Variation in genetic structure and gene flow across the range of Sequoiadendron giganteum (giant sequoia)

During this century, climate warming and altered precipitation patterns will lead to habitat changes that may be beneficial to some long-lived tree species and detrimental to others. Paleoendemics, with limited and disjunct distributions will face the greatest challenges, as migration rates will be too slow to keep pace with rapid environmental change and populations at the receding edges are eroded through mal-adaption. Giant sequoia (Sequoiadendron giganteum (Lindl.) Buchholz) is an iconic Sierra Nevada tree species with populations that tend to be small and highly fragmented (particularly in the northern range), making them especially vulnerable to environmental change. Maintenance of genetic variation is an important determinant of population persistence that, in part, depends on gene flow within and between populations. The research presented here describes: 1) the distribution of genetic diversity among population pairs distributed across the range of giant sequoia, and 2) the effective rates of gene flow across a highly fragmented habitat. In 2015 and 2016, DNA was extracted from foliage collected from eight groves distributed across the range of giant sequoia, with the exception of Mariposa grove (leaf tissue from this grove originated from a clonal orchard at the University of California’s Russell Research Station). For Placer rove, DNA was obtained from foliage of all trees and from seeds after germination. We used 11 microsatellite loci (DeSilva and Dodd 2014) to compare allelic diversity between population pairs, using the software STRUCTURE (Pritchard et al. 2000). We also used STRUCTURE to infer levels of historic gene-flow through assessing the degree of admixture between spatially separated population pairs. We inferred dispersal distances and size of effective mating groups within groves using SPAGeDI (Hardy and Vekemans 2002). We found a prominent north-south divide at the Kings Canyon in spatial connectivity across S. giganteum groves. For the northern populations pairs, assignments of individuals to genetic clusters closely follow the grove of origin, indicating strong genetic structure (fig. 1). Rates of admixture (individuals showing evidence of shared genotypes from the two clusters) were low. This is surprising given the close proximity of members of each pair (i.e., ~4 km between Calaveras groves, ~3 km between Merced and Tuolumne , and ~7 km between Nelder and Mariposa groves. This indicates that even over short distances, groves in the northern range act as independent genetic units, with limited inter-grove gene flow. This is in contrast to populations in the southern part of S. giganteum range. For example, Long Meadow and Freeman Creek exhibited weak genetic and high rates of admixture (all individuals show equal ancestry from both assigned clusters) (fig. 1). These data indicate that these two southern groves exhibit little or no genetic divergence indicating recent or ongoing gene flow. This finding confirms the pattern noted in a range-wide study (Dodd and DeSilva 2016) that southern populations form a continual genetic unit. Given the distance between these two groves (about 20 km), we believe that genetic connectivity in the southern S. giganteum range may be maintained through a stepwise model of gene-flow.