WASHINGTON (Reuters) - Wildfires sparked by lightning near Canada's Hudson Bay are threatening the habitat of polar bears, encroaching on the old tree roots and frozen soil where females make their dens, a conservation expert on the big, white bears said on Thursday.

Polar bears are more typically threatened by the melting of sea ice, which they use as platforms for hunting seals, their main prey. But those who live near Hudson Bay spend their summers resting up on shore when the bay thaws, living in dens dug in the frozen soil among the roots of stunted spruce trees.

Fires in this area are rare, said Steven Amstrup, a former polar bear specialist with the U.S. Geological Survey and now chief scientist at the nonprofit conservation organization, Polar Bears International.

"It's a cool, wet environment that doesn't burn very often," Amstrup said by telephone from Washington state. "It's not an environment where the forest is adapted to fires very much."

Unusually hot, dry weather in Manitoba, Canada, and lightning strikes caused several fires through Wapusk National Park across known polar bear dens in July, said Manitoba Conservation Officer Daryll Hedman.

High temperatures this week in Churchill, Manitoba, were in the upper 50s to mid-60s F (15 to 18 C), with overnight lows above freezing. The U.S. National Snow and Ice Data Center indicated in a map of Arctic ice cover released this week that the vast majority of Hudson Bay is ice-free.

Female polar bears in the western Hudson Bay population use dens under the root crowns of small, slow-growing spruce trees that grow in permafrost soils along the banks of rivers and lakes. Some dens have been used for over 100 years.

"Not only is the permafrost no longer permanent, tree roots needed to stabilize the den structure are disappearing," Amstrup said. "The kinds of habitats where mother polar bears in this area give birth to their cubs are simply disappearing as the world warms."

CUBS VERY VULNERABLE

Historically, the soil in most areas around Hudson Bay is frozen solid below a surface layer about one foot thick that thaws and re-freezes seasonally, Amstrup said.

In recent years, that top layer of soil has gotten increasingly thicker, so the thawing goes deep enough to defrost the soil around the tree roots, making the openings that the bears dig collapse, Amstrup said.

When the trees burn, as some may this summer, their roots die out and further damage the polar bear dens, he said.

Unlike other polar bear populations, where pregnant females use dens dug in snow, Hudson Bay females come ashore to these tree-root dens to rest and give birth, remaining in the dens until the following spring.

"They're essentially food-deprived in the summertime. They come ashore and basically just rest and try to save energy. They crawl into these dens, it's cool in there, they're not harassed by insects and they basically just rest until the snow comes and until they give birth," Amstrup said.

Without these earthen dens, he said, females would not be able to conserve their energy as well. And if snow does not come early enough, cubs could be born out in the open, where they would be exceedingly vulnerable.

"Cubs are born at about a pound and a half (680 grams), blind, nearly hairless, essentially immobile and totally helpless," he said in a follow-up email. "Their survival depends upon the shelter of the den to protect them from the elements."

Polar bears have government protection in Canada and the United States. The U.S. polar bear population in Alaska is listed as threatened under the Endangered Species Act because of increasing damage to their icy habitat by climate change.

(Editing by Sandra Maler and Phil Berlowitz)