The High Alpine Tundra is Surprisingly Rich

High species richness at higher elevations may be evidence of the "species pool hypothesis" that posits plants have had a longer, uninterrupted time to evolve into diverse species and adapt to high-alpine, high-latitude environments.

Early studies of elevation and plant species richness–the number of different species found in a given area–suggested that species richness was greater at lower elevations. This was thought to mirror the global trend of greater diversity at lower latitudes (moving toward the equator). As you move closer to the poles or high mountain areas, the reasoning went, fewer plant species can survive in the colder, harsher climate.

But researchers have also noted the remarkable richness of plant species in alpine areas of Alaska and northern Canada. In some places, there appears to be a greater diversity of plants in high-alpine tundra than in the lush forest below. But does that trend hold under scrutiny? And what might cause that apparently “backwards” pattern?

A 2014 study published in the Journal of Biogeography suggests an answer by analyzing comprehensive plant surveys of 1,022 different plots in Denali National Park and Preserve. The plots stretched from forested lowlands at the northern edge of the park to the lofty tundra of the Alaska Range, encompassing a full spectrum of elevations in Interior Alaska.

Indeed, species richness was higher at higher elevations, reaching a peak above 1,000 meters before diminishing again at extreme elevation. The researchers hypothesized that this trend is related to the natural history of the region via the “species pool hypothesis.” Since the area was covered by glaciers and tundra for most of the past 140,000 years, evolutionary lines of plants occurring in this area adapted to these treeless environments have had longer uninterrupted time and space to evolve into diverse species. Meanwhile, forested areas were periodically eliminated from the ice age landscape in this region, so the “species pool” was diminished accordingly and the newly warm, wet lowlands were populated from a less diverse reservoir of plants.

To test this hypothesis, researchers classified the plant species into different biogeographical groups. Some species were circumpolar, meaning they grow all around the Arctic, including Greenland. Others were incompletely circumpolar, amphi-Beringian (growing on the American and Asian sides of the Bering Strait), North American, or endemic only to Alaska and the Yukon Territory.

If the “species pool” hypothesis were true, then much of the high diversity in alpine regions should be driven by endemic and amphi-Beringian species, relatively new species that evolved on the expansive tundra of the last ice age. It turned out this was exactly the case–along with circumpolar species adapted to cold climates, endemic and amphi-Beringian species had the strongest positive correlation between elevation and richness. North American species on the other hand, many of which may have migrated northward at the end of the ice age, showed the opposite trend.

While these correlations don’t show definitive causation, they’re strong evidence in support of the species pool hypothesis. The results are also a good reminder that evolutionary history profoundly influences today’s ecology.

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

Abstract

We 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.

Methods

We 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.

Results

Plot‐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 conclusions

Our 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.

Roland, C. A. and J. H. Schmidt. 2015. A diverse alpine species pool drives a "reversed" plant species richness-elevation relationship in interior Alaska. Journal of Biogeography 42(4): 738-750.