Impact Of Latitudinal Gradient,Fire,and Succession On Genetic Diversity Of Eastern White Cedar(Thuja Occidentalis L.)

Eastern white cedar （EWC; Thuja occidentalis L.） is an important late-successional treespecies in the Boreal forest. It is listed as an endangered species by many states/provinces inNorth America. Conducting in-depth research on its population dynamics, genetic structureand diversity, is essential for the maintenance of ecosystem stability, the conservation of itsgermplasm and genetic resources, and the sustainable forest management. The long-termobjective of this study was to understanding the relative influence of climate, naturaldisturbances, forest succession and fragmentation upon the dynamics and genetic structure ofEWC populations in the boreal forest. Specifically, we used molecular markers to:1) examinethe impact of latitudinal gradient and fragmentation on its population genetic diversity;2)investigate the relative importance of the mode of regeneration along a post-fire succession aswell as the effect of disturbance history on its genetic structure;3) estimate the effects oflandscape features on its genetic diversity, and the conservation value of fire residuals. Weintroduced a spatial-temporal approach, as well as integrating climate factors, stand history,disturbance factors, and genetic data. The main conclusions are as follows:1) There were no significant latitudinal effects on population genetic diversity (Genediversity, H_s; Allelic richness, AR; Population differentiation, F_st) along the latitudinalgradient from the boreal mixed-wood to northern coniferous forest. Increased populationisolation and fragmentation was not correlated with a significant decrease in geneticdiversity.Positive Fisvalues and heterozygote deficiency were observed in populations fromthe marginal (F_is=0.244; P_HW=0.0042) and discontinuous zones (F_is=0.166; P_HW=0.0042)indicating inbreeding. However, populations from the continuous zone were in HWequilibrium (F_is=-0.007; P_HW=0.3625). Bayesian and NJT （neighbour-joining tree） analysesdemonstrated the presence of a population structure that was partly consistent with thegeographic origins of the populations. In general, there was substantial genetic differentiationamong EWC populations (F_st=0.077). Several marginal populations experienced recentpopulation decline, the remaining populations showed a signal of recent expansion. Overall,the impact of population fragmentation on the genetic structure of EWC is the presence of a positive inbreeding coefficient along the latitudinal gradient. The fragmented populations ofEWC appear well-buffered against effects of inbreeding on genetic erosion.2) The percentage of asexual regeneration slightly increased with stand development（1916,22.8%;1823,27.0%;1760,30.9%） along a250-year-long post-fire successionalgradient, while genotypic diversity decreased. Most gene dispersal was realised within site（pollen,88.5%; seed,79.4%）. The youngest site （1916） received a great portion （75.8%） ofgenes mediated by pollen and seed dispersal from older sites （1823,1760）. Mean seeddispersal distance was estimated to be1079.4m, with a maximum distance of7.2km, whilethe pollination between mates occurred over a mean distance of536.6m （maximum=4.4km）. Fine-scale spatial genetic structure （SGS） analysis showed contrasting patterns of SGSbetween saplings and adult trees. SGS was high and significant in saplings, and weaker inadult trees. Clonal growth increased SGS in saplings over short distances, together with SGSintensity （Sp）. Fine-scale SGS among saplings （excluding clonal individuals） in the firstdistance interval gradually weakened along the post-fire successional gradient. The distancefrom seed sources and the time needed for post-fire regeneration to produce seed-bearing treesare among the factors that controlled EWC abundance along the post-fire succession, togetherwith the abundance of suitable microsites.3) There were high levels of gene flow among small EWC fire residuals from thefragmented terrestrial landscape, naturally fragmented EWC islands from the lacustrinelandscape, and mainland EWC old forests from the non-fragmented terrestrial landscape. Ourresults revealed that a source-sink dynamic pattern of gene flow. Small EWC fire residualsmaintained high genetic diversity by receiving replenishment from mainland populations.There were significant differences in several genetic estimates among the three landscapetypes with small EWC fire residuals having the lowest allelic richness （AR=5.06）, highestpopulation differentiation (F_st=0.052), and lowest number of private alleles （PA=5）. Bothclimate （wind） and landscape features shaped the genetic structure of EWC in the landscape.Our study provides empirical evidences on the top conservation value of mainland EWCold-growth forest or its large patches. Therefore, appropriate protection measures should betaken before they turn to small patches that characterised with reduced genetic variation, asthey can continue to provide crucial habitat reserves for fire sensitive species.