CHAPTER 3

WHOLE TREES

LANDSCAPE AND ORNAMENTAL TREES

Landscape and ornamental trees are an important part of human life. They provide shade and beauty around homes, schools, markets and shopping areas, places of work, along streets and highways, in city parks and other areas. They also help conserve energy and the quality of air, water and soil.

Virtually all species of temperate broad-leaved trees are important landscape and ornamental plants and fulfil one or more functions in landscape design. As good as all countries have some level of nursery industry that offers for sale planting stock for landscape and ornamental purposes. Small-scale nursery operations are often an excellent opportunity for small business or family-run enterprises.

HISTORICAL ASPECTS

Trees have been used as ornamental plants since the earliest of times. More than 4 000 years ago, the Egyptians wrote about trees being transplanted with a ball of soil around their roots. Some trees were moved up to 2 400 km by boat. In Greece, Theophrastus (370-285 BC) and Pliny (AD 23-79) gave instructions for tree planting and care. Many books on the care of trees and woody shrubs have been written since those early times.

During the Middle Ages, botanical gardens contained primarily plants of medicinal importance. Later, the gardens of private estates contained many exotic plants introduced via trade and travel. Many of these gardens are now public and are great sources of information and recreation.

By the early 1700s, trees were being planted with some frequency in the cities and estates of Europe. During the early settlement of North America, trees were cut to make room for farms and communities. During the late 1700s, however, trees were being planted in town squares. Unfortunately, after the trees were planted, few received care, except perhaps on large estates. As settlers migrated west into the open prairies, they planted seeds of fruit trees and other trees to shelter their homes from high winds.

In the early 1900s, national research institutes in Europe and North America began to study fruit and forest trees and by the 1950s, these institutes began working on problems associated with landscape and ornamental trees. The need for this research was accelerated with the introduction of several major pests and diseases that caused serious problems with both forest and ornamental trees (e.g. Dutch elm disease (Ophiostoma ulmi) and white pine blister rust (Cronartium ribicola). Experiment stations, botanical gardens, arboretums and some large plant nurseries have long been involved in the introduction and evaluation of landscape materials that are able to tolerate the rigours of the urban environment. These events led to the development of the science of arboriculture - the planting and care of trees and other woody plants [Harris, 1976].

BENEFITS

Ornamental trees provide a basic contact with nature and heighten pleasure in human surroundings. Their value is difficult to quantify in economic terms but some of the aesthetic benefits they can provide are:

1. A variety of colour, form, texture and pattern;

2. Softening of harsh architectural lines;

3. Formation of vistas, frame views, provision of focal points and definition of spaces;

4. Trees can make enticing play areas;

5. Cooling shade, pleasant fragrances, intriguing sounds and serene settings;

6. They can create the impression of a well-established place in new residential areas and reduce the raw “unfinished” look.

Ornamental trees can add to the value of real estates, although there are few examples of accurate assessments. One study in the eastern United States indicated that trees increased the appraised value of undeveloped land by 27 percent and that of 0.2 ha residential lots with houses by 7 percent. Industry officials have found that attractive buildings and landscapes result in above-average labour productivity, lower absenteeism and easier recruitment of workers with hard-to-find skills [Harris, 1976].

Trees can have considerable effects on the microclimate of areas of heavy human population. They absorb heat as they transpire, provide shade that reduces solar radiation and reflection can reduce or increase wind speed, and can increase fog precipitation and snow deposition [Harris, 1976]. Trees can have a significant beneficial effect on the cost of winter heating and summer cooling of buildings. They break up urban “heat islands” by providing shade. It has been estimated that the shade provided by strategically placed trees near a residential home can reduce air conditioning costs by 30-50 percent, and trees planted as windbreaks around buildings can reduce winter heating energy use by 4-22 percent [Ciesla, 1995; Sampson, 1992].

DESIRABLE CHARACTERISTICS

Most temperate broad-leaved trees are used as ornamental and landscape plants in some form. Desirable characteristics of the trees selected for this purpose include genetic variability, foliage colour and density, overall form, growth rate (both fast and slow growing plants may be desired) and ability to grow and survive under a wide range of climatic and soil conditions.

Genetic variability

Genetic variability within species is a desirable trait to have in trees and plants used as ornamentals. Many trees have one or more distinct varieties. Varieties are considered to be one step below the species level in the taxonomic hierarchy. For example Salix alba var. vitellina is a variety of S. alba found in Switzerland that has especially good form and brilliant yellow shoots that are very decorative [FAO, 1979]. They are characterized by having distinct characteristics of foliage or form but are inseparable at the species level. Varieties appear in nature, are genetically stable and reproduce from seed. Cultivars are mutations or distinct forms of plants, initially found in nature, and propagated asexually by cutting or grafting with the objective of maintaining those characteristics for a saleable plant. Cultivars are not part of the classic Linnean taxonomic hierarchy. Cultivar names have been developed by the nursery industry to reflect the characteristics of the plant, the location where it was discovered, its discoverer, etc., but are not always latinized [Harrison, 1975]. For example, a purple leafed form of Fagus sylvatica, first cultivated in Germany during the eighteenth century, is designated as F. sylvatica Purpurea [Edlin, 1985]. Another well-known cultivar of this tree is F. sylvatica Pendula, a weeping form with long branches [Samson and Samson, 1986]. Certain cultivars of Salix alba grown commercially in the Netherlands are designated Liempe, Belders and Lievelde. These cultivars are produced vegetatively and are male trees that have good form and are fast growing [FAO, 1979]. Cultivariants are cultivars that appear somewhat different from their vegetative parents due to propagation from non-typical foliage [Harrison, 1975].

Foliage colour

Another desired characteristic of deciduous temperate broad-leaved trees used as landscape or ornamentals is the foliage colour. Several trees such as various cultivars of Fagus sylvatica and Acer palmatum have deep reddish-purple foliage colour throughout the growing season. Their colours can add interest and variety to a landscape during the entire growing season [Dirr, 1990].

Brilliant foliage colour during the autumn season is another desirable characteristic of landscape and ornamental trees. Many deciduous broad-leaved trees produce brilliant displays of fall colour. For example, the foliage of several oaks, e.g. northern red oak (Quercus rubrum), scarlet oak (Q. coccinea) and pin oak (Q. palustris), turn a brilliant scarlet colour in autumn. Other trees with bright autumn foliage colour include Acer saccharum (red-orange), A. rubrum (bright red), Betula spp. (yellow), Fraxinus pennsylvanica (bright yellow or violet depending on cultivars), Cornus florida (deep red), Liquidambar styraciflua (deep red), Pistacia chinensis (bright red) and Liriodendron tulipifera (bright yellow) [Ferguson, 1982].

Growth

Rapid growth is a desired characteristic of trees established in new developments so that the harshness of new construction is minimized in as short a time as possible. Silver maple (Acer saccharinum) is a popular fast growing shade tree but is susceptible to iron chlorosis when planted on alkaline or iron poor soils [Dirr, 1980]. Other fast-growing trees used in new landscapes include Alnus spp., Catalpa spp., Paulownia tomentosa, Populus spp., Robinia pseudoacacia, Salix spp., Sapium sebiferum and Ulmus spp. [Ferguson, 1982].

In other situations, trees with relatively slow rates of growth may be required. Examples include small gardens or patios where a fast-growing tree would quickly outgrow available space. Examples of slow-growing trees which would work well in limited space include Acer palmatum, Cercis canadensis, Cornus spp., certain cultivars of Prunus, Pistacia chinensis and Ostrya virginiana [Ferguson, 1982].

Colourful flowers and fruits

Flowering trees are especially popular in early spring when some species produce abundant flowers before leafing out. Two outstanding examples are the Japanese flowering cherries (Prunus serullata and P. yedoensis). Cultivars of these trees were given by the Japanese Government to the United States Government and were planted among several famous monuments in Washington, DC, where the annual cherry blossom display is world famous. Other trees that produce abundant and colourful spring flowers are Amalachier canadensis, Crateagus spp., Cornus florida, Sorbus spp. and flowering crab apples (Malus spp.).

Summer flowering trees continue to provide colour and variety after the spring blossoms have passed. Summer flowering trees include Catalpa spp., Liriodendron tulipifera, Magnolia grandiflora and Oxydendrum arboreum [Ferguson, 1982].

Another desired trait is trees that produce flowers in spring and brightly coloured but not necessarily edible fruits later in the season. A good example is the flowering dogwood (Cornus florida), which typically produces white blossoms in spring (Figure 3.1) and brilliant red fruits in autumn. Several pink flowering cultivars (rubra and Cherokee Chief) are also available [Dirr, 1990]. Other trees in this category include Crateagus spp., Prunus spp. and Diospyros khaki [Ferguson, 1982].

Figure 3.1 The flowering dogwood (Cornus florida) is a popular ornamental tree because it produces attractive flowers in spring and brilliant red fruits in autumn.

Ability to tolerate harsh conditions

Landscape and ornamental trees are often established under harsh environmental conditions. These include cities where there are high levels of air pollution, exposure to high levels of reflected heat, and limited open soil surface for air and water. Some temperate broad-leaved trees that are able to tolerate these conditions include Acer spp., Aesculus carnea, Carpinus betulinus, Catalpa spp., Celtis occidentalis, Fraxinus spp., Platanus spp., Quercus spp., Tilia cordata and Ulmus spp.

Ability to withstand salt spray is a factor that must be considered when landscaping near the seashore. Some trees, which can tolerate seashore conditions, include Acer platanoides, Acer rubrum, Arbutus spp., Carpinus betulinus, Eucalyptus spp., Nyssa sylvatica, Salix alba var. vitellina and Ulmus parvifolia [Ferguson, 1982].

USES

The purpose of any landscape is to look pleasing to the eye. Some special uses of broad-leaved trees and shrubs in landscape design are described in the following sections:

Shade and roadside trees

One of the major purposes of broad-leaved ornamental trees is to provide shade. Many temperate broad-leaved trees characteristically have spreading crowns, dense foliage and provide excellent shade. Planting of trees along roadsides in towns and cities provides not only welcome shade on warm summer days, but they also soften the sharp edges of homes, office buildings, factories and other structures.

Figure 3.2 - Plane trees (Platanus x acerifolia) provide welcome shade in a marketplace in Hefei, Anhui Province, China.

Many temperate broad-leaved trees are popular shade and street trees throughout the world. The species of Platanus (family Platanaceae) are popular street and shade trees in many areas. Not only do they provide good shade, but also their flaky green and white bark is attractive and eye-catching. Platanus occidentalis is widely used in many North American towns and cities and P. orientalis is often used in the Near East as a shade tree. In some European and Asian cities (e.g. Rome), a tree known as the London plane tree (Platanus x acerifolia), a hybrid between P. occidentalis and P. orientalis, is a popular tree for planting along streets, shopping malls and markets [author’s observation] (Figure 3.2). This tree can tolerate harsh city conditions and drought and is often pollarded. It does not have bright fall colouring. Another popular street and shade tree is the European lime or linden (Tilia cordata). This tree is widely planted in temperate climates. In Berlin, Germany, the famous street Unter den Linden, which has many eighteenth and nineteenth century monumental buildings, is named for linden trees, which line the street. Other popular street trees include the horse chestnut (Aesculus hippocastanum) and the Norway maple (Acer platanoides). The latter has been so widely planted in parts of North America that it is said to have been “overused.” This tree requires a lot of room to grow and its roots tend to buckle roadways and sidewalks [Dirr, 1990].

Another popular North American shade tree is the American elm (Ulmus americana). Its popularity stems from its graceful, vase-like crown form (Figure 3.3). This tree was once widely planted throughout the eastern and mid-western states but, unfortunately, most of these trees have succumbed to the Dutch elm disease, an introduced disease caused by the fungus Ophiastoma (= Ceratocystis) ulmi, which attacks the trees vascular system and causes a rapid death [Manion, 1991].

Specimen and character trees

The purpose of a specimen or character tree is to have a shape or form that will be attractive to look at throughout the year. They can be used in a variety of different situations such as marking the edge of a vista or characterize a particular space. The size of a specimen tree must be in relative proportion to its surroundings. If the space available is tall and narrow, a tree with a pyramidal or spire-like crown is most suitable.

There are many examples of temperate broad-leaved trees that are excellent specimen or character trees. The pronounced, pendulous form of the weeping willow (Salix babylonica) or the golden weeping willow (S. alba Tristus) provide interesting silhouettes, and the branches turn a yellow-gold colour in spring just prior to leafing out. The massive Fagus sylvatica and its various cultivars are popular specimen trees for college campuses, city parks and golf courses. In the southern United States, two popular specimen trees include Magnolia grandiflora, an evergreen with large white flowers that is known as the “symbol of the south” and the stately, spreading live oak (Quercus virginiana), a tree that graced many old southern plantations and is the major street tree in several southern cities including Savannah, Georgia [Dirr, 1990; Miller and Lamb, 1985]. Other temperate broad-leaved trees used as specimen and character trees include honey locust, Gledistsia triacanthos, Lombardy poplar (Populus nigra Italica) and tulip tree Liriodendron tulipifera [Ferguson, 1982].

Figure 3.3 American elms (Ulmus americana) on the campus of Colorado State University, Fort Collins, Colorado. This graceful tree was once widely planted throughout the eastern and mid-western United States but many have succumbed to the Dutch elm disease.

Shelters, screens and buffers

Shelter plantings are usually designed to protect adjoining areas from effects of wind or frost. The subjects they protect can range from other plants, to homes or greenhouses. Protection may take one of two forms - either overhead protection with overstorey plants or side protection. Trees are also important for screening unsightly areas such as industrial areas. The tall, columnar form of the Lombardy poplar (Populus nigra Italica) makes it an excellent choice for this purpose. The broad, spreading form of Salix babylonica also makes it a good screen tree. Another tree used for this purpose is Acer campestre, which can be grown in hedges [Ferguson, 1982].

Wall trees

Trees with restricted root systems can be used for close planting to walls and homes without fear that the roots will damage foundations. Temperate broad-leaved trees that are good candidates for wall plantings include Betula spp., Carpinus betulus Fastgiata, Crateagus phaenopyrum, Ilex opaca and Laurus nobilis [Ferguson, 1982].

PESTS AND DISEASES

A wide variety of pests and diseases affect shade and ornamental trees, many of which can cause significant damage. Because shade and ornamental trees are commonly planted under conditions vastly different from those in their natural ranges, they are often more prone to stress and subsequent invasion by pests and diseases. The widespread death of elms by the Dutch elm disease in Europe and North America has already been mentioned and is a catastrophic example of the loss of a highly valued group of shade trees. The recent introduction into the United States of a long-horned wood-boring beetle (Anoplophora glabripennis), an insect native to China, has resulted in the removal of large numbers of Acer platanoides and other shade trees in New York City [Haack et al., 1997] and more recently, in Chicago. In many European cities, infection of plane trees (Platanus acerifolia) by Ceratocytis fimbriata, a vascular fungus native to North America, has caused widespread death of many trees [author’s observation].

BONSAI

Bonsai is a technique for retaining the essential growth form of a tree but reducing it to pocket size. A more in-depth treatment of the culture and art of bonsai was presented by the author in the FAO Non-Wood Forest Products Series No. 12 document [Ciesla, 1998].

TEMPERATE BROAD-LEAVED TREES USED IN BONSAI

Like conifers, many temperate broad-leaved trees are popular for bonsai culture (Table 3.1). Many are popular for their hardiness. Some species, such as various cultivars of Acer palmatum or Fagus sylvatica, are selected because of their deep red to pink foliage colour. The Japanese hornbeam (Carpinus japonica) offers a symphony of bronze to yellow foliage colours in autumn. Several temperate broad-leaved hardwoods, such as some members of the family Rosaceae, are popular because of their seasonal flowers [Samson and Samson, 1986].

Table 3.1 Some temperate broad-leaved trees used in bonsai culture

¹ Various cultivars used
Sources: FAO, 1979; Samson and Samson, 1986