Interior Alaska's hot and dry summer of 2013, coupled with an invasion of insect pests that proliferated in number this year, has taken a steep toll on the region's birch trees, experts say.

Had it not been for the heavy rains that swept in during the second half of this summer, a large number of the trees might have been doomed, said Glenn Juday, a professor of forest ecology at the University of Alaska Fairbanks

"If we had not had something like a near-record rainfall, we would have seen mass tree deaths," Juday said.

Record and near-record heat in Fairbanks last year caused the birches -- deciduous trees that make up nearly a fifth of the Interior Alaska boreal forest -- to shrink their roots and branches, he said. In those conditions, the trees did what they had to do to conserve water, he said: They had dieback of the root systems and dieback of some of the top branches.

"The tree is strategically pulling back from supporting tissue when it's in an extreme drought," Juday said. Effects on the trees are dead branches and sparse leaves that lose most of their green color long before the arrival of fall, Juday said. The chlorophyll -- the chemical compound that makes leaves green and is needed for photosynthesis -- is retained only along the main veins of leaves, causing other parts of leaves to fade to yellow and brown.

Dry Interior conditions continued into about mid-2014, until a deluge arrived late in the summer.

The birches were ultimately rescued by those heavy rains, and some younger trees were able to put out a second or even third flush of growth once the moisture arrived, Juday said. Still, some trees, with their shrunken roots, were not able to take full advantage of the added moisture, he said.

In a future drought year, the trees might not be rescued by heavy rains, and it would be reasonable to expect that large numbers would die, he said.

On top of the heat, Juday said, the birch trees have suffered from an infestation of amber marked birch leaf miners, invasive insects that have steadily worked their way from the U.S. East Coast north and west into Alaska.

The Interior Alaska infestation that started around Eielson Air Force Base has spread, and now trees are afflicted "all across the Yukon Flats, all across the Tanana Valley lowlands, all across the south-facing slopes," Juday said. Some areas are particularly hard hit. At Bonanza Creek, site of an ecological research station about 20 miles west-northwest of Fairbanks, "it appears that a large majority of the trees were being affected," he said.

Leaf miners were introduced into the Anchorage area a little more than a decade ago, and it was believed to be just a matter of time before they migrated north, Juday said.

Does the warmer weather have anything to do with the infestation?

"Well, it got warmer, and here it is," he said. But the insects might have moved in anyway, he added.

Juday is now preparing to examine the rings of birch trees to see what two consecutive summers of unusual conditions have done to growth.

Glen Holt, a regional forester for UAF's Cooperative Extension Service, said his office was flooded this summer with calls about birch problems -- about four times the normal number. The dominant concerns were leaf problems and top die-backs, he said in an email.

Many Fairbanks-area birch trees are particularly vulnerable to stresses because they are old, he said.

The Cooperative Extension Service has advised property owners to monitor the health of their birch trees and to protect their roots by refraining from piling wood, soil or other materials next to them.