Warming temperatures are causing plants across alpine and arctic environments to stay green longer and reproduce earlier, scientists find

Across the tundra, warming temperatures are causing plants to stay greener longer and flower earlier—and that could reshape life there, according to new research led by the University of Colorado Boulder.

These findings, which were published today in Nature Communications, synthesized 30 years of experimental warming data from 18 different tundra (arctic and alpine) sites across the globe and found that not only are leaves coming out earlier and staying on the plants longer in this critically understudied biome, but their reproductive cycles are not responding in the same way.

This change could not only have cascading effects through the ecosystem, affecting everything from pollinators to the animals that depend on these plants for food, but could also change the balance of carbon between the land and the atmosphere. 

“Plants are responding to climate change in unique ways, and because we are seeing that different parts of their life cycle are responding differently—like their flowers and their leaves, this is going to have pretty interesting impacts on different organisms that are interacting with these (plant) species, like pollinators and herbivores,” said Courtney Collins, a postdoctoral researcher in the Institute of Arctic and Alpine Research (INSTAAR) at CU Boulder and the lead author on the study.

“This research shows how difficult it is to make broad scale predictions about what’s going to happen with global climate change, because even with 30 years of data at 18 sites, there’s still very complex responses that are happening.”

These data were collected as part of the International Tundra Experiment (ITEX), a network of researchers from across the globe, including all of the arctic countries, that looks at a warming climate’s effects on tundra.

While ITEX has been going more than 30 years, CU Boulder didn’t get involved until 1995, when Marilyn Walker (former professor at CU Boulder and now the global lead for hybrid power systems at UL) started researching how alpine plants were changing in response to warming at the Niwot Ridge Long-term Research site. Jane Smith, a postdoctoral research scholar at CU Boulder, and Chiara Forrester, a PhD student in ecology and evolutionary biology (EBIO), also took part through the years.

These researchers, along with the other ITEX researchers, collected the data by deploying passive, open-top warming chambers to the various sites and placing them on already growing plants. They then raised the temperature through solar radiation anywhere from one to three degrees Celsius to get an idea of how these plants would respond to this kind of temperature increase—the minimum expected with climate change for this part of the world.

“The tundra is warming much more rapidly than other parts of the world. In some places, it’s happening at twice the rate of warming (of the rest of the globe), and so these changes are occurring extremely fast and they’re happening as we speak,” said Collins.

“These biomes are changing, and so we really need to understand what’s going on so we can help to make long-term predictions and also mitigate these changes.”

What they found is that both the leaf and reproductive phenophases (the annual life cycle phases, like flowering and fruiting, that occur in a plant’s life) responded strongly—but inconsistently—at every site in the experiment, which could cause the growing season for these plants to get roughly 3% longer.

“There is no question that climate is changing across the globe. But the consequences of these changes to our systems are not clear cut,” said Katharine Suding, a professor of distinction in EBIO and the senior author on the paper.

“This study is cool because it demonstrates that warming will affect the timing of plant growth and flowering, pushing it earlier, but that different stages of plant growth and flowering don’t change in unison.”

These inconsistent changes could potentially affect more than just the plants. The researchers think this longer season could have ripple effects throughout the tundra ecosystem on everything from the pollinators relying on the flowers to the herbivores eating the leaves, and also on the global climate itself as the plants could potentially affect the carbon cycle.

“This is currently the largest synthesis of experimental warming across the whole tundra biome, which is one of the most understudied biomes in terms of plant phenology compared to other ecosystems globally,” said Collins. 

“I think this really helps give the tundra research more exposure to the public and also to climate modelers who are thinking about changes across the globe.”

Other authors on the paper include Sarah C. Elmendorf and Emily Ogburn from CU Boulder; Robert D. Hollister and Ulf Molau from Grand Valley State University; Greg H. R. Henry and Christopher W. Kopp from the University of British Columbia; Karin Clark from the Government of the Northwest Territories; Anne D. Bjorkman from the University of Gothenburg; Isla H. Myers-Smith from The University of Edinburgh; Janet S. Prevéy from the U.S. Geological Survey; Isabel W. Ashton, Alessandro Petraglia and Carolyn Livensperger from the National Park Service; Jakob J. Assman from Aarhus University; Juha M. Atalo from Qatar University; Michele Carbognani from the University of Parma; Chelsea Chisholm and Zoe A. Panchen from ETH Zurich; Elisabeth J. Cooper from The Arctic University of Norway UiT; Ingibjorg Svala Jonsdottir from the University of Iceland and The University Centre in Svalbard (UNIS); Kari Klanderud from Norwegian University of Life Sciences; Marguerite Mauritz from University of Texas at El Paso; Jeremy L. May and Steven F. Oberbauer from Florida International University; Eric Post from the University of California Davis; Christian Rixen from the Swiss Federal Institute for Forest, Snow and Landscape Research WSL; Heidi Rodenhizer and Edward A. G. Schuur at Northern Arizona University; Philip Semenchuk from The University of Vienna; Heidi Steltzer from Fort Lewis College; Ørjan Totland from The University of Bergen; and Jeffrey M. Welker from The University of Alaska Anchorage and The University of Oulu.