Levels of Genetic Variation in Trees: Influence of life history characteristics

In a previous study, levels of genetic variation, as measured by isozyme analyses, were compared for 113 taxa of vascular plants. Each species was classified for 12 life history and ecological traits and three measures of genetic variation were calculated. Plants with large ranges, high fecundities, an outcrossing mode of reproduction, wind pollination, a long generation time, and from habitats representing later stages of succession tended to have more isozyme variation than species with other combinations of characteristics. This paper discusses the results of the previous study and examines the available isozyme data for similar trends in forest trees. Special consideration was given to differences in genetic variation among 20 conifer species that have many of their life history characteristics in common. Successional stage, habitat type, cone type and historical events were associated with differences in genetic variation among the conifer species. These results are discussed in terms of expectations from current population genetics theory. F orest trees have been the subject of many quantitative genetic investigations (Libby and others 1969, Stern and Roche 1974). These studies have concentrated on morphometric and physiological charac­ teristics such as survival, growth initiation, height and diameter growth, hardiness to environmental stress, and various leaf, stem, fruit, and wood characteristics. As a result, the distribution of quantitative genetic variation is better understood in certain tree species than in most other naturally occurring plants. Studies that use biochemical techniques to measure genetic variation in forest trees generally have lagged behind those that use quantitative traits. A few workers have used secondary plant compounds, such as terpenes, in studies of species (Zavarin and Snajberk 1969, Zavarin and others 1969), racial (Smith and others 1969) and population (Adams 1975a, 1975b) differentiation. But, because the genetic basis of variation in these compounds is poorly understood, they are of limited use in population genetic studies. Since the early 1970's, electrophoretic techniques have been used in genetic studies of forest tree populations. These techniques offer a number of advantages over other biochemical or quantitative approaches: (a) genetic inheritance of electrophoreticaIly­ detectable traits can be easily demonstrated; (b) most isozyme loci are codominant and gene frequencies can be calculated without the necessity of genetic crosses; (c) estimates of genetic variation can be compared directly between populations or between species. The relatively few isozyme studies of plant populations have been the subject of a number of recent reviews (Gottlieb 1977, Brown 1979, Presented at the Symposium on Isozymes of North American Forest Trees and Forest Insects, July 27, 1979, Berkeley, Calif. Associate Professor of Biology, University of Kansas, Lawrence, Kans. Associate Professor of Biology, University of Colorado, Boulder, Colo. Hamrick 1979, Hamrick and others 1979). Generally, these reviews have concluded that plants contain somewhat more variation than invertebrate animals and considerably more variation than most vertebrates. Furthermore, different plant species contain varying amounts of genetic variation. Trees, for example, have been found to contain significantly more variation than herbaceous plants (Hamrick 1979, Hamrick and others 1979). This paper reviews the results of a previous study (Hamrick and others 1979), which compared the isozyme variation of plant species with different combinations of life history traits and examines the available isozyme data for similar trends in forest trees. Specifically, we address the following questions: • Do plant species with certain combinations of life history traits contain higher levels of genetic variation? • Does being a woody plant directly affect levels of isozyme variation? • Can differences in genetic variation among tree species be explained by their life history and ecological characteristics?