Even modest climate change can lead to profound changes in the northernmost forests
Even relatively modest global warming and its associated precipitation shifts could dramatically alter Earth’s northernmost forests, which form one of the planet’s largest nearly intact forested ecosystems and are home to much of the planet’s terrestrial carbon.
That’s the key finding of a unique five-year experiment, led by a University of Michigan ecologist, that used infrared lamps and soil heating cables to assess the expected near-term effects of climate change on thousands of seedlings of nine tree species growing in far northern forests, which are known stand as boreal forests.
The boreal forests of North America mainly contain conifers such as spruce, spruce, and pine. They are primarily found in Canada and Alaska, but can also be found in parts of northeastern Minnesota, a small portion of Michigan’s Upper Peninsula, and northern Maine. The boreal forests are bordered to the north by tundra and to the south by temperate forest.
In the experiment, young trees at two University of Minnesota forest sites in northeastern Minnesota were heated in the open 24 hours a day, from early spring to late fall, without the use of greenhouses or growth chambers. Two levels of potential 21st-century global warming were used: about 1.6 degrees Celsius (about 2.9 degrees Fahrenheit) and about 3.1 C (about 5.6 F) above ambient temperature.
In addition, before some storms, movable tarpaulins were placed over half of the plots to collect rainwater and mimic precipitation shifts under a changing climate. As a control, some trees were grown at ambient temperatures and humidity levels.
The study found that even modest global warming (1.6 C) caused major problems for many species, including reduced growth and increased mortality. In addition, reduced rainfall amplified the negative effects of warming on the survival of several boreal species.
“Our results indicate problems for the health and diversity of future regional forests,” said UM forest ecologist Peter Reich, lead author of the study, which will appear in the journal Aug. 10. Nature.
“The current southern boreal forest could reach a tipping point with even modest global warming, resulting in a major shift in composition with potentially adverse effects on the health and diversity of regional forests,” said Reich, director of the Institute for Global Change Biology. at the University of Michigan School of Environment and Sustainability.
“Those impacts could reduce the capacity of our forests to produce wood, to host other plant, microbial and animal diversity, to dampen flooding, and – perhaps most important of all – to scrub carbon from the air and to hold it in wood and soil.”
According to scientists, plants in mid-to-high latitudes are likely to experience both positive and negative effects from 21st century climate change. In some places, especially in the far north, a longer growing season can encourage tree growth if there is a lot of moisture.
In other locations, warmer and drier conditions can reduce tree growth and survival. Observational studies show that both positive and negative trends in boreal forest survival and growth are already taking place.
But direct experimental tests of the effects of climate warming on boreal forests in a range of soil moisture conditions are rare and generally limited in scope, scope and duration, according to the authors of the new study.
The Nature report fills many of these knowledge gaps. The study used joint manipulation of temperature and rainfall to examine the likely short-term effects of climate change on the mortality and growth of saplings at the two field sites.
“In the experiment, we subject forest plots to temperatures that we won’t see for the next 40 or 50 or 60 years to understand what those upcoming temperatures will do,” Reich said.
The researchers found that warming alone, or in combination with reduced rainfall, increased juvenile mortality of all nine tree species and severely reduced growth in several northern conifer species — balsam fir, white spruce and white pine — common in boreal forests.
At the same time, modest warming boosted the growth of some broadleaf hardwoods, including some oaks and maples, which are scarce in the boreal forest but much more common in temperate forests in the south.
However, the new study concludes that hardwoods are likely too rare in the southern boreal forest to quickly fill the void left by disappearing conifers. Therefore, the expected near-term climate change is likely to shift the current boreal forest to a “new state” with a modified composition.
“That new state is probably a more impoverished version of our current forest at best,” Reich said. “Worst-case scenarios could be high levels of invasive woody shrubs, which are already common at the temperate-boreal border and are rapidly moving north.”
The experiment was conducted at two University of Minnesota field stations. Reich, who joined the UM faculty in 2021, has a joint affiliate in Minnesota and continues to work together on the forest warming project.
For the experiment, more than 4,500 seedlings from nine native tree species — five broadleaf and four coniferous leaf species — were planted in existing herb, shrub and fern vegetation at the study sites. The nine tree species are balsam fir, white spruce, fig tree, white pine, red maple, sugar maple, paper birch, bur oak and red oak.
The movable sails resulted in approximately 30% less rainfall on randomly selected plots over the course of the growing season. In fact, because rainfall during the five years of the experiment was higher than average, the low rainfall treatment represented the average dry years of the past century, and the control treatments typically represented wet years during the same period. Therefore, the low rainfall treatments were in no way extreme.
The other authors of the Nature paper are Raimundo Bermudez, Rebecca Montgomery, Karen Rice, Sarah Hobbie and Artur Stefanski of the University of Minnesota, and Roy Rich of the Smithsonian Environmental Research Center.
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Peter Reich, Even modest climate change can lead to major transitions for boreal forests, Nature (2022). DOI: 10.1038/s41586-022-05076-3. www.nature.com/articles/s41586-022-05076-3
Quote: Study: Even modest climate change could lead to dramatic changes in northernmost forests (2022, Aug. 10) Retrieved Aug. 10, 2022 from https://phys.org/news/2022-08-modest-climate-northernmost-forests .html
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