Genetic Status of Bur Oak (Quercus macrocarpa) in New Brunswick

D.A. McPhee, J. Loo
Atlantic Foretry Centre - Canadian Forest Service

Bur oak as Isolated Peripheral Populations

In the province of New Brunswick, Canada, the range of bur oak has been reduced significantly from its historic distribution. The remaining stands are separated from the fringe of the present continuous range of the species by approximately 750 km, and are approximately 220 km from the next nearest population located in Maine. NB populations exist as a few small isolated stands, many of which are threatened. Conservation of peripheral populations may contribute to the protection of the evolutionary process and the systems that are likely to produce future adaptive potential. Outlying populations sometimes contain atypical genetic variation in response to harsher environments at the edges of the ecological range for the species. With climate change models predicting a temperature increase as high as 3 0C, plant species ranges could shift on the order of 500 km in this century. Northern outliers of a typical southern species, such as the bur oak in NB, are already adapted to the northern photoperiod. Given the continuing reduction in size and number of stands, there is cause for concern for the future of bur oak in the province. Populations are at risk due to the genetic consequences related to small, isolated, peripheral stands. Also, as a result of the genetic variation that might now exist in these isolated stands, they can be viewed as a genetic commodity.

Isozyme Study

In order to inform development of a conservation strategy to maintain bur oak on the NB landscape, a genetic diversity study was conducted using isozymes. The technique of starch gel electrophoresis, which identifies isozyme variation at putative loci, was used. Six NB stands were compared to three isolated stands in the New England states, four stands on the fringe of the species' contiguous range, and five within the current species range. The stands used in this study are separated by a minimum of 10 km and consist of at least 40 mature trees. Twelve enzyme systems with a total of 19 polymorphic loci were analyzed using bud tissue from up to 50 trees per stand. When stand size was less than 50, all mature trees were sampled.

Statistics

Genetic diversity was estimated as: the proportion of loci that are polymorphic (P), mean number of alleles per locus (A), mean effective number of alleles per locus (AE), mean observed heterozygosity (HO) and the mean expected heterozygosity (HE). Total genetic diversity (HT) of a population can be divided into two components: the

component due to variation within the subpopulations (HS), and the component due to variation between subpopulations (DST). The proportion of genetic diversity (GST) due to the among-population component was also calculated.

Results

The data collected from 19 isozyme loci indicate that although the NB populations are small and under threat, they still maintain high levels of genetic diversity similar to other populations within the species= contiguous native range. The average number of alleles was slightly higher for the populations within the contiguous range (2.47) compared to the isolated NB populations (2.26). However, the total effective numbers of alleles (1.595 and 1.545) were similar. Estimates of gene diversity among subpopulations (DST) were small across all loci, as were the proportions of the total variation explained by differences among populations (GST).

Discussion

The presence of the small, isolated, genetically diverse populations of bur oak at such a vast distance from its nearest neighboring population begs the question as to how they have come to exist in NB. Two scenarios have been proposed to explain their presence. The first lies in the distant past, with the rapid northern expansion of tree species in the post-glacial period. Pollen records clearly indicate that this rapid expansion of oak species ranges was followed by a subsequent contraction within northeastern North America due to changes in climatic conditions. This scenario places the bur oak of NB as remnants of the range expansion-contraction phenomenon. Genetically this scenario implies that although the species currently exists as only a few small stands, it may have once occurred in much greater numbers, and that the genetic consequences that act on small population have not yet affected the bur oak of NB. A second possible scenario is that North American First Nations peoples who were known to carry supplies of acorns for considerable distances as a source of food may have planted bur oak in N.B. The seed may have come from various locations in the species= contiguous range, resulting in considerable genetic diversity in the NB founder populations.

Given the relatively high levels of genetic diversity remaining in the NB populations, it appears that conservation efforts for this species stand a good chance of success. Stands should be protected where they exist to prevent further loss. Stands should be expanded where possible, and new ones established using NB seed.