For millennia, black spruces (Picea mariana) have been a common presence across Arctic forests in North America, but they are now being threatened by more frequent and severe wildfires.
Like many other trees in these forests, the spruce 鈥� an evergreen conifer 鈥� depends on fire for regeneration. As fire moves across the landscape, the trees鈥� cones, which are covered in a waxy coat, heat up and slowly open to release their seeds onto the new seedbed.
But climate change is leading to more frequent and severe fires, as well as warmer, drier conditions in these boreal forests. at Wilfrid Laurier University in Ontario, Canada, and her colleagues have found that these rapid environmental changes are hindering the black spruces鈥� ability to regenerate.
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The team looked at data from 1538 field sites affected by forest fires across the entire boreal region of North America. Collectively, the sites experienced 58 fires between 1989 and 2014. By comparing the pre- and post-fire composition of tree species growing at the sites, the researchers could evaluate how well black spruce populations bounced back after the fires.
They found that the black spruce was able to regenerate after forest fires at 62 per cent of the sites. Tree regeneration failed entirely at 18 per cent of sites, while black spruces at the remaining areas were replaced by competing trees, such as aspens and poplars.
鈥淭he frequency of this [complete replacement with other trees or the failure to regenerate at all] was fairly surprising, given that black spruce is a fire-embracing species that requires fires for regeneration,鈥� says Baltzer.
The study points to a couple of potential reasons for the black spruce鈥檚 problems. First, boreal forests are covered in a blanket of organic soil in which black spruces thrive. However, with fires burning more intensely due to climate change, this soil layer is thinning, creating conditions that are better suited to allow competing species to flourish. Second, black spruces can take up to 50 years to become reproductively mature. Other species reach maturity sooner, leaving the spruce disadvantaged.
These findings could have serious implications for carbon storage and biodiversity. High-latitude forest systems store between 30 and 40 per cent of all terrestrial carbon, with boreal forests locking up more carbon than any other forest system in the world, including the tropics. Black spruces help store a huge amount of this carbon, not just in the trees themselves, but also in the organic soil they maintain, which is where most of the carbon is locked away in the boreal system. Increasing fire activity will release a lot of this carbon into the atmosphere.
鈥淭here鈥檚 lots of questions about how transitions away from black spruce to dominance by other species will alter carbon sequestration and storage in boreal ecosystems,鈥� says Baltzer.
Black spruce forests also support a vast range of wildlife, such as caribou, which feed and live within these forests. With caribou populations declining across North America, there is real concern for their habitat availability in the future, says Baltzer.
鈥淲hen we see [tree] species that have been stable and dominant on the landscape reach these tipping points because of climate-warming-induced changes, it must be a big red flag to all of us,鈥� she says. 鈥淭he only real solution to these large-scale ecosystem changes that we鈥檙e seeing, in boreal systems and elsewhere, is the reduction of greenhouse gas emissions and for governments to make hard choices that will put us at net zero.鈥�
PNAS
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