The distribution of plants around the globe changes in response to changes in climate. Most people would be surprised to learn that plant fossils found on Ellesmere Island indicate that tropical forests once existed there. Here on the coast of British Columbia we imagine that Western Red Cedar has been present ‘forever’, but the pollen record indicates that this iconic tree only became abundant recently. A high mountain plant, American bistort – once abundant on southern Vancouver Island, growing at low elevations in open tundra-like habitat that is now forested – no longer occurs here, though it does occur in abundance at higher elevations on the Olympic Peninsula.
Uncovering the ancestral history of populations of particular species is one aspect of the discipline of biogeography – synthesizing information from biology and geography. Biogeography’s broad goals are to understand the history behind the patterns of distribution of different plants on the landscape. Important factors shaping plant distributions include climate change, sea level changes, the extent of glacial ice, the positions of the continents, and mechanisms of dispersal.
During the Pleistocene (Ice Age), global climate cooled and warmed many times. In response, ice sheets, some covering large parts of the Northern Hemisphere, formed and melted repeatedly as did valley glaciers in southern mountain ranges. It is commonly held that during the last ice advance, British Columbia was covered by an ice sheet until about 12,000 years ago. After the ice melted, plants and animals returned to BC as their habitats became available.
We are particularly interested in learning about the migration routes into BC of the original tundra colonists that now occur only on the highest mountains, though they likely spread throughout the region when colder climates meant that even at low elevations the habitat was tundra-like. These immigrants became the ancestors of our alpine species as forest covered lowlands and mid-slope elevations. Swedish plant geographer Eric Hultén proposed that many of these tundra species migrated from northern unglaciated lands of Yukon and Alaska, part of “Beringia”. Or did they migrate from south of the glacier limits in the northern US states? Maybe they came from both sources and directions. Biogeographical studies now show that a few survived the Ice Age and spread from the Brooks Peninsula (Vancouver Island) and parts of Haida Gwaii (the Queen Charlotte Islands). Recently we have raised another possibility, that previously unsuspected regions of BC also escaped the last glaciation and the plants that survived there were also ancestors of our remarkable mountain flora.
Fossil plant (Image 1) and animal (Image 2) remains document the occurrence of species at particular locations in the past. However, such remains usually cannot tell us how they got there. The most powerful tool to reveal their migration routes is through genetic analysis. We are studying DNA differences among individuals of the same species in selected plants from the northern hemisphere, especially to uncover their “wanderings” through time. The study of human migrations uses the same approach.
In 2001, we selected ten widespread alpine-arctic tundra species for intensive sampling throughout BC (Image 3) and adjacent areas. In 2010, thanks to a generous grant from the Walton Innovation Fund, we expanded the scope of this project to include samples from the Altai Mountains of southern Russian (Image 4). We returned to Russia again in 2011, this time further east (Image 5).
These Russian forays helped us understand the relationship of populations on both sides of the critically located Bering Strait. In 2012, we made additional collections from Alaska (Image 6) within the well-recognized ice age refugium that has never been glaciated. The collections from Russia and Alaska occur along the likely migration route of tundra species via the Arctic, between the mountains of Central Asia and the mountains of western North America.
The first species we analyzed, was mountain sorrel (Image 7) and the results were immediately exciting. Our analysis indicates that there are six lineages in this species and that these lineages likely diverged from each at least a million years ago. The species survived the last Ice Age in multiple refugia (ice-free areas) including southern Siberia, Beringia, the western US and, contrary to expectations, within the borders of BC. This last conclusion is based on the presence of two genetic types within the borders of BC that are not found north or south of BC. Some populations of mountain sorrel in northern BC contain individuals of mixed ancestry, including descendants that arrived from both the north and the south. Populations from eastern Russia and those from Alaska share many genotypes, as would be expected since during full-glacial periods, sea level was lower and a very broad land mass connected eastern Asia to North America. The absence of any shared genotypes between southern Siberia and North America indicates a very long period of separation. Fewer genotypes occur in southern BC than in northern BC and for the most part, the same genotypes do not occur in both areas, suggesting different landscape histories. Surprisingly, few genotypes occur in the Arctic, an indication that the species spread there relatively recently.
We have completed analyses of five more species. One of them, Sibbaldia, has a very different migration history. The genetic evidence indicates that it arrived in North America hundreds of thousands of years ago, and then two separate groups evolved and became isolated from each other. One lineage now occurs only in the Sierra Nevada mountains of California and the other occurs in the mountains of the remainder of western North America. Most of the genetic diversity of this species occurs in the US Rocky Mountains, indicating that the species has inhabited that region longer than any other location in western North America. More recently it spread northward through British Columbia into the arctic.
The migration patterns revealed by this study could not have been predicted without our genetic studies. These studies tell us much more than simple distributions of species, the traditional approach to such questions about migration corridors and routes. They identify fascinating possibilities about the origin of BC’s plant and animal species and bring into question the concept of a province-smothering ice sheet at the end of the Ice Age.
Ken Marr has been Botany Curator with the Royal BC Museum since 2001. His PhD is from the University of British Columbia, where he also did post-doctoral studies before heading to China for two-and-a-half years to study ethnobotany at the Xishuangbanna Tropical Botanical Garden (Chinese Academy of Sciences), specifically crop domestication of several Cucurbits.
With his graduate students, Curator of Botany and Earth History Richard Hebda has written more than 120 scientific papers and 250 popular articles. He has been co-author or co-editor of eight books and major reports, and serves as the province’s expert adviser on Burns Bog and science adviser on paleontology.