Mediterranean ecosystems comprise a high proportion of endemic taxa whose response to climate change will depend on their evolutionary origins. In the California flora, relatively little attention has been given to the evolutionary history of paleoendemics from a molecular perspective, yet they number among some of the world's most iconic plant species. Here, we address questions of demographic change in Sequoiadendron giganteum (giant sequoia) that is restricted to a narrow belt of groves in the Sierra Nevada Mountains. We ask whether the current distribution is a result of northward colonization since the last glacial maximum (LGM), restriction of a broader range in the recent past (LGM) or independent colonizations in the deeper past. Genetic diversity at eleven microsatellite loci decreased with increasing latitude, but partial regressions suggested this was a function of smaller population sizes in the north. Disjunct populations north of the Kings River were divergent from those south of the Kings River that formed a single cluster in Bayesian assignment tests. Demographic inferences supported a demographic contraction just prior to the LGM as the most likely scenario for the current disjunct range of the species. This contraction appeared to be superimposed upon a long-term decline in giant sequoia over the last 2 million years, associated with increasing aridity due to the Mediterranean climate. Overall, low genetic diversity, together with competition in an environment to which giant sequoia is likely already poorly adapted, will pose major constraints on its success in the face of increasing aridity.

Citation: DeSilva, R. and R. S. Dodd. 2020. Fragmented and isolated: Limited gene flow coupled with weak isolation by environment in the paleoendemic giant sequoia (Sequoiadendron giganteum). American Journal of Botany 107(1): 45-55. PREMISE:Patterns of genetic structure across a species' range reflect the long-term interplay between genetic drift, gene flow, and selection. Given the importance of gene flow in preventing the loss of diversity through genetic drift among spatially isolated populations, understanding the dynamics of gene flow and the factors that influence connectivity across a species' range is a major goal for conservation of genetic diversity. Here we present a detailed look at gene flow dynamics of Sequoiadendron giganteum, a paleoendemic tree species that will likely face numerous threats due to climate change. METHODS:We used microsatellite markers to examine nineteen populations of S. giganteum for patterns of genetic structure and to estimate admixture and rates of gene flow between eight population pairs. Also, we used Generalized Dissimilarity Models to elucidate landscape factors that shape genetic differentiation among populations. RESULTS:We found minimal gene flow between adjacent groves in the northern disjunct range. In most of the southern portion of the range, groves showed a signal of connectivity which degrades to isolation in the extreme south. Geographic distance was the most important predictor of genetic dissimilarity across the range, with environmental conditions related to precipitation and temperature explaining a small, but significant, portion of the genetic variance.CONCLUSIONS: Due to their isolation and unique genetic composition, northern populations of S. giganteum should be considered a high conservation priority. In this region, we suggest germplasm conservation as well as restoration planting to enhance genetic diversity.

In an era of unprecedented climate change, the adaptive potential of populations has become an increasingly important topic to conservation biologists, raising questions of landscape partitioning of adaptive variation and management strategies to maintain population viability. Given the rapid rate of climate change, new beneficial mutations are expected to play a limited role for species with

Research Highlights: Patterns of dispersal shape the distribution and temporal development of genetic diversity both within and among populations. In an era of unprecedented environmental change, the maintenance of extant genetic diversity is crucial to population persistence. Background and Objectives: We investigate patterns of pollen dispersal and spatial genetic structure within populations of giant sequoia (Sequoiadendron giganteum). Materials and Methods: The leaf genotypes of established trees from twelve populations were used to estimate the extent of spatial genetic structure within populations, as measured by the Sp statistic. We utilized progeny arrays from five populations to estimate mating parameters, the diversity of the pollen pool, and characteristics of pollen dispersal. Results: Our research indicates that giant sequoia is predominantly outcrossing, but exhibits moderate levels of bi-parental inbreeding (0.155). The diversity of the pollen pool is low, with an average of 7.5 pollen donors per mother tree. As revealed by the Sp-statistic, we find significant genetic structure in ten of twelve populations examined, which indicates the clustering of related individuals at fine spatial scales. Estimates of pollen and gene dispersal indicate predominantly local dispersal, with the majority of pollen dispersal <253 m, and with some populations showing fat-tailed dispersal curves, suggesting potential for long-distance dispersal. Conclusions: The research presented here represent the first detailed examination of the reproductive ecology of giant sequoia, which will provide necessary background information for the conservation of genetic resources in this species. We suggest that restoration planting can mitigate potential diversity loss from many giant sequoia populations.

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