The population genetics of two temperate rainforest trees, Lagarostrobos franklinii (Hook f.) Quinn (Huon pine), and Atherosperma moschatum Labill. (Sassafras)

Abstract

The population genetics of two temperate rainforest tree species endemic to south eastern Australia were studied. Both species are long-lived and members of ancient families. There are parallels between the two species even though one was a gymnosperm and the other an angiosperm. For example, both species reproduce both vegetatively and sexually. Lagarostrobos franklinii (Huon pine) (Podocarpaceae) is mostly dioecious and wind pollinated, while Atherosperma moschatum (sassafras) (Monimiaceae), is monoecious or dioecious and insect pollinated. Both have potential for long distance seed dispersal, L.franklinii by water and A. moschatum by wind. The population genetics of both species was studied from stands throughout their geographic range using isozyme analysis. Most genetic diversity was found within rather than among sites. Genetic diversity among sites was low but generally consistent with expectations for each species (Hamrick and Godt 1979). Atherosperma moschatum had much more diversity among sites than Huon pine, with its mainland sites differentiating significantly from its Tasmanian ones. In Huon pine, most differentiation was found in isolated sites. Diversity within sites was also low in Huon pine but was much greater in sassafras. The structure of genotypes within stands was examined using spatial autocorrelation. In both species trees of like genotypes were found to be clustered at short distances. This genetic substructuring was found regardless of population size, density, distance from other stands, level of inbreeding, history, etc. Most sites deviated from Hardy-Weinberg expectations with deficiencies of heterozygotes, and high levels of allelic fixation, and were effectively inbred. The size structure and floristics within stands were investigated and used to assist in the interpretation of the patterns of genetic variation, inbreeding and stand dynamics found in each species. There was much variation in size structures and regeneration modes between sites in both species and neither appeared to require large scale disturbances for regeneration. The two species varied in the relationships between site environmental/ecological similarity and genetic similarity. In both species there was as much diversity in genetic variability and size structure in small isolated stands as there was in stands within larger assemblages. The proportion of trees contributing to the reproductive population, as well as the proportion of each gender type within that population, were estimated for Huon pine stands. On average thirty percent of Huon pine trees greater than one metre tall were reproductively active in the mast year recorded, and overall there were equal proportions of male and female trees. The relationships between reproduction and gender expression, with size structure, density, floristics, inbreeding and genotypes were investigated. Stands were also compared to identify if there were geographical or climatic trends in the distribution of these characteristics. Reproduction was found to increase with increasing tree size and also with more open canopies. Sites with similar proportions of females were found to also have similar species compositions. The distribution of reproductive trees and gender types within stands was investigated using spatial autocorrelation. The results were compared with genotypic distributions within the same stands. Although there was no direct correlation between gender type and genotype, both genotype and gender type were clustered at the same spatial scale, suggesting that such clustering may have a strong vegetative component. Huon pine seed production was estimated at one site and seed dispersal was investigated. Very large quantities of seed were shed. Seed dispersal laterally was negligible, but potential for dispersal down water courses was great as it stayed afloat for extended periods. Huon pine seed germination was investigated both in the field and under experimental conditions. Germination generally was slow, and with a low success rate. However seed in the field germinated at particular daylengths (regardless of temperature) in two consecutive seasons. Both species showed evidence that vegetative reproduction and localised pollen and seed dispersal have led to the development of family clusters, leading to inbreeding, and local fixation of allelic proportions. However infrequent long distance gene flow has probably reduced population differentiation. The population viability of each species was discussed