GENE DIVERSITY AND GENETIC STRUCTURE IN A NARROW ENDEMIC, TORREY PINE (PINUS TORREYANA PARRY EX CARR.).

Abstract

Recent reviews have suggested that tree species are the most variable of organisms, as measured by proportion of polymorphic loci or average heterozygosity (Hamrick, 1979; Hamrick et al., 1979). Average heterozygosity is greater than 0.30 for several conifers. By contrast, annual herbaceous species have a mean heterozygosity of 0.13 (Hamrick et al., 1979). Conifers have several mechanisms that promote outcrossing, and are expected to maintain high levels of genetic variation. Despite these mechanisms, it is uncertain whether the breeding system is capable of maintaining variability in small populations and in the absence of migration. While the genetic consequences of reduced population size have long been understood in theory, empirical evidence is scarce in plants, particularly in tree species. The potential to maintain high levels of genetic variation in reduced and scattered populations is important because of its implications for genetic resource conservation, and in fact, for the ability of species to respond to environmental change and avoid extinction. Because most conifers are commercially valued and exploited for lumber and paper products, much of the original forest in North America has been destroyed, and the tendency under management will be to reduce native populations to scattered relicts. One way of forecasting the fate of species reduced in numbers is to make use of natural experiments, by examining the genetic structure of species that occur in disjunct populations (Shaffer, 1981). We chose to investigate how much variation might be preserved in conifers by observing a narrow endemic, Torrey pine (Pinus torreyana Parry ex Carr.). The investigation also contributes to an understanding of the biogeography of the California Channel Islands. Torrey pine has the smallest population of any known pine. The 1973 count in the 445 ha (1,100 acre) Torrey Pines State Reserve on the Pacific Coast at San Diego, California was 3,401 mature trees (Calif. Dept. Parks Rec., 1975). Including seedlings, the Reserve's naturalist estimated ca. 7,000 individuals in 1979 (H. Nicol, pers. comm.). The population includes only two other small stands, contiguous with the Reserve. Another population occurs on Santa Rosa Island, one of the Northern Channel Islands off the California coast near Santa Barbara. There may be 2,000 individuals on the northeast coast of the island. Climatic, edaphic, and floristic characteristics of the two sites were summarized by Haller (1967). The San Diego and Santa Rosa Island populations are separated by 280 km and the island is 40 km from the mainland. It is highly unlikely that there has been any significant opportunity for gene exchange within recent centuries. Nor is Torrey pine likely to exchange genes with its closest relatives, digger pine (Pinus sabiniana Dougl.) and Coulter pine (Pinus coulteri D. Don), both of which are allopatric. Controlled crosses with Digger pine succeed only with difficulty and there have been no hybrids with Coulter pine despite several attempts (Critchfield, 1966).