GENETIC IMPLICATIONS OF ARBORETUM SEED EXCHANGE

Abstract

The exchange of seed among arboreta all over the world, desirable as it is from the standpoint of plant introduction and testing, as well as international cooperation, presents serious problems from a genetic point of view. Although persons engaged in tree improvement research are particularly concerned about this, it is also of fundamental importance to anyone responsible for the taxonomic accuracy of specimens in an arboretum. We must bear in mind that seed collected from an arboretum tree or other plant may not produce plants of the mother species, because the male parent may have been another species. Species hybrids are common among trees, and in an arboretum related species are frequently planted close together. Even if there are several specimens of a taxon in a planting, there is no assurance that the pollen has not come from another species, unless the seed resulted from artificial pollination. The problem is particularly serious in the case of collections from plants of infraspecific taxa, such as subspecies, varieties, forms, etc. These readily intercross. If there is only a single plant of the species, its seed, if viable, is either hybrid or the result of self-pollination. Self-pollination, although yielding plants of the same species as the mother, often results in retrogression through the phenotypic expression of hidden defects. Inbreeding depression may result in stunted or very slow growing plants. In most tree species, inbred plants are probably not specimens. It is also very likely that a very high percentage of self-pollinated seed will be sterile, yielding no plants as all. Thus, in many cases it is a waste of time to collect seed from a single specimen because the seed is either nearly all empty or, if not, yields only inbred plants. In spite of these fundamental biological facts, numerous arboreta continue to exchange seed indiscriminately, and many continue to publish seed lists. Such seed lists and exchanges are widely distributed by the various arboreta in many parts of the world. Moreover, most American arboreta engage in the practice of seed exchange, whether or not a list is published. Another associated problem has developed in recent years with the advent of the scientific selection and breeding of forest trees. This is the problem of a lack of information concerning the original, indigenous locality from which the seed or cuttings were obtained for arboretum trees. Studies of racial variation show that most tree species with a wide botanical range vary greatly in genetic constitution as a result of natural selection in different environments. Thus there is no such thing as a typical red for example. Red oak from Kansas is quite different from red oak from Ohio or Maine. These differences may be reflected by variation in tree form, leaf form, seed size and shape, growth rate, resistance to drought, insects and diseases, time of flushing, growth cessation, leaf coloration and leaf fall, etc. This complicates matters for most arboreta, which do not have space for a wide variety of ecotypes for each species. In some cases, a limited effort has been made to include samples of this diversity, as at the Michaux Quercetum, started by the Morris Arboretum in 1953, which includes numerous population samples of many American oaks, carefully identified'as to geographic origin.