FIFTY TREES FROM FOREIGN LANDS

ELBERT L. LITTLE, JR.

Since ancient times mankind has sought out from distant parts of the earth new and better farm crops, garden plants, and trees. Many of these strange trees, introduced from seeds brought back by explorers and travelers through the years, have been highly successful for shade, ornament, fruits and nuts, shelterbelts, lumber, and other uses where the conditions for growth are not too different from those in their native lands. Discovery of the New World made possible great interchange of trees and other plants between East and West. Much of the early botanical exploration of North America was made by horticultural collectors who were hunting new plants for European gardens.

With settlement of the climatically diverse portions of the United States came introduction of trees from far away. Naturally the colonists from Europe brought their familiar shade trees, most of which succeeded also in eastern United States where the climate is similar. Examples are Norway spruce, white poplar, European white birch, and sycamore maple. The Yankee Clippers and afterwards botanical explorers brought back from temperate portions of Asia other kinds, such as ginkgo, Chinese scholartree, particled goldenrain-tree, and royal paulownia. The subtropical regions of Florida, southern Texas, southern Arizona, and California have obtained a wealth of exotic trees from tropical lands throughout the world.

After some years of testing, the good points and limitations of these introduced trees, such as their degree of hardiness to winter temperatures, soil and moisture requirements, drought resistance, tolerance to city smoke and dust, and susceptibility to insects and disease, have become known. Indeed, some of these exotics have been so successful that they have escaped from cultivation and have become naturalized, propagating themselves in waste places, roadsides, and woods as if wild. However, the native trees in any locality, having become adapted through the ages, usually are preferable to untested exotics.

At present more than a thousand kinds, or species, of foreign trees, not counting their numerous horticultural forms, are grown in the United States for shade and ornament. Additional thousands not yet popular have been introduced in arboretums and botanical gardens or have been planted infrequently as specimen trees. For their size, the subtropical regions from Florida to California have more different kinds of exotic trees than do any of the temperate regions of the United States. Because of the richness of tropical floras over the earth, many hundred kinds of trees have become available to these warmer regions.

Home owners in all parts of the United States now have wide selections of foreign trees for planting. The less familiar exotics command attention in contrast to the common native shade trees. Improved horticultural varieties, such as those with drooping branches, columnar crown, odd-tinted or cutleaf foliage, or distinctively colored flowers, are available. Aristocratic trees rich in history and legend may be planted. Among these is the ginkgo, a peculiar living fossil from China saved from extinction by plantings around temples through the ages. The cedar-of-Lebanon, so closely associated with the Holy Land and the source of the beautiful wood used in King Solomon's Temple, deserves to be planted more but is not hardy in the far North. The Italian cypress, the classical cypress of the ancient Greeks and the Romans, whose columnar shape is displayed in formal gardens, can be grown in Southern and Pacific States.

For a tropical atmosphere, hardy trees that are representative of their relatives from warmer lands can be grown northward. Examples are silk-tree ("mimosa"), with its fernlike foliage and pink, ball-like blossoms; ailanthus, with its coarse, compound leaves; and royal paulownia, with its big leaves and striking clusters of large violet flowers. Certain nut trees and fruit trees can serve a double purpose for shade or ornament also.

For forestry purposes, such as in reforestation of large areas through establishment of plantations, native trees generally have been more satisfactory than introduced trees. Scotch pine and Norway spruce, the most popular foreign forest trees, have been planted chiefly in the Northeastern States. California has its plantations of eucalyptus from Australia. In the prairie-plains shelterbelts, several exotic trees, such as the following, have been successful: Siberian elm, Chinese elm, Russian-olive, Russian mulberry, Austrian pine, white willow, and ailanthus.

In this article 50 species of the commonest and most popular trees from foreign lands are described briefly and illustrated, primarily for their identification. Though emphasis has been given to the trees widely planted for shade and ornament in temperate regions of the United States, about a fourth of the species selected are tropical trees restricted to the subtropical regions along the southern border from Florida to California.

Several are no longer widely recommended, because of objectionable features or susceptibility to disease or insects or because some better kinds are available, though they may be suitable for special purposes. Nevertheless, they have been planted so frequently that they merit inclusion in a list used for identification purposes. Nearly half of these 50 species have already become naturalized and grow in their adopted home as if native.

The description of each tree species contains the approved common and scientific names as well as other names frequently used. Size is stated as large (more than 70 feet tall), or medium-sized (30 to 70 feet tall), or small ( less than 30 feet tall). Leading characteristics useful in identification, such as form of the tree, bark, leaves, flowers, and fruits, are briefly described in nontechnical terms. However, some horticultural varieties with unusual or extreme characteristics may differ from the general descriptions. Though many trees reveal their geographic origin in their names, the native home is stated along with the regions in the United States where the species is grown.

Notes on special uses and desirable qualities as well as objectionable points are included. The drawings of leaves and fruits by Leta Hughey, botanical artist, Forest Service, will aid in naming trees or specimens.

The degree of hardiness of introduced trees and shrubs to cold weather in winter is expressed by division of the United States into hardiness zones, climatic zones based upon average annual minimum temperatures (U. S. D. A., Atlas of American Agriculture, Climate, page 9, figure 1928). Alfred Render (Manual of Cultivated Trees and Shrubs, edition 1, 1927; edition 2, 1940), Donald Wyman (Hedges, Screens, and Windbreaks, 1938), and other authors on horticultural subjects have adopted these hardiness zones, citing the northernmost zone where each species can be grown.

On the next page is a revised map of hardiness zones of the United States, based upon the map of average annual minimum temperature for the 40-year period from 1899 to 1938 (U. S. D. A.. Climate and Man, Yearbook of Agriculture, page 707. 1941).

Ten zones for North America have been designated by number, beginning with zone 1, with average annual minimum temperature exceeding 50 F. and representing the treeless zone of northern Canada and Alaska. The nine zones of the United States with their limits of average annual minimum temperature (Fahrenheit) are. from north to south: Zone 2, 50 to 35 ; zone 3, 35 to 20 ; zone 4, 20 to 10 ; zone 5, 10 to 5 ; zone 6, 5 to 5 ; zone 7, 5 to 10 ; zone 8, 10 to 20 ; zone 9, 20 to 30 ; zone 10, above 30 . In mountainous regions of western United States the zones become crowded into narrow bands according to altitude. To conform to the base map, the narrow zone 6 is omitted in the West, where zone 5 extends from 10 to 0 and zone 7 from 0 to 10 .

As factors other than latitude affect the coldest temperature in winter, the hardiness zones do not extend across the United States from east to west uniformly in parallel strips. The zones curve southward in the interior, where extremes of temperature are great, and shift to the south also in mountainous regions, where higher altitudes have a cooling effect. Zone 2, the coldest, is restricted to the Canadian border region from Wisconsin to Montana. Along the coasts the southern zones project farthest north, owing to the moderating influence of the oceans and ocean currents.