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Selection and Breeding of Small Trees

George H. Ware

Research Associate
The Morton Arboretum
Lisle, IL 60532

A paper from the Proceedings of the 9th Metropolitan Tree Improvement Alliance (METRIA) Conference held in Columbus, Ohio, August 8-10, 1996.

Abstract

The challenge of the urban-tree planting site emerges from the widespread prevalence of soil adversities and space restrictions, both above and below ground. Geographical, ecological, and edaphic origins of small trees in nature often give indications or clues as to their success in difficult urban places. Arboreta and botanic gardens, with their access to foreign sources of plants and seeds, are possibilities for obtaining new plants. Collections also offer opportunities for creating potentially useful hybrids. Small trees may be placed in categories according to their coping capacities on urban sites. There is a dual challenge in dealing with urban adversities: choosing the most appropriate tree and creating an accommodative site. Proper transplanting and follow-up care are vitally important.

Introduction

Restricted spaces have constraints that pose unending challenges for selecting, planting, and managing trees in urban areas, especially in "downtown" situations or in densely built neighborhoods. There are countless kinds of situations where limitations dictate use of small-stature trees that have space requirements matching available space, both above and below ground. Because of adversities posed by urban sites, the palette of suitable small trees is often limited. An important challenge is the search for small trees suitable for inhospitable sites, with a concurrent challenge to select or create accommodative planting sites. There is growing urgency to find tough small trees for tight places.

What are some sources of "new" or little-known small trees? Natural landscapes with native trees offer good possibilities for finding suitable species for restricted spaces. A lot of native species are in use in urban landscapes, but there are also numerous species that are little known. Genetically dwarfish forms of commonly planted species have not been utilized to any great extent. Many arboreta and botanic gardens have active acquisition programs and some have foreign contacts who respond to requests for specific items.

In Search of Suitable Small Trees

The Morton Arboretum has been assembling collections of woody plants since 1922. Good records show remarkable survivability of many kinds of trees and shrubs planted several decades ago. Some of these survivors are small-stature trees or large shrubs that can be easily groomed into tree forms. Arboretum groupings are notable for their aesthetics and presentability, but they also provide splendid opportunities to determine toughness, durability, and urban-use potential. Additionally, they provide ideas and clues as to directions for searching for new and better urban trees. Arboreta and botanic gardens contain overlooked woody plants that should be given greater attention as prospective urban landscape plants.

An example is the mulberry group. White mulberry (Morus alba), native to China, is not highly regarded as an urban tree. It is weedy and invasive, but it is a "coper", spontaneously appearing and enduring in all kinds of urban spaces. But there are other mulberry species from Asia that offer promise as urban trees. The toughness of white mulberry is also exhibited in three other hardy mulberry species: northern Chinese mulberry (M. australis); Mongolian mulberry (M. mongolica); and Russian or Tartarian mulberry (M. alba var. tatarica). Northern Chinese mulberry and Mongolian mulberry are small trees. Male trees (mulberries are dioecious) and hybrids of these two small tree species offer examples of urban-use promise of Arboretum-tested trees. Russian mulberry is a small-leaved relative of common mulberry, but its ultimate size and qualities are not well known. Native red mulberry (M. rubra), though not a small tree, is worthy of closer investigation as an urban tough tree. In this discussion, a small tree is considered to be one that attains a maximum height of 10 meters under midwestern U. S. climatic conditions.

There are other examples of tough-tree groups where special selections of small trees can be made. White poplar (Populus alba) is a rampantly growing large tree producing countless annoying root sprouts. There is a small-tree variety (P. alba var. globosa) that is a tenacious performer on inhospitable parking lot islands. In one instance, trees are in good shape after 25 years, maintaining a well-behaved globe form. Similarly, there is a globe-shaped tree-of-heaven (Ailanthus altissima) clone that copes well with harsh sites. Male selections of the weedy European buckthorn (Rhamnus cathartica) are successful plants in planter boxes of any size. These are examples of an approach to selecting tough small trees from groups known to contain good copers. Some other genera with small-tree representatives are: Robinia, Fraxinus, Ulmus, Acer, Tilia, Quercus, Malus, and Pyrus.

Forest Understory

Tough small trees may be located by longtime observation, random and fortuitous finds, and plantation testing. There is also a rationale using ecological and geographical bases for determining the potential usefulness and success of small urban trees. Trees have their origins in forests and woodlands where collective influences and longtime adaptations create microenvironments quite different from the concrete-dominated, limited urban spaces where trees are expected to survive and serve both functionally and aesthetically.

Forests and woodlands occur in a great variety of situations that have evolved over millennia. It is in these tree-testing laboratories that tree species, both large and small, with urban-use potential, have originated. Trees that tolerate adversities in nature are candidates for surviving inhospitable urban situations. Difficult situations for trees in nature are floodplains, clay soils, limestone regions, forest edges, and the driest regions of a species' natural geographic range.

Small trees that occur as understory trees in forests are to some extent dependent upon the favorable microclimate created by the larger overstory trees. Temperature and moisture fluctuations are less than those in open areas. Evenness of soil moisture conditions is maintained by the shelter provided by the large-tree canopy. Understory trees may be easily stressed by marked fluctuations of the open-area planted-tree environment both above ground and below ground. In the forest, successful competition with large trees is managed by small trees with their superficial root systems in forest-floor porous organic soils with favorable oxygen levels. This special genetic and ecological niche for small trees may translate to a narrow range of environmental tolerance, and increased vulnerability to adversities of open urban landscapes.

Inhospitable urban landscapes require trees that come from tough situations in nature. Longtime ecological selection and attunement in what we might call "nature's tough-tree testing laboratories" provide necessary qualities or "credentials" for survival of small trees on urban landscapes.

Flood Plains

Floodplains and swamps where prolonged flooding in springtime and excessively dry soils in summer permit survival of only those trees with genetic adaptions to a broad spectrum of soil-moisture conditions. Trees that can tolerate the low oxygen levels found in the soils of swamps and floodplains can tolerate the low oxygen levels of compacted, rubble-laden fill material that often comprises what is called urban soil. Floodplain species are the most commonly planted street trees of many towns.

Provenances from Edges of Species Ranges

Many trees from dry and droughty border sections of their large geographic ranges are able to cope with urban sites. Trees from more climatically favorable parts of the ranges may not have inherent qualities to withstand the difficult conditions often found on these urban sites. Abundant rainfall in the eastern United States provides favorable conditions for forests. Forest-tree distribution in border areas of geographic ranges may be limited by insufficient rainfall. Trees from these border areas have been stress tested by nature for millennia and provide sources of the tough genetic material from which urban trees may be selected.

Pioneer Species

Another approach to choosing successful urban trees for adverse environments is to seek out pioneer species; that is, those plants that colonize open fields or newly formed land surfaces such as barren areas left behind following coal or gravel removal. Pioneer plants have the capacity to endure a great deal of environmental adversity, both above and below ground. By their success in nature's laboratories, these plants demonstrate their capacity to cope in adverse man-made situations.

Restricted urban planting spaces vary from sidewalk planting pits to patches of lawn. Limitations are often posed by sidewalks, curbs, access drives, walls, utilities, light poles, fire plugs, and other man-made structures. Too often, trees are placed in spaces that are provided after buildings and parking-lot construction have been completed. Space for urban trees should be planned early, rather than adding trees in whatever spaces can be developed following completion of construction projects.

The main goal in selecting trees for limited spaces is to produce a long-term harmonious relationship between the tree and its environment, both above and below ground. Limited planting space requires the selection of trees that can survive.

It is widely accepted that more than 80 percent of urban tree problems begin underground. Attention must be given to the substratum where the root system (one-half of the functional tree) must cope successfully. Some of the adversities of urban soils are periodic excessive wetness, excessive dryness, compaction, alkalinity, salinity, root confinement, and grass competition. Successful establishment of urban trees involves a dual approach: site improvement for better accommodation of root growth and selection of the most suitable tree for survival and successful performance. Proper transplanting and follow-up maintenance are also important ingredients.

"Coping" Categories

Lists of small trees usually do not indicate degree of stress tolerance or "toughness" for selecting suitable possibilities. The graded lists given here are based upon experience in the Chicago region; however, the information is applicable for a much larger region.

Small trees that require favorable soil situations: pagoda dogwood (Cornus alternifolia), flowering dogwood (C. florida), Japanese maple (Acer palmatum), fringe-tree (Chionanthus virginicus), service-berry (Amelanchier), goldenrain-tree (Koelreuteria paniculata), gray birch (Betula populifolia), witch-hazel (Hamamelis virginiana), redbud (Cercis canadensis), and saucer magnolia (Magnolia x soulangiana),

Some trees that tolerate moderate soil adversity: hedge maple (Acer campestre), Amur maple (A. ginnala), Tartarian maple (A. tatarica), nannyberry viburnum (Viburnum lentago), black-haw viburnum (V. prunifolium), Japanese tree lilac (Syringa reticulata), and European spindle tree (Euonymus europaeus).

Tough trees for small places include crabapples (Malus) and pears (Pyrus). Notable among the crabapples is the diminutive Sargent crabapple (Malus sargentii). It is generally grown from seed and shows considerable variation. Cultivars of Pyrus calleryana such as 'Bradford' have been much overplanted but are among the best copers in very limited planting places. Birch-leaf pear (P. betulaefolia) also is a promising hardy flowering pear. Korean flowering pear (Pyrus fauriei) is a tough little tree (to 5 meters) that must be groomed to form a single-stem tree. Because it blooms after other pears have bloomed, it extends the flowering period of the group. Mongolian linden (Tilia mongolica) has a compact rounded crown with very small leaves and irregularly toothed margins. Height is up to 10 meters. Small-stature hackberries (Celtis reticulata, C. bungeana, and C. pumila) have been in Arboretum collections for decades but have not been tested as urban trees. Similarly, dwarf Chinese ash (Fraxinus bungeana) specimens are longtime survivors.

Several species and cultivars of hawthorn are widely planted with good success: Washington hawthorn (Crataegus phaenopyrum), thornless cockspur hawthorn (C. crus-galli var. inermis), and 'Winter King' hawthorn (C. viridis 'Winter King'). Foliar rust problems appear to be fewer on these hawthorns than on other species.

Recent Morton Arboretum accessions of rare elm species from China include some very promising tough small trees. One of these, Hebei elm (Ulmus lamellosa), develops mottled bark that enhances its appeal. Another small tree, cork-bark elm (U. propinqua), has distinctive pubescent dark red twigs and produces red to wine-colored autumn foliage. Szechuan elm (U. szechuanica) develops a broad umbrella form with reddish emerging branchlets in the spring. Because these elms may be hybridized, the potential for developing even more new trees is good.

Large Shrubs

Small trees may come from unusual sources. Large shrubs may sometimes be groomed as small trees. A notable example is Cornelian cherry dogwood (Cornus mas). Several species of Viburnum may be groomed as large shrubs or small trees. A cut-leaf cultivar of silver maple (Acer saccharinum 'Skinneri') has a dominant central trunk with pendulous branches. When the tall central trunk is cut back, the finely dissected foliage produces a willow-like look. As a pond-bank tree, this maple is a good substitute for willow.

Conclusions

Interest in new trees for urban landscapes continues to grow, especially among municipal arborists. Stress-tolerant small trees are very much a part of the picture. The International Society of Arboriculture Research Agenda for the 90's stipulate the following:

(1) Develop species resistant to specific diseases and pests,

(2) Develop species resistant to urban stress and pollution,

(3) Develop species with specific forms and structures.

The search for tough trees, the selection of longtime survivors, and the development of new hybrids all are responses to the desiderata set forth in the Agenda.


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