Evolutionary History and Genetic Conservation of Fraser Fir (Abies fraseri [Pursh] Poir.)

Abstract

Fraser fir (Abies fraseri [Pursh] Poir.) is a glacial relict species endemic to high peaks in the Southern Appalachians. A conifer with considerable ecological and economic importance, it has been devastated by the infestation of the balsam woolly adelgid (Adelges piceae Ratz.), an exotic insect from Europe. Fraser fir is a potentially informative and interesting model species for the study of forest tree population genetics and ecology because of (1) the fragmented nature of its populations, (2) the threat to the continued existence of its natural stands from the adelgid and from global climate change, and (3) its unclear relationship with other North American Abies species. These characteristics are also compelling reasons for conserving its genetic composition.

I developed a series of stage-structured population matrix models for Fraser fir to simulate genetic dynamics in a long-lived forest tree species with overlapping generations. The results suggest that Fraser fir populations are large enough and its life cycle is long enough to avoid significant genetic drift, absent repeated adelgid infestations. The model results indicate that other forces, including natural selection and inter-population pollen exchange, are more likely to have influenced the genetic structure of the species.

I used microsatellite molecular markers to assess the genetic structure and genetic diversity of Fraser fir populations. This analysis found only slight differentiation among populations, but greater inbreeding and less observed heterozygosity than in other conifers. No genetic diversity measure was correlated with population size, suggesting that smaller populations did not suffer more extensively from detrimental genetic effects following post-Pleistocene fragmentation. While some gene flow may occur between populations in close proximity, the genetic architecture of the species is more likely a function of its post-glacial migratory history. Unexpectedly, some of the smallest Fraser fir populations were the most genetically diverse by some measures, and the largest and least isolated populations were among the least diverse.

Using the same microsatellite loci, I detected a relatively small amount of differentiation among Fraser fir, balsam fir (Abies balsamea [L.] Mill.), and intermediate fir (A. balsamea var. phanerolepis Fern.), suggesting that these taxa should be treated as varieties of the same species. Balsam fir appears to consist of three demes, suggesting the possibility of one large central fir refuge during the Pleistocene, with smaller refugia to the east and west. Fraser fir, with highly exserted and reflexed cone bracts, may represent an adaptive extreme of balsam fir that, during post-Pleistocene isolation, lost genetic contact with relatives lacking exserted bracts. The results also indicated the probable introgression of subalpine fir (A. lasiocarpa [Hook.] Nutt.) genes into balsam fir.

I conclude that in situ conservation of Fraser fir's genetic composition is currently adequate, but may be insufficient in the face of global climate change and repeated adelgid infestations. A concerted ex situ strategy is needed to thoroughly conserve the genetic diversity of the species. I developed such a strategy for Fraser fir with two objectives: (1) to preserve its natural population genetic diversity in case Fraser fir populations are extirpated or degraded, and (2) to conserve and make available Fraser fir genetic resources for the breeding of an economically important tree species. The ex situ gene conservation strategy has four central components: (1) a seed bank representing all the Fraser fir populations, (2) existing elements of tree breeding efforts, (3) conservation plantings, and (4) an archive of Fraser fir DNA.