A diverse alpine species pool drives a ‘reversed’ plant species richness–elevation relationship in interior Alaska

Abstract

AbstractAimWe evaluate whether vascular plant species richness in interior Alaska is highest in the alpine zone. We test the proposition that historical dynamics have influenced the sizes of species pools inhabiting different segments of the landscape by quantifying the contrasting responses of five phytogeographical elements within the flora to changes in elevation and topography.LocationDenali National Park and Preserve, Alaska, USA.MethodsWe developed Bayesian hierarchical models for total plant species richness and the richness of five phytogeographical elements with data from a systematic‐grid sample containing 1022 plots. We also used rarefaction and extrapolation techniques on these data to estimate species richness for more extensive landscape segments, including across seven elevation bands in the study area.ResultsPlot‐level species richness had a unimodal response to elevation, reaching its peak in the alpine zone. Overall plant species richness across more extensive areas was also higher in the alpine zone, even though this zone occupies less area overall in interior Alaska. Species richness of Alaska endemic and amphi‐Beringian phytogeographical elements showed particularly strong positive responses to elevation, whereas that of the North American and incompletely circumpolar elements declined with elevation.Main conclusionsOur results document a unique pattern in the study of variation in species‐pool sizes with elevation, revealing that sparsely vegetated alpine areas contained a larger species pool than adjacent extensive lowlands. This pattern was driven by pronounced increases in phytogeographical elements whose distribution patterns suggest that they evolved or persisted in the region during full‐glacial intervals. The persistence of tundra and/or similar open, treeless vegetation in interior Alaska contrasts with the simultaneous periodical elimination of boreal lowland ecosystems in the region. We posit that this has resulted in a regional species pool relatively enriched in alpine species and impoverished in boreal lowland taxa, causing a ‘reversed’ relationship between species richness and elevation. Increased habitat heterogeneity in high‐relief areas of the mountainous landscape are also likely to have contributed to the reported patterns.