Genetics and the Physiological Ecology of Conifers

Jeffry B. Mitton

Publisher Summary

This chapter focuses on the genetic variability associated with the physiological differences among genotypes in Rocky Mountain conifers. It illustrates genetically based differences in physiology by considering variation among genotypes in survival, growth, and resistance to herbivores and presents the mechanistic studies needed to understand the relationships between genetic and physiological variation. In addition to the marked elevation distribution of conifer population size in mountainous regions—especially in the Rocky Mountains—many conifers have other life-history traits and ecological circumstances that promote high levels of genetic variation. Data collected from numerous provenance studies and allozyme surveys confirm this phenomenon. Provenance surveys show that Douglas fir, ponderosa pine, and lodgepole pine populations in the Rocky Mountains differ by 300 m in elevation and likely exhibit adaptation to different environments, while allozyme studies in conifers reveal populations with high levels of variation within populations and only slight variation among populations. Molecular techniques being employed these days help examine variation in the nuclear, mitochondrial, and chloroplast genomes. Provenance studies reveal adaptation to local environments in most of the Rocky Mountain conifers but don’t reveal the localization of the genes that produce morphological, physiological, and phenological responses to environmental gradients in conifer populations. Examination of the physiological and demographic consequences of allozyme loci and variation of nuclear, mitochondrial, and chloroplast DNA help answer whether mitochondrial and chloroplast genomes contribute to the adaptive variations in conifers.

Natural selection acts on the diversity of genotypes, adapting populations to their specific environments and driving evolution in response to changes in climate. Genetically based differences in physiology and demography adapt species to alternate environments and produce, along with historical accidents, the present distribution of species. The sorting of conifer species by elevation is so marked that conifers help to define plant communities arranged in elevational bands in the Rocky Mountains (Marr, 1961). For these reasons, a genetic perspective is necessary to appreciate the evolution of ecophysiological patterns in the coniferous forests of the Rocky Mountains.

The fascinating natural history and the economic importance of western conifers have stimulated numerous studies of their ecology, ecological genetics, and geographic variation. These studies yield some generalizations, and present some puzzling contradictions. This chapter focuses on the genetic variability associated with the physiological differences among genotypes in Rocky Mountain conifers. Variation among genotypes in survival, growth, and resistance to herbivores is used to illustrate genetically based differences in physiology, and to suggest the mechanistic studies needed to understand the relationships between genetic and physiological variation.

I Conifers Have High Levels of Genetic Variation

Evolutionary theory predicts that species that have large population sizes and broad geographic ranges will have high levels of genetic variation. This prediction is based on several relationships. First of all, the number of new mutations arising each generation increases with population size. In addition, the rate of genetic drift is inversely proportional to population size, so that more mutations are expected to accumulate in large populations. Jointly, these relationships predict that genetic variation will increase with population size. For example, the expected number of alleles at a locus k is k = 4Nu + 1, where N is the population size and u is the neutral mutation rate (Hard and Clark, 1989). Similarly, the predicted heterozygosity, H, also increases with population size:

=4Nu/(4Nu+1).

In addition to population size, many conifers have other life-history traits and ecological circumstances that promote high levels of genetic variation. For example, many species of conifers, such as ponderosa pine (Pinus ponderosa), lodgepole pine (Pinus contorta), and Douglas fir (Pseudotsuga menziesii), have immense geographical ranges that place them in a wide variety of natural environments. The degree of environmental variation experienced by a species is expected to increase with the geographic range of the species, and the selection among heterogeneous environments is expected to enhance genetic variation (Hedrick et al., 1976; Hedrick, 1986; Gillespie and Turelli, 1989). We can easily imagine that the Douglas fir growing in sympatry with pinyon pine (Pinus edulis) in the Chiricahua Mountains of southern Arizona will be selected to have traits very different from those favored in the Douglas fir growing in sympatry with Sitka spruce (Picea sitchensis) in the temperate rain forest of the Olympic Peninsula.

Forest biologists detect genetic variation with diverse methods. Trees planted as seeds into common gardens in provenance studies yield estimates of heritability for virtually any character that can be measured. Heritability is the proportion of total phenotypic variation attributable to genetic variation. By planting seeds from diverse sources into a common garden, the environmental differences among collection sites are eliminated, and therefore apparent differences can be attributed to genetic differences among collection localities. The use of several common gardens in contrasting environments allows inferences of how genotypes respond to a diversity of environmental conditions. Although common garden studies reveal differences among genotypes and patterns of performance along environmental gradients, individual genes are not identified. Specific genes or sequences of DNA from seeds and needles collected in the field can be examined with electrophoretic surveys of proteins and with a variety of molecular techniques. Surveys of electrophoretically detectable genetic variation of proteins, or allozyme variation, have been used to measure the genetic variation and describe the geographic variation of many species of conifers (Hamrick et al., 1979; Hamrick and Godt, 1990; Loveless and Hamrick, 1984; Mitton, 1983). The DNA from the nucleus, from mitochondria, and from chloroplasts can be sequenced, or examined for restriction fragment length polymorphisms (RFLPs) (Wagner, 1992a,b), including examination of the variable number of tandem repeats (VNTRs), also popularly referred to as DNA fingerprints.

A Provenance Studies

Provenance studies of many species of conifers have identified substantial proportions of genetic variation for characters such as germinability, growth, disease resistance, times of bud break and bud set, susceptibility to frost damage, and growth form (Wright, 1976). One indication of the magnitude of variation among individuals is the range of values for maximum photosynthetic capacity, gleaned from the review of Ceulemans and Saugier (1991) (Table I). Clear differences distinguish species, but these data illustrate that there is substantial variation within species as well. In both sweet chestnut and black cottonwood, the maximum photosynthetic capacity varies by a factor of two among individuals.

Table I

Range of Average Photosynthetic Capacities for a Variety of Treesa

aPhotosynthetic capacity values refer to maximum CO2 exchange rates at saturating light, 20–25°C, and 330 ppm CO2.From Ceulemans and Saugier (1991).

In addition to demonstrating substantial proportions of genetic variation underlying phenotypic variation, provenance studies have revealed geographic variation of the genetically determined...