Article Patterns of Fine-Scale Spatial Genetic Structure and Pollen Dispersal in Giant Sequoia (Sequoiadendron giganteum) Rainbow DeSilva * and Richard S. Dodd   Citation: DeSilva, R.; Dodd, R.S. Patterns of Fine-Scale Spatial Genetic Structure and Pollen Dispersal in Giant Sequoia (Sequoiadendron giganteum). Forests 2021, 12, 61. https://doi.org/10.3390/f12010061 Received: 10 December 2020 Accepted: 4 January 2021 Published: 7 January 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional clai- ms in published maps and institutio- nal affiliations. Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Deptartment of Environmental Science, Policy, and Management, University of California, Berkeley, CA 94720, USA; dodd@berkeley.edu * Correspondence: rainbow222@berkeley.edu Abstract: Research Highlights: Patterns of dispersal shape the distribution and temporal devel- opment 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 persis- tence. 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 predom- inantly 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. Keywords: pollen dispersal; spatial genetic structure; bi-parental inbreeding; giant sequoia 1. Introduction Propagule/regeneration dispersal is a key ecological process that influences the evolu- tion of genetic diversity both within and among populations [1–4]. For sessile organisms such as forest trees, how far propagules disperse away from the parent tree has many consequences for the distribution of genetic diversity at fine-spatial scales [5]. Moreover, the characteristics of the dispersal kernel play a large role in determining the extent of long-distance dispersal (gene flow), and thus modulate large scale patterns of genetic diversity and structure across a species range [6–8]. Since an adequate pool of genetic di- versity, on which selection can act, is critical for the success of populations under changing environments, understanding dispersal dynamics is important for successful management of species. At the scale of a population, dispersal dynamics shape the clustering of related in- dividuals on the landscape. Fine-scale spatial genetic structure (FSGS) can be defined as the non-random arrangement of genotypes on a landscape [5,9]. In plants, FSGS is caused by the interplay of many evolutionary forces, but of key importance is dispersal limita- tion, which creates patterns of isolation by distance between parents and offspring [5,10]. When related genotypes aggregate together in space, this can increase rates of bi-parental Forests 2021, 12, 61. https://doi.org/10.3390/f12010061 https://www.mdpi.com/journal/forests