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Tree introduction


JONATHAN WRIGHT is Associate Professor of Forestry Michigan State University, East Lansing, Michigan.

An extract from Genetic of forest tree improvement, to be published shortly by FAO

An exotic is strictly defined as an introduction from a foreign country. That narrow definition includes white spruce transferred a few miles from Ontario, Canada, to Michigan, United States but excludes western white pine transferred 2,000 miles from Idaho to Michigan. For the purposes of this chapter it is preferable to define an exotic or an introduced species as one which is grown outside the limits of its natural range.

Introduced species form the heart of agricultural production in all civilized countries and will therefore probably play a large role in forestry. This dependence of agriculture and horticulture upon introduced plants and animals is apparent if one examines the menu for an ordinary meal or one's clothing. Over 99 percent of the food and cloth (aside from modern synthetic fibers produced from wood and coal) used by a temperate-zone civilized man is prepared from plants or animals grown outside their native ranges. For example, if an American or north European were to subsist entirely on "native" foods, he would have to do without beef, mutton, cane or beet sugar, wheat, rye, barley, carrots, potatoes, maize, beans, or almost everything else included in his daily food.

Species introduction has already proved its value in Australia, New Zealand, South Africa, and the United Kingdom. In parts of these countries exotic plantations furnish the majority of the lumber cut. They do this because they fill a niche which the native species cannot fill. In some instances they produce four times as much lumber as the natives. In other instances they furnish valuable softwood lumber whereas the natives are hardwoods.

In well-timbered areas such as the United States and northern Europe there has been a prejudice against introduced species. Part of this prejudice is due to the poor performance of early, unplanned introductions. No wonder, because the "average" ecotype of the "average" foreign species stands little chance of success. A good selection program is necessary to choose the best type for a locality.

Another part of the prejudice against exotics is due to the prevailing belief that native types must be perfectly adapted to their habitats because of generations of natural selection. There are several reasons why this argument may not be valid. Modern, man-changed environments are not the same as those in which natural selection operated. There is always a lag between an environmental change and genetic adaptation to the new environment. No region, no matter how species-rich, has all the genes necessary for the evolution of a perfectly adapted type. Natural selection has favored types which survive, not the types which the forester values most.

Species introduction is best considered as a part of forest genetics for three reasons. First, the differences between species and ecotypes are a matter of degree. Therefore, tests of new species and new ecotypes are best organized on the same basis, as a part of a tree breeding program. Second, the choice of the proper geographic ecotype has much to do with the success of an introduced species in forestry. Third, most species hybrids involve at least one exotic species. Development of a new hybrid into a commercially promising variety for forest planting depends just as much on knowledge of the exotic species as on proper hybridization technique. Finally, the present generation of foresters is largely dependent upon tree breeders for most of their detailed knowledge concerning foreign species which might prove to be valuable as introduced trees.

Literature on exotics

The literature on exotics is best developed in the United Kingdom, Australia, New Zealand, and South Africa. For many years those countries have relied heavily on introduced species in their reforestation programs, and the majority of technical papers in their forestry journals (Forestry, Irish Forestry, Scottish Forestry, Australian Forestry, New Zealand Journal of Forestry, etc.) as well as technical bulletins released by their experiment stations deal with the silviculture or utilization of exotic species. Also, most of their annual reports have sections on "exotic forestry" containing statistics and details not applicable to native woodlands. In 1967, this literature was summarized in a series of reports prepared for the Seventh British Commonwealth Forestry Conference held in New Zealand and Australia now published as one of the volumes of Exotic forest trees in the British Commonwealth.

Rol and Pourtet's two-volume book is the best general source of information on exotic forest trees in France. It is a well-annotated catalogue of the forestry plantings at the Arboretum des Barres, near Nancy. Professor Pavari's book is as complete a survey of the use of exotics as has been prepared for any country. It contains summaries of exotic forest plantings and descriptions of single arboretum specimens for almost all species tested in Italy.

Aughanbaugh's annotated check list describing the many specimens represented in the arboretum of the Ohio Agricultural Experiment Station at Wooster is unique for the United States although partial lists have appeared for some of the arboreta. Recent papers by Harkness and Wright cover the growth and forestry possibilities of specific genera (Abies, Picea and Pinus) in the northeast.

In 1956, FAO published a monograph on Eucalyptus. That monograph contains descriptions of all the Eucalyptus species planted outside their natural ranges, together with voluminous data on climatic preferences, growth potentialities, uses, etc. It is by far the most comprehensive treatment of its sort available for any genus.

In several other countries the literature on exotics consists of numerous short papers about single species or about small regions.

Factors governing successful tree introduction

Performance and characteristics of exotics in their native habitats

The performance and characteristics of a species in its native habitat has provided the best clue as to the potential usefulness of that species in its new habitat.

Monterey pine (Pinus radiata D. Don) is the best example of this generalization. Its hardness, growth rate, stem form, genetic variability, and good quality are essentially the same in New Zealand, Australia, and South Africa as in the native stands in California and Baja California. Only in maximum height achieved 160 feet (48 meters) in New Zealand, 120 feet (36 meters) in native stands - was a new characteristic evident in the exotic southern hemisphere forests. If the Monterey peninsula type of climate were as widely distributed in California as in the Southern Hemisphere, this species would be as much appreciated in its homeland as south of the equator.

The native habitat/strange habitat correlation is equally true for wood and growth properties. Unfortunately, this has usually been overlooked until new introductions were nearly ready for harvest. Thus, there are large areas of Eucalyptus globulus Labill. E. saligna Sm., Larix decidua Mill., and Tsuga heterophylla (Raf.) Sarg. which are not producing high quality wood. They are now being harvested at small profit and being replaced by better species. This fact could have been foretold by a letter to the wood anatomist in the country of origin.

Similarly, a promising species such as Ginkgo biloba L. is overlooked because the reports of its excellent wood quality are hidden in Chinese literature. Its excellent wood quality (similar to a white pine) shows up in the United States as well as in China.

To summarize, acquaintance with a species in its native habitat is one of the best guides to a successful introduction.

Economic importance as a native species

The potential importance of a species as an exotic is not closely related to its economic importance in its native range. European beech (Fagus sylvatica L.) is an important hardwood in northern Europe because it is one of the few hardwoods present. It grows well in the United States but would hardly become important in the next 50 years because of planting difficulties, the excess of Fagus grandifolia Ehrh. forests, and the large number of more desirable hardwood species. Red pine (Pinus resinosa Ait.) is the most important pine in large parts of the northern United States and southern Canada. However, it has been biologically unsuccessful in foreign countries.

Pinus radiata D. Don, Larix leptolepis (Sieb. and Zucc.) Gord., and Larix decidua Mill. have such small or such inaccessible natural ranges that they are less important in their homelands than when planted abroad.

On the other hand Pinus sylvestris L., P. strobus L., P. ponderosa Laws., Pseudotsuga menziesii (Mirb.) Franco, and Eucalyptus camaldulensis Dehn. are important at home and abroad.

Size of natural range

The size of a species' natural range is not strongly correlated with its potential usefulness as an exotic. In Japan, for example, the climate suitable for Larix leptolepis (Sieb. and Zucc.) Gord. is found only on a few isolated mountains. The species does well on large areas of flatlands in northcentral Europe and the northeastern United States.

Similarity of climates in regions of origin and of use

Minimum winter temperature is probably the most important single factor ]limiting successful interchange of trees between regions. It is the basis for Rehder's classification of trees by potential use zones in the United States.

Monterey pine (P. radiata D. Don) is subjected to minimum winter temperatures of about 20º F. in its native range and is considered as a promising forest tree only where the minimum winter temperature is that high or higher. The importance of low winter temperature is also illustrated by the Eucalyptus species, whose relative hardiness as exotics can be forecast from minimum temperatures reached in native stands. Almost all trees from southern or Pacific Coast parts of the United States fail to survive severe winters encountered in the eastern United States.

Damage from growing season frost has been the apparent limiting factor when moving northern species southward. Larix sibirica Ledeb., Abies sibirica Ledeb. and Abies balsamea (L.) Mill. are northern trees which can withstand severe winter cold. Yet, they regularly show spring frost damage when tested in England, France, and on the middle Atlantic coast of the United States. Similarly, many northern U.S. species suffer frost damage when tested along the relatively mild Gulf (Coast.

Low summer temperatures usually favor growth because of the favorable influence on moisture relations. Nearly all eastern United States species are subjected to much warmer summers in their native habitats than in areas of northern Europe, Australia and New Zealand where they thrive.

Warmer-than-normal summer temperatures may be limiting factors responsible for the relatively poor showing of species from northern Europe and the northern United States tested in southern Europe and the southern United States. The evidence on this point is not yet clear as many adverse reports on northern species were certainly due to the fact that the northern species were tested in comparison with inherently more productive southern types.

Low total annual precipitation has been a limiting factor in a number of instances. There are relatively few cases of successful transplantation from a moister to a drier region unless irrigation was practiced.

Seasonal distribution patterns govern the planting of many species in South Africa. Pinus radiata D. Don and P. pinaster Ait., both native to winter-rainfall areas. have been used in the winter-rainfall areas on the west coast. Pinus palustris Mill., P. caribaea Morelet, P. elliotii Engelm, P. taeda L., P. patula Schlecht. and Cham., and other species from summer-rainfall areas of the southern United States and Mexico have been used mostly in the summer-rainfall areas of the east coast. In part the lack of adaptability of winter-rainfall species to summer-rainfall areas seems to be a disease problem. The failure of P. pinaster and P. radiata in summer rainfall areas has frequently been associated with diseases which are favored by hail damage or high humidity.

There has been relatively little opportunity to determine whether too much precipitation has been a serious deterrent to enforced tree migration. Many arid-land species when tested in moist parts of the eastern United States have grown as well as in their native habitats but are still not useful because faster growing species are available. Similarly, the native forests of truly high-rainfall areas, such as the Olympic peninsula of Washington in the United States, parts of Queensland, Australia and parts of New Zealand, are already so productive that there has been little concern with plant introduction.

The excellent performance of the poplars and eucalypts grown under irrigation in near-desert conditions in Spain and the Near East indicates that low relative humidity has rarely been a limiting factor. Similarly, variations in daylength between the native habitat and the country of planting have probably been of minor importance. Most of the north Europe/eastern United States transfers involve changes from about 50º N. to about 40º N. and Japan/eastern United States transfers usually involved a shift from about 35º N. to about 40º N.

The general relationship between the climates of the region of origin and of successful growth is such that one should make an approximate but not identical match between the two when planning new introductions.

Differences in plasticity

Trees vary in their capacity to thrive under conditions different from those prevailing in their natural habitats. Some, such as Acer saccharum Marsh., A. rubrum L., Picea breweriana S. Wats., P. engelmannii Parry, Pinus resinosa Ait., and Quercus alba L., grow to large sizes within their native ranges, but show no real promise in the foreign countries in which they have been planted. Others, such as Acer platancides L., Picea omorika (Pancic) Purkyne, P. pungens Engelm., Pinus strobus, L., P. sylvestris L., and Quercus rubra L. have grown as well when planted abroad as in their native habitats. The latter are termed plastic species.

There is no good general explanation for differences in plasticity. There are no general morphological or physiological characters which permit identification of the plastic species prior to actual testing. Both the plastic and nonplastic groups include species with small and large ranges, slow and fast growth rates, small and large sizes.

Most forest regions include plastic and nonplastic species. Northern Europe is an exception. It contains only adaptable species which grow well under a greater-than-expected range of conditions. This is probably a consequence of the extreme depauperization of the flora during the Pleistocene and the subsequent rapid recolonization by species which could survive under varied conditions. The modern forest flora consists of a relatively few species: Acer campestre L., A. platanoides L., A. pseudoplatanus L., Alnus incana Moench., A. glutinosa (L.) Gaertn., Betula pendula Roth, B. pubescens Ehrh, Fagus sylvatica L., Fraxinus excelsior L., Picea abies (L.) Karst, Pinus sylvestris L., Populus alba L., P. nigra L., P. tremula L., Sorbus aucuparia L., Taxus baccata L., Ulmus glabra Huds., U. procera Salisb. All these have been among the most successful introductions in the northern United States and other cool temperate regions. Evidently their rapid repopulation of northern Europe after the retreat of the glaciers was not an accident.

There are limited soil-preference data for the most commonly planted exotics in the United Kingdom, Germany, New Zealand, South Africa, and parts of Australia where exotics have been planted on a large scale for many years. The data are based upon immature plantations and are less exact than for well-studied native species. In general, they are sufficiently precise as to show that a given species is best adapted to one of about three drainage, texture or fertility classes. Drainage is usually the most important. There is generally a good natural land/foreign land correlation as regards soil preference of a species. In parts of Australia, addition of phosphorus has increased the growth of Pinus radiata D. Don. In parts of Great Britain, addition of nitrogen has increased the growth of Picea sitchensis (Borg.) Carr.

In France, Italy, Japan, the United States and other regions where the majority of the exotic testing work is still in the arboretum- or pilot plantation-stage there have been few opportunities to observe growth under varied site conditions. This is even true of such a commonly planted species as Pinus sylvestris L. in the United States, because almost all older plantations are of unknown provenance and the poor performance of any particular planting must be regarded as due to a combination of site and genotype.

Size of genus

The special value of monotypic genera in tree introduction work was recognized by J. R. Schramm when he was director of the Morris Arboretum of the University of Pennsylvania, Dr. Schramm reasoned that monotypes should be free of pests because they have no close relatives from which pests could transfer easily and because most insects and diseases are limited to small groups of related species. His reasoning is borne out in practice, and the monotypes as a group are remarkably free of serious pests wherever they are planted. The author is personally acquainted with seven of them - Cercidi phyllum japonicum Sieb. and Zucc., Ginkgo biloba L., Maclura pomifera (Raf.) Schneid., Metasequoia glytostrobus Hu and Cheng., Pseudolarix amabilis (Nels.) Rehd., Sciadopitys verticillata Sieb. and Zucc., and Thuyopsis dolobrata Sieb. and Zucc.-which have been used as ornamentals in Pennsylvania and southern Michigan. In each of these the freedom from pests extends even to minor leaf-feeding insects which cause the leaves of most trees to be full of holes by late autumn.

The quality of being monotypic does not insure that a species has exceptional hardiness, growth form, or wood quality.

Possibilities as a hybrid parent

Many species deserve introduction because they can be used as parents of hybrids. In Korea, for example, the American Pinus rigida Mill. and P. taeda L. are being planted on a relatively small scale as pure species. Yet their hybrids are being produced by the thousand and will ultimately play a very important role in Korean forestry.

In the southern hemisphere Pinus radiata D. Don far overshadows its relative P. attenuata Lemm. as a useful forest tree. The latter would not warrant further testing if it could be used only as a pure species. However, the hybrids are sufficiently promising that a large-scale provenance testing program of P. attenuata is easily justifiable.

In the midwestern United States P. sylvestris L. and P. nigra L. are-important exotic forest tree species in their own right. Their Asiatic relatives (P. thunbergii Parl., P. densiflora Sieb. and Zucc., P. yunnanensis Franchet, etc.) have not shown sufficient promise to be planted as pure species. But some Eurasian hybrids are so promising that none of the Asiatic members of the series Sylvestres can be neglected.

Genetic variability within species

Almost any wide-ranging species contains enough genetic variability that the performance of a single biotype in a foreign country is a very rough indication of a species' potential in that country. This point is illustrated best by reference to P. sylvestris L. in the United States. By mischance most of the northeastern plantings of this species were from a particularly illsuited group of provenances from Germany. The resulting thousands of acres are almost worthless, and the species has a poor reputation. Excellent provenances are available. If they had been used, the species would enjoy an excellent reputation.

Ninety-five percent of present knowledge of the performance of exotic species is derived from trees of unknown geographic origin. Because the variability within species is so great, any single tree or single plantation which performs 75 percent as well as a native type should be taken as evidence that the species concerned is capable of out-producing good natives.

Exchange relationships among regions

The major regional exchange relationships for forest trees are summarized in Table 1. In some cases there has been a mutual exchange of valuable trees, as between the various countries with Mediterranean-type climates. In other cases the exchange has been onesided, as between northern Europe and the Pacific northwest.

The boreal forests of northern Eurasia and Canada have contributed little to other regions because of the paucity of their flora and the great reserves of native forests in the countries for which they might act as donors. The large tropical and subtropical regions of the world have large numbers of potentially useful species but their donor possibilities have not been explored. The donor possibilities of the Mexican highlands and the mountainous areas of western China seem great but comparatively little known.

Suggested plan for introducing and testing exotics

The factors discussed in the previous sections can be summarized as a series of rules.

1. Study the climates of the world and select regions with climates similar to the region in which you live. The climatic matches need not be exact because some factors compensate for small deficiencies in others.
2. Study the intrinsic growth and wood properties of the tree species in those regions, and plan to introduce and test all those with desirable properties. The list of trees to be tested will contain 50 to 100 species for cool regions, such as Scandinavia or eastern Canada; 500 to 1,000 species for warm temperate regions, such as the eastern United States and central Europe; more than 1,000 species for humid tropical regions.
3. Study the monotypic genera because of their probable freedom from pests and the north European species because of their probable adaptability to a variety of site conditions.
4. Include close relatives of the most promising species to prepare for future hybridization.
5. Study the site preferences of the species to be introduced and plan the tests accordingly.
6. Introduce from 3 (small-range species) to 20 (large range species) provenances of each species. Follow up with larger provenance tests of promising species. Test only known-origin material.
7. Test the new species under three or four different conditions.
8. Progress from small-plot plantings which test possibilities of single trees to large-plot plantings which test stand performance.


Recipient region

Donor regions



United States (northeast)

China (west)

Japan (Hokkaido)

Europe (north)


India (north)


Japan (Honshu)


Mediterranean basin (mountains)

U.S.A. (southeast and southwest)

Rocky Mountains

U.S.A. (Pacific coast)


Canada (Pacific coast)

United Kingdom

Europe (north and west)

Asia (north)

Japan (mountains of Honshu)


Mediterranean basin

Himalayan Mountains

U.S.A. (Pacific coast)

Japan (lowlands)

Canada (Pacific coast)

Mediterranean basin



France (south)

China (west)

Europe (north)

Germany (south)

Europe (Central [mountains])



Japan (mountains of Honshu)





U S A (mountains of west)


U.S.A. (east, north)




China (west)

Italy (south)

Mediterranean basin

Europe (north)

New Zealand

Mexico (mountains)

Japan (mountains of Honshu)

South Africa

U.S.A. (California)

U.S.A. (northeast)

United States (California)

" (southeast)


East Africa



Mediterranean (lowlands)




U.S.A. (California lowlands)


A program such as outlined here would require decades to accomplish if the hit-or-miss methods of the past were used. However, it could be accomplished in a reasonable length of time if well planned and if new developments in experimental design were used.

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