With the increasing demand for more traditional wood products, including paper and fuelwood, there is a need and considerable opportunity in some parts of the United States to grow and utilize fast-growing, short-rotation woody trees. Various species and hybrids of poplars (Populus spp.) and several other native hardwoods have played this role. However, poplars are highly specific as to their site and growing requirements which restrict their potential use to selected areas. The same is true for the other hardwoods, such as American sycamore (Platanus occidentalis L.) and green ash (Fraxinus pennsylvanica Marsh.). Since the late 1960s, a number of additional woody species has been selected and tested under accelerated growing conditions. For this discussion, I will restrict my comments to two genera, Eucalyptus (Eucalyptus L.'Herit) and Royal paulownia (Paulownia Sieb. and Zucc.). Both are exotics, having been introduced in the United States during the 1800s. Both are fast growing hardwoods with many useful qualities. The full potential of these genera are not fully understood nor appreciated, yet there is considerable information that suggests that with a focused effort, both genera would make a useful contribution.
Eucalypts are restricted in range because all members of this genus are susceptible to low temperatures. Yet, with careful selection of species and proper seed source, there are genotypes that can be of considerable value and use. Paulownia has had relatively poor acceptance because it is considered difficult to grow and often has poor form. There is virtually no industrial experience with this genus in the United States because of relatively low volumes now available. Yet in Asia, especially in Japan and the People's Republic of China, it is widely used in fine furniture. It is widely grown in its native land of China for its timber value, and as a windbreak tree.
Early introductions were essentially random. Although a number of plantings took place in California during the late 1800s, most of the species and seed sources were eventually found to be unsuitable for timber products. Still, the various species found a use as a windbreak tree or as an ornamental. By the 1960s, more than 150 species were being grown in California alone as ornamentals because of their decorative flowers and pleasing form and shape.
A Eucalypts species introduction and testing center was established in south Florida in 1899 by what is now the USDA Forest Service. The goal of this center was to identify an array of new, fast-growing hardwood species. A few years later, a similar center was established in southern California. Both centers lasted approximately 10 years; but sudden low winter temperatures in both California and the southern United States abruptly ended the wide use of Eucalyptus at that rime. Since then, a large series of new species introductions have been carried out, usually with limited success. The collections were often poorly identified and seed source sampling was inadequate.
Investigations of the last 25 years have clearly demonstrated considerable genetic variation within this genus. Although no truly cold-hardy species have been identified, it is still possible to select for a degree of low temperature tolerance in certain species and seed sources. In field studies in California, a number of high-elevation seed sources from Australia withstood, in 1973, a cold wave occurring in late December and January when the temperatures went below freezing on 10 consecutive nights with a low of -7°C. In Florida, selected seed sources have tolerated temperatures as low as -12°C. With the proper genetic and management studies, it should be possible and feasible to match species and seed source to the appropriate growing conditions. In recent years, this has led to reports of fiber yields of 15 to 25 dry tons per hectare. For example, Eucalyptus amplifolia Naud. has been coppiced cultured through four rotations in northern Florida with annual yields reaching 23 dry tons per hectare. Somewhat similar high yields have been reported for Eucalyptus saligna Sm. in Hawaii. In each case, the key was the careful identification and selection of genetic material (species and seed sources) appropriate for given site conditions and the application of compatible management techniques. It is apparent from a number of current field tests that the full potential of this genus has not been exploited and many more seed source (provenance) investigations need to be initiated within a narrow range of growing conditions. While it is very unlikely that true cold-hardy Eucalypts will be found, there are many situations in which this genus will be useful. At this time, only approximately half of the species have been tested to any degree in the United States. There are some subgroups of the Eucalypts in which virtually little is known, even in Australia where it is native. What is needed is a better coordinated effort and a better understanding of the considerable variation in this genus.
Many report this species as a weed tree for its poor form and rather large shade leaves. This is not surprising since there has been very little genetic selection of seed source for its adaptability to given sites, nor selection for form or growth rate. In the last 15 years, studies have been initiated to select from Paulownia tomentosa populations within the United States for improved growth and form. Still, current efforts to produce Paulownia spp. commercially have not been completely successful. In addition to a lack of appropriate plant material (seeds), optimum management techniques have not been applied.
While reviewing forest genetics activities in the People's Republic of China, both natural and planted stands of Paulownia spp. were studied. Currently, seven species and six varieties are commonly recognized. However, the Chinese list nine species of Paulownia. Many of the species tend to be subtropical and are not suitable for the continental United States. In 1986, four provenances each of Paulownia elongate (S.Y. Hu), P. fargessii (Franch), P. fortunei (Seem) Hemsl. and P. tomentosa collected in China were planted near Birmingham, Alabama; Tillman, South Carolina; Morganton, North Carolina; and the Garrett-Allegheny County area of Maryland. These species and provenances were selected as being good candidates for the southern and southeastern United States. On the whole, seedling survival after 2 years has been good with height growth ranging from 0.5 to 8 meters. It is too early to make any conclusion but it does appear that other Paulownia species can be grown in the United States. At 5 years, we will have an opportunity to evaluate form as well as height growth. As a comparison, P. elongate in a park in Peking had a height of 10.6 meters at age 6 while a roadside tree of P. fortunei near Kweilin, Kuangsi, had a height of 20.8 meters at age 11.
From the experience in China, Paulownia spp. prefer warm climates with 24°C to 30°C being the most favorable. P. tomentosa can withstand temperatures as low as -20°C, and P. elongata -15°C. Both P. fortunei and P. fargessii appear to be less cold hardy than P. elongate. Cold hardiness will need to be further determined under a number of growing conditions. Additional studies will be needed to determine appropriate management techniques for this genus. Yet, its rapid growth, attractive flower, and excellent wood quality make it a genus that needs to be considered for further use in the United States.