individuals that share a large portion of their genetic material. The genetic similarity among individuals results from gene exchange in prior and current generations. New species arise under the following sequence of conditions: 1) the flow of genes between populations is cut off, 2) the separate populations experience different selective pressures, and eventually, 3) individuals cannot interbreed once the populations are rejoined. However, genetic separation is not always complete, and members of different species sometimes produce fertile offspring. All species are classified in a hierarchical scheme that is designed to reflect evolutionary relationships. Species are named according to strict rules.

Important terms:

binomial nomenclature	geographical isolation
hybridization	isolating mechanisms

Principles of Nomenclature

1. Scientific names are Latin or Latinized forms of other languages.

2. Scientific names of species follow the binomial system of nomenclature. Each species name consists of two words. The first word designates the genus. The second word is always used with the first word and refers to the species within the genus, e.g., Alnus rubra.

3. Each plant name is associated with the abbreviated name of its author, e.g., Magnolia acuminata L. (L. refers to Linnaeus).

4. Scientific names should be underlined or italicized.

5.Hybrids are designated in one of two ways:

a. if the original parents are known, both names are used with the female parent first, e.g., Populus tremuloides x P. alba;

b. if the original parents are not known, a new name is given, e.g., Platanus x acerifolia.

6. Infraspecific names are formed by adding Latin words to the species name, prefaced by an abbreviation of the type of infraspecific category:

a. subspecies: e.g., Pseudotsuga menziesii subsp. glauca;

b. variety: e.g., Quercus garryana var. breweri

c. form: add the abbreviated f. before the form name, e.g., Pseudotsuga menziesii subsp. glauca f. pendula.

7. Cultivated varieties (cultivars) are strains selected for desirable characteristics and are often produced by horticultural or agricultural techniques. The names of cultivars may be written in two ways:

a. Chamaecyparis lawsoniana ‘Glauca’

b. Chamaecyparis lawsoniana cv. Glauca

8. Plant classification has a hierarchical organization (example for Douglas-fir)

Level of Classification Name (key features)

KINGDOM Plantae (many cells, photosynthesis)

DIVISION Spermatophyta (seeds plants)

SUBDIVISION Gymnospermae (naked seeds)

ORDER Coniferales (cone bearing)

FAMILY Pinaceae

GENUS Pseudotsuga

SPECIES Pseudotsuga menziesii (Mirb.) Franco

SUBSPECIES Pseudotsuga menziesii

subsp. glauca (Beissn)

References:

1. Davis, P. M., and V. M. Heywood. 1963. Principles of Angiosperm Taxonomy.
2. Grant, V. 1971. Plant Speciation. Columbia University Press.
3. Jameson, D. L., (Ed.) 1977. Genetics of Speciation. Benchmark Papers in Genetics., Vol. 9. Dowden, Hutchinson and Ross, Stoudsbug PA.
4. Levin, D. A. 1979. The Nature of Plant Species. Science 204:381-384.

 

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THE SPECIAL BIOLOGY OF TREES

Basic statement:

Trees carry out the same life functions as smaller plants. However, their large size and long-life spans make these functions more challenging than for many other plants.

Key ideas

We will consider the special biology and stresses of trees. For example, 1) Trees have many parts that require communication (among buds, among branches, between roots and shoots). 2) Trees can move water from the soil to more than 100 feet above the ground without expending energy. Human devices that move water to these heights use pumps that must be fueled. Scientists still debate the forces that move water through trees. 3) Trees must endure large environmental fluctuations in different seasons and throughout their life span. They use a combination of phenotypic and genotypic solutions to this challenge. 4).... We will add to the list.

Important terms:

evapotranspiration	apical dominance	root/shoot ratio
compartmentalization	auxin	senescence

Reference

Wilson, B. F. 1971. The growing tree. University of Massachusetts Press, Amherst MA.

 

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THE GEOMETRY OF TREES

Basic statement:

The open growth system of trees permits a variety of branching patterns, leaf placements and overall crown shapes. The geometric forms of some trees is so distinctive that they can be identified by their silhouettes alone. Different tree forms have been addressed by several models that seek to explain the adaptive value of branching patterns.

Key ideas:

Henry Horn developed a model to explain the differences in branching pattern and leaf placement of angiosperm trees. Light and heat are the primary selective agents in this model. The model suggests that leaf-size and leaf-placement patterns are strategies to maximize photosynthesis and minimize heat stress at different stages of forest succession. Other models by Leopold and by Stevens provide conceptual frameworks to explain stem form and branching pattern. All models stress the adaptive nature of tree architecture.

Important terms:

monolayer	deliquescent
multilayer	excurrent

References:

1. Hallé, F., R. A. A. Oldeman, and P. B. Tomlinson. 1978. Tropical Trees and Forests: An Architectural Analysis. Springer-Verlag, Berlin.
2. Horn, H. S. 1975. Forest succession. Scientific American 232:90-98.
3. Stevens, P. S. 1974. Patterns in Nature. Little, Brown & Co., Boston/Toronto.
4. Zimmermann, M., and C. L. Brown. 1974. Trees, Structure and Function. Springer, N.Y.

 

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LIFE-HISTORY STRATEGIES

Basic statement:

The biological environment is an important agent of natural selection. Changes during forest succession are thought to select for different life-history features of tree species. These life-history features, in turn, affect strategies of forest management and uses of trees in urban settings.

Key ideas:

Tree species are found in recurring combinations called forest communities. Each community is broadly associated with a set of physical conditions (e.g., climate, soil type) and/or disturbance regimes (e.g., fire, flooding). Even in constant environments, forest communities change gradually over an often predictable sequence of stages. The life-history characteristics of species comprising this sequence also change in a predictable pattern. These characteristics include attributes such as the amount of energy allocated to growth vs. reproduction, the numbers and sizes of seeds, and the time spent in juvenile and adult life stages. Because the amount of energy derived from photosynthesis is finite and less than that necessary to meet all of the possible life-history demands, energy allocated to one process is not available to another. Thus, different strategies of allocating limited resources are selected in early vs. late stages of succession.

Important terms:

allocation	succession	intolerant
life-history traits	pioneer	tolerant

References:

1. Charnov, E. L., and W. M. Schaffer. 1973. Life-history consequences of natural selection: Cole's result revisited. Amer. Nat. 107:791-793.
2. Matthews, J. D. 1963. Factors affecting the production of seed by forest trees. Forestry Abstracts 24:i-xiii.
3. Stearns, S. C. 1977. The evolution of life history traits. Ann. Rev. Ecol. and System. 8:145-171.

 

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CONIFER ADAPTATIONS

Basic statement:

Conifers are characterized by low species diversity and limited morphological variation. Nevertheless, much of the Northern Hemisphere is dominated by conifer species. What is the reason for their success in this topographically and climatically diverse region?

Key ideas:

Conifers are most common in regions of harsh climate--where substantial portions of the year are cold and/or dry and, hence, growing seasons are short. Their success in harsh environments is the result of several factors. Conifers can maintain high rates of photosynthesis at relatively low temperatures. Their wood consists of narrow water-conducting cells (tracheids) that reduce the frequency of air bubble formation when stems thaw in the spring. Bubbles break the flow of water from roots to the leaves and result in foliar damage. Conifer needles have thick, waxy coatings and sunken stomates, which prevent excessive water loss. The sapwood column of conifers is large and acts as a short-term reservoir that supplies water to foliage during drought periods. The evergreen leaves of most conifers allow them to be opportunistic--to photosynthesize immediately whenever cold or drought periods are broken. Conifers can therefore photosynthesize in late fall and early spring, when rainfall is abundant, while deciduous angiosperms must try to photosynthesize in the summer, when rainfall is limited. Although the climate of the Pacific Northwest is relatively mild, this region is dominated by conifers. The combination of mild, wet winters and dry, cool summers appears to give a competitive advantage to conifers over deciduous angiosperms in this region.

Important terms:

tracheid	sunken stomates
vessels	winter desiccation

References:

1. Franklin, J. F., and M. A. Hemstron. 1981. Aspects of Succession in the Coniferous Forests of the Pacific Northwest. In: Forest Succession, Concepts and Application. D. C. West, H. H. Shugart and D. B. Botkin (Eds.). Springer Verlag, New York.
2. Waring, R. H., and J. F. Franklin. 1979. Evergreen coniferous forests of the Pacific Northwest. Science 204:1340-1386.
3. Zimmerman, M. H., and C. L. Brown. 1971. Trees, Structure and Function. Springer Verlag, New York. Chapter 4.

 

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REPRODUCTIVE SYSTEMS OF TREES

Basic statement:

Reproduction insures the continuity of life. In trees, as in other plants, reproduction involves a mixture of strategies. Some reproductive mechanisms foster a diversity of progeny; others insure uniformity. Each strategy has practical and evolutionary implications.

Key ideas:

Reproductive systems of trees can be broadly categorized as sexual or asexual. Sexual reproduction involves the formation of sex cells by meiosis and their union (fertilization) to form the first cell of a new individual. Each step results in new combinations of genes. Thus, sexual reproduction is characterized by a diversity of progeny and is well suited for survival in fluctuating environments. Unlimited variation is disadvantageous, however, and is constrained by several factors (e.g., low chromosome numbers). Asexual reproduction (reproduction without fertilization) results in uniform progeny. The most obvious mechanism of sexual reproduction in trees is propagation via vegetative parts. Asexual reproduction is advantageous in 1) stable environments, 2) conditions in which physical damage is frequent and severe, and 3) harsh environments where sexual reproduction often fails. Plants may have a mixed strategy of both sexual and asexual reproduction.

Important terms:

agamospermy	dioecious	meiosis	gamete	vegetative propagation	stem sprouts
monoecious	mitosis	fertilization	zygote	layering	root suckers

References

1. Baker, H. G.. 1960. Reproductive methods as factors in speciation in flowering plants. Cold Spring Harbor Symp. Quant. Biol. 24:177-191.
2. Bawa, K. S. 1980. Evolution of dioecy in flowering plants. Ann. Rev. Ecol. Sys. 11:15-39.
3. Freeman, D. C., K. T. Harper and W. K. Ostler. 1980. Ecology of plant dioecy in the intermountain region of western North America and California. Oecologia 44:410-417.
4. Solbrig, O. T. 1976. On the relative advantages of cross and self fertilization. Ann. Missouri Bot. Garden 63:262-276.

 

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POLLINATION AND SEED DISPERSAL

Basic statement:

Pollen grains and seeds are the mobile life stages of trees. Both stages have very high mortality rates. However, rather than being the weak link in the chain of survival, pollen grains and seeds serve vital functions that cannot be accomplished during the longer, dominant adult stage of trees.

Key ideas:

Pollen grains are produced in the anthers and transported to the surface of the stigma by air, animals or water. Pollen grains are small compact structures made up of several cells, one of which is the sperm cell. The sperm cell is delivered to the ovary, where it fertilizes the egg cell, contained within the ovule. The wide array of flower structures, sizes, and color reflects different adaptations to pollinating agents. Wind-pollinated flowers are typically small and have large anthers and stigmas. They often lack petals, which interfere with the wind. By contrast, insect-pollinated plants have prominent petals, which function to attract insects and direct them to the reproductive structures. Insect-pollinated plants often have other attractants, such as aroma, and "rewards," such as nectar. All of these features enhance the potential for successful movement of genes (contained in sperm) between individuals in the process of sexual reproduction.

Seeds consist of an embryo surrounded by nutritive material and enclosed in a protective covering. They are the primary mode of dispersal in tree species. Seed dispersal allows trees to expand into favorable environments. Over short time periods (decades) this includes the ability of early successional species to find open ground for establishment. Over very long time periods (centuries to thousands of years), seed dispersal allows trees to migrate in response to changing climate. Seed dispersal has genetic consequences, as wide dispersal enhances the chance that mating will occur between genetically unrelated individuals. Seed dispersal is also an important agent of gene flow between neighboring populations. As with other types of adaptations, seed adaptations in trees include a variety of traits (biochemical, structural, developmental).

Important terms:

endosperm	epicotyl	honey guides	pollen tube
hypocotyl	cotyledon	nectaries

References:

1. Pijl, L. van der. 1972. Principles of Dispersal in Higher Plants. 2nd ed.
2. Harper, J. L., et al. 1972. The shapes and sizes of seeds. Ann. Rev. Ecol. and System. 1:327-356.
3. Proctor, M., and P. Yeo. 1972. The pollination of flowers. Taplinger Publishing Company, New York, 418 pp.

 

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COEVOLUTION

Basic statement:

Interactions between plants and animals are far more complex than the simple observation that animals eat plants. Reciprocal selection has produced intriguing and widespread dependencies among trees and their animal associates.

Key ideas:

The fossil record reveals one of the best examples of coevolution: the contemporaneous expansion of angiosperms and insects during the Cretaceous period. Coevolutionary interactions in the current environment are only a recent topic of scientific investigation. Investigations are being carried out using a combination of approaches: observation, correlation, experimentation and modeling. Coevolutionary interactions under current study involve such processes as pollination, defoliation, seed predation, seed dispersal. Traits involved in these interactions are diverse (e.g., biochemicals, mechanical structures, timing of life-history events).

Important terms:

Coevolution

References:

1. Ager, A. A., and R. F. Stettler. 1983. Local variation in seeds of ponderosa pine. Can. J. Bot. 61:1337-1344.
2. Janzen, D. H. 1969. Seed eaters versus seed size, number, toxicity, and dispersal. Evolution 23:1-27.
3. Lanner, R. M. 1982. Adaptations of whitebark pine for seed dispersal by Clark's Nutcracker. Can. J. For. Res. 12:391-402.
4. Levin, D. A. 1976. The chemical defenses of plants to pathogens and herbivores. Ann. Rev. Ecol. System. 7:121-159.

 

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TREE RESPONSES TO CLIMATE CHANGE

Basic statement:

Vegetation changes on all temporal and spatial scales in response to natural climate variations. Forest communities are transient assemblages and tree adaptation cannot keep pace with changes in the selective environment.

Key ideas:

Forest communities of the last ice age do not have counterparts on the modern landscape. Conversely, species that dominate present-day forests were rare during glacial periods. Because glacial periods are longer than interglacial periods, forest trees have been selected predominantly under conditions very different from today. Tree species dominating modern forests first became common 8,000-10,000 years ago, when they expanded northward from ice age refugia. Species spread at different rates and in different directions, reaching their current range limits only by 3000-5000 years ago. Thus, present-day forests are recent assemblages and should not be considered stable over evolutionary time scales. Although less extreme, the effects of environmental changes over the past several centuries and decades are evident at vegetation ecotones.

Important terms:

dendrochronology	Little Ice Age	Holocene
greenhouse warming	pack rat middens	palynology

References:

Brubaker, L. B. 1988. Vegetation History and Anticipating Future Vegetation Change. In: Ecosystem Management for Parks and Wilderness, J. K. Agee and D. R. Johnson, Eds. University of Washington Press, Seattle. pp. 41-61.

 

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BIOLOGICAL INVASIONS

Basic statement:

Humans are increasingly moving about the world. As they do so, they may either accidentally or intentionally introduce organisms, including plants, into places in which they have never occurred. Sometimes these introductions are harmless, but many times the introduced organisms negatively affect native species and ecosystems.

Key ideas:

In the last few centuries a number of plants have been introduced accidentally through the inclusion of weed seeds in imported crop seeds, on or in imported animals, or in soil from other countries that was loaded onto ships as ballast and then dumped here in exchange for cargo. Non-native plants have also been introduced intentionally for ornamental and agricultural purposes. Most introduced plants do not establish and reproduce, but many do form extensive "natural" colonies and are considered "non-native invasive plants." These invasive plants compete with native species for resources such as light, water, nutrients, pollinators, and seed dispersers. They may also alter hydrology, geomorphic processes, and disturbance frequencies and intensities in native ecosystems, all of which can severely affect native species. Non-native invasive plants are considered to be a significant threat to biodiversity.

Important terms:

Non-native invasive plants	restoration
Biological invasions

References:

1. Mooney, H. A., and J. A. Drake. 1986. Ecology of biological invasions of North America and Hawaii. Springer-Verlag, New York.
2. U. S. Congress, Office of Technology Assessment. Harmful non-indigenous species in the United States. OTA-F-565. U. S. Government Printing Office, Washington DC.

 

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HUMAN INFLUENCE ON TREE EVOLUTION

Basic statement:

Over the past several thousand years people have been the most influential biological agent of natural selection. The numbers of domesticated plants and animals is a prime example of the power of human selection. Although less obvious, the influence of human activity on tree species has been no less important.

Key ideas:

The influence of humans on trees was initially inadvertent, resulting from the various ways people used the forested landscape (e.g., hunting, pollarding, selective cutting). In recent times, we have targeted many of our efforts toward specific goals in tree improvement. Taking advantage of our understanding of evolutionary processes, we have begun to change the characteristics of trees for our own purposes. At the same time, our inadvertent influence on tree species continues and may be unrelated to the direct use of forests (e.g., acid rain). Discovering the nature of these influences and seeking a rationale for humans as agents of natural selection is a challenge for us now and in the future.

Important terms:

hybridization	selective breeding
genetic engineering	cloning

References:

1. Gates, D. M. 1993. Climate Change and Its Biological Consequences. Sinauer Associates Inc., Sunderland MA.
2. Harris, L. D. 1984. The Fragmented Forest. University of Chicago Press.
3. Krugman, S. L. 1984. Policies, strategies, and means for genetic conservation in forestry. In: C. Yeatman, D. Kaflor and G. Wilkes (Eds.), Plant Genetic Resources, a Conservations Imperative. Westview Press. p. 71-78.
4. Libby, W. J. 1973. Domestication strategies for forest trees. Can. J. For. Research 3:265-276.

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AESTHETIC VALUE OF TREES

Basic statement:

The basis for the aesthetic appeal of trees is probably linked to the variety of shapes, textures, and colors they represent, and the effect of those features on human awareness.

Key ideas:

We will explore our own ideas about the aesthetic and other intangible values of trees, keeping in mind the biological origin of traits that evoke our responses. For example, we like groupings of trees with different colors and forms. The color of tree foliage varies greatly, depending on pigment content: from dark green of coast redwood and big leaf maple, to yellowgreen of western redcedar and American elms, to the fall-season browns of oaks and the brilliant reds of sugar maple. The different shapes of trees are derived largely from their branching patterns: angular and pointed geometry of conifers and excurrent angiosperms, rounded forms of deliquescent angiosperms. Trees evoke a feeling of permanence and stability by the fact that their life spans and sizes are so much greater than ours. Yet trees provide interesting change and reflect the passage of time by the seasonal cycle of leaf and flower production and fall. We will add to this list.

Reference:

Stettler, R. F. 1978. On plants and their essence. The University of Washington Arboretum. Winter 41(4):17-24.

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TREES FOR THE URBAN ENVIRONMENT

Basic statement:

Throughout most of the United States, trees are an integral part of cities. While the urban environment does not favor tree growth, the reverse is even more likely: trees can significantly improve life in the city.

Key ideas:

During the past century, Americans have moved from rural environments to cities. In the process, we brought certain aspects of the rural environment, namely trees, to the urban setting--even where the local climate does not support the growth of trees. What is the reason for our dependence on trees? What benefits do we derive from them? What are the consequences of the urban environment to trees? What are selection criteria for successful urban trees? What is the formula for the successful greening of our cities?

Important terms:

greenbelt

References:

1. Black, M. 1978. The Reforestation of Seattle. University of Washington Arboretum Bulletin 41:26-29.
2. Fussel, E. 1965. Frontier: American Literature and the American West. Princeton University Press, N.J.
3. Marx, L. 1964. The Machine in the Garden. Oxford University Press, New York.
4. Miller, R. W. 1988. Urban Forestry: Planning and Managing Urban Greenspaces. Prentice-Hall, Englewood Cliffs NJ.
5. Nash, R. 1982. Wilderness and the American Mind. 3rd Ed. Yale University Press, New Haven.
6. Pollan, M. 1991. Second Nature: A Gardener's Education. Dell Publishing, New York.

 

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TREES AND HUMAN BEHAVIOR

Basic statement:

The value of trees in urban settings includes their effects on human actives, attitudes and health.

Key ideas:

What are the benefits of the urban forest? We know about the environmental returns of having trees and green spaces in cities - better water quality, cleaner air, reduced pollutants and energy savings. In addition, social scientists have discovered other benefits, such as job satisfaction, improved quality of life, stress reduction, faster healing and restoration of mental abilities. Social science research confirms that urban nature is a fundamental human need, essential for the health and well-being of people!

References:

1. Dwyer, J.F., H.W. Schroeder, P. H. Gobster. 1994. The Deep Significance of Urban Trees and Forests. In R.H. Platt, R.A. Rowntree, P.C. Muick (Ed..), The Ecological City: Preserving & Restoring Urban Biodiversity. Amherst: University of Massachusetts Press.
2. Lewis, C. A. 1996. Green Nature/Human Nature: The Meaning of Plants in our Lives. Chicago: University of Illinois Press.
3. Schroeder, H. W. 1989. Environment, Behavior, and Design Research on Urban Forests. In E. H. Zube & G. T. Moore (Ed.), Advances in Environment, Behavior and Design - vol. 2. New York: Plenum Press.

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