FLAGSTAFF, Ariz. (AP) — Northern Arizona has missed out on a white Christmas, and if the lack of snowfall continues, scientists say there will likely be more far-reaching effects on the region's pine trees.

Without enough winter moisture, scientists tell the Arizona Daily Sun, the trees will be more susceptible to bark beetles and disease, all of which lead to tree mortality.

"This is super dry for us, so if it continues there's going to be a lot of concerns I'm sure," said John Anhold, a forest entomologist with the U.S. Forest Service.

The latest drought maps show drought and abnormally dry conditions have taken hold of significant portions of the Four Corners region of Arizona, New Mexico, Colorado and Utah. With the exception of Colorado, the other states are worse off now than they were at this same time last year.

For Flagstaff, the below-normal precipitation expected for this winter will also affect the city's water sources and supply balance going into next year, while the dry weather has already been a game-changer for prescribed fire operations this fall.

On the Coconino National Forest alone, it has allowed crews to do low-intensity understory burns on about 50 percent more acreage than normal, said Victor Morfin, the forest's fuels specialist.

The fuels were already so dry in October that crews had to switch to burning at night, when relative humidity is higher, to prevent the fire from getting too hot, he said.

The Forest Service is taking a break from prescribed burning over the holidays but if the weather continues to be warm and dry, Morfin said the agency will ramp up burning again in January.

Trees respond to drought in several ways, said George Koch, a biology professor at Northern Arizona University.

As conditions dry out, they start to close up tiny pores on their needles and leaves called stomata. That helps slow water loss to evaporation but prevents the intake of carbon dioxide, which plants need to photosynthesize, Koch said.

Drought also causes damage to trees' water transport systems. Warm, dry air increases evaporation from leaves and needles while dry soils make it more difficult for the plant to pull up water through its roots.

Without enough moisture, ponderosas also struggle to produce pitch, or resin, that works as their main defense. Without that barrier, insects are free to crawl into a tree and reproduce with little resistance.

Anhold said it wouldn't be until late summer or fall that the effects of a beetle outbreak would become visible as needles turn yellow, then red and then start to fall off.

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Information from: Arizona Daily Sun, http://www.azdailysun.com/