Research and Analyses
Research questions. Micranthes (Saxifragaceae), a clade of small-flowered herbs comprising 75 species, is an ideal group for investigating the biotic impact of climate change in montane and Arctic ecosystems, which are especially vulnerable to a warming global climate[1,2]. Micranthes has a disproportionately high occurrence in cold habitats (30% of the clade), a relatively recent divergence (<20 MYA), and numerous striking geographic disjunctions (especially between Asia and North America)[3–10]. These attributes make this clade an exceptional model system for exploring the evolution and geographic spread of cold-adapted plants in the context of climate change.
Question #1. An Arctic perspective on temperate disjunctions: what patterns are seen using representatives of an understudied, yet pivotal, biome to investigate well-known plant distributions? Two fascinating biogeographic disjunctions are exhibited in both the chloroplast and nuclear phylogenies of Micranthes: Eastern Asia-North America (EA-NA) and Cascades/Sierra Nevada-Rocky Mountains (CSN-RM). Although these disjunctions have been previously explored, many questions remain and Micranthes provides new insights into these patterns.
The Temperate Flora—which includes one-third of Micranthes—is notoriously discontinuous, being well-established in East Asia, Eurasia, eastern North America, and, to a lesser extent, in western North America . Our preliminary results strongly suggest that the most prominent disjunction pattern within Micranthes is between eastern Asian and western North America taxa, rather than the more common eastern Asia – eastern North America or western North America – eastern North America patterns [16,17]. Therefore, we are excited to explore the eastern Asia – North America disjunction in the context of the distinctive pattern seen in Micranthes.
Similarly, Micranthes provides a unique system for investigating the poorly known CSN-RM disjunction. This pattern has been noted in over 100 plant and animal groups, but there have been very few studies on this distribution in a systematic or biogeographic context. Within Micranthes two phylogenetically, morphologically, and ecologically disparate taxa, M. bryophora and M. tolmiei, show this pattern, with each having distinct western and eastern populations.
Research Question #2. How did the cold-adapted Micranthes respond to historic climate change? We are witnessing the devolution of an entire biome[14–17]. The Arctic, which has undergone an increase in surface temperature almost three times that of the global average in recent decades, is also experiencing immense biotic and abiotic changes[18–20]. Yet, the ecological consequences of climate change in this region, far exceeding those in temperate and tropical biomes, are comparatively underreported. This is in part due to the Arctic often being regarded as a simple, species-poor system, when in fact it has been shown that its biotic entities are strongly interconnected and that individual species play pivotal roles in supporting ecosystem function[21–23]. Much of the research in the Arctic has focused on the fauna, yet the responses of vegetation to warming have been shown to be complex, significant, and intricately involved in other ecosystem processes[16,17,21]. Furthermore, the strong environmental filters in the Arctic such as a short growing season and low average annual temperature allow only a limited number of hardy species to survive. Finally, the Arctic is an ideal natural system for looking at the effects of climatic changes because it has a dramatic and well-documented history of repeated climate oscillations over a short period of time.
Phylogenetics. Our research questions are being addressed through state-of-the-art phylogenetics and analyses. Through our first round of high-throughput sequencing—targeting 596 putatively single copy nuclear genes and 88 plastid genes and spacers, across 57% of the species in the group—we have established a much-improved phylogenetic framework. With this dataset we developed and honed workflows employing the latest phylogenomic and biogeographic methods and demonstrated the utility of Micranthes for further research. With this research our goals are to use Micranthes to: 1) provide a new perspective on the roles of vicariance, dispersal, and refugia in shaping north temperate biogeographic patterns and 2) develop Micranthes as a model system to explore the radiation and distribution of high-elevation and high-latitude plants in the context of historic climate change. With increased sampling and an improved phylogenetic framework, compiled with other data we are gathering, Micranthes is in an excellent position to emerge as a model Arctic/alpine system. Presently, my fieldwork has resulted in collections from 131 populations of Micranthes. Additional samples not obtained from the field were retrieved at herbariums or from colleagues (32 samples). In total, I have ~60 species, with most being represented in at least two populations, to be included in my final phylogenetic analysis.
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