The dry winter has dashed hopes of a white Christmas and squashed snowplay opportunities around Flagstaff. But the lack of snowfall, if it continues, will likely have a more far-reaching and long-lasting impact on the region’s pine trees.
Scientists say that without enough winter moisture, the trees risk a breakdown in crucial water transport systems and will be more susceptible to bark beetles and disease, all of which lead to tree mortality.
“This is 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 Forest Service.
The below-normal precipitation expected for this winter will also affect Flagstaff’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, the dry fuels allowed fire crews to do low-intensity understory burns on about 50 percent more acreage than normal, said Victor Morfin, forest fuels specialist on the Coconino.
Here’s what else a dry fall and winter means for forests, fires and water.
Trees respond to drought in several ways, said George Koch, a biology professor at Northern Arizona University who studies the interactions between plants and their environments.
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 the kicker is it also prevents the intake of carbon dioxide, without which plants can’t photosynthesize, leading to carbon starvation, 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. Those opposing forces severely stretch tiny columns of water in the trees’ tissues. If stretched enough, the columns will break and air bubbles will form, clogging the tree's "water pipes" and depriving leaves and needles of water. The process, called hydraulic fracture, is one of the main causes of tree mortality during drought.
Those duel forces of drought and starvation could definitely start to happen after just one dry winter, Koch said.
“If this winter does remain very dry I would definitely expect some tree mortality,” he said.
Without enough moisture, ponderosas also struggle to produce pitch, or resin, that is their main defense against bark beetle species in the area, said Anhold, of the Forest Service. Without that as a barrier the insects are free to crawl into a tree and reproduce with little resistance, he said.
It wouldn’t be until late summer or fall that the effects of a beetle outbreak would become visible in area trees as their needles turn yellow, then red and then start to fall off, Anhold said. He is part of a team that does annual aerial surveys of the region's forests to assess things like bark beetle damage.
When it comes to beetle infestations, the good news is precipitation can quickly turn things around, Anhold said.
“When moisture comes back that seems to turn off bark beetle activity,” he said.
But if this winter continues on its current dry trajectory, summer rains would have to be substantial — a couple months of heavy rain at least — for soil moistures to recover to where they should be after a normal winter and normal monsoon, Koch said. Snowmelt is so important for ponderosas, especially bigger trees, because it seeps deep enough into soils that it reaches the trees’ roots, Koch said. Summer rains, on the other hand, dump lots of water over a shorter period of time, which tends to run off instead of penetrating deep into the soil.
“For mature trees, even if we got a big monsoon I don't think it’s going to bail them out,” Koch said.
Although a stressor on trees, the dry fall and winter have been a boon to prescribed fire operations. Fire managers on the Coconino National Forest were able to carry out planned ignitions on 18,791 acres between September and now, which is significantly more than the 10,000 to 15,000 acres they can usually accomplish each fall, Morfin said.
The lack of moisture meant fuels were still dry enough to burn into November and December, lengthening the burn window to a couple of months instead of the normal two to six weeks, Morfin said.
In fact, in October the fuels were already so dry 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.
In the case that the area does start getting more snow, the Forest Service will switch to burning piles of forest slash that were produced during mechanical tree thinning, he said.
Flagstaff’s water managers plan for dry spells, which is why the city has a variety of water supplies, including groundwater, surface water and reclaimed wastewater, said Brad Hill, director of the city’s water services division.
The division creates its water management plan for the year each spring after staff have an idea of how much water Upper Lake Mary gained from snowmelt, Hill said. They also have to consider an agreement with the National Park Service and the Forest Service to keep Upper Lake Mary at a certain minimum level for wildlife, he said. Because the lakewater is the most renewable source as well as the cheapest, it is used as the baseload supply, with groundwater making up the difference, Hill said. This week, the breakdown was 30 percent surface water, 70 percent groundwater.
The lake is about 58 percent full, which is high for this time of year thanks to a large influx of snowmelt last year, Hill said. Though dry conditions have taken hold, the city won’t change its plan for the volume pumped from the lake until this spring, Hill said. If it ends up being a low-snow winter, the city will have to adjust what it pulls from Lake Mary in 2018, he said.
While potable water use this fall is about the same as last year, the amount of reclaimed water the city is pumping up to Arizona Snowbowl is up dramatically from last year, Hill said. Since Nov. 1, Snowbowl has received 156 acre feet, or 50.8 million gallons, of water for snowmaking. Last year at this time, the city had delivered just 35 acre feet, or 11.4 million gallons, to the ski resort, Hill said.