ARIZONA — The law of unintended consequences strikes again.
The Forest Service won’t undertake controlled burns this spring, to prevent firefighters from gathering in groups in which they might spread COVID-19.
That makes sense – protect the first responders.
But it comes with a price tag: Controlled burns and managing natural fires in the cool wet months remains the single best weapon right now when it comes to preventing the kind of megafire that consumed Paradise, California.
Moreover, such projects decrease tree mortalities, while restoring forest health.
So in recent years, the Forest Service has embraced both managed wildfires and prescribed fires as the best way to start thinning the dense thickets of trees on millions of acres in Northern Arizona.
Prescribed burns and managed fires have so far treated far more acres than project like the 4-Forests Restoration Initiative. So skipping a whole season of burning threatens forested communities. Mechanical thinning efforts have nearly stalled thanks to the lack of a market for the wood scraps, saplings and downed wood. The Arizona Corporation Commission last year crippled efforts to develop a market for biomass-fueled power plants by not issuing a biomass mandate for power companies.
One study published recently in the scientific journal Ecological Applications hints at the cost of the delay in both thinning and low-intensity burning.
Such projects reduced by about 15 percent the tree mortality in the Sierra Nevada Range between 2012 and 2016, according the University of California at Davis research.
The researchers monitored tree deaths in 2017 on 10 experimental plots in the central Sierra Nevadas. The researchers studied the effects of both drought and pine beetle outbreaks on ponderosa pine, sugar pine, white fir and incense cedar. They compared deaths from drought and beetles among trees in burned or thinned plots to those in untreated areas.
Some 45 percent of the trees in the untreated areas died, compared to 30 percent of the trees in the treated plots.
Lead author Christina Restaino in a release said “There are currently too many straws in the cup. Denser forests use more water. We’re learning that fuel treatments used to reduce fire risk have multiple benefits. Forests that are more open and less dense are stronger in the face of insect outbreaks too. By thinning forests, we can reduce water stress and make forests more resilient to drought and climate change.”
Aspen loss linked to drought
Another study linked the mass die-off of 17 percent of Colorado’s aspen forests from 2000-2003 to decreased rain and snow combined with warmer temperatures.
The dry, warm conditions hampered the ability of the trees to pump water out of the ground and into their leaves, leading to tree death that persisted for a decade after the drought, according to the study published in Global Change Biology by researchers from Carnegie Institution.
The researchers traced certain radio isotopes in the water that revealed whether it came from shallow sources or deeper down. They discovered the aspen relied heavily on water close to the surface. This made high summer temperatures especially damaging, since the heat rapidly dried out the soil close to the surface.
Yet another study found that trees pursue different strategies to cope with drought – and the relentless competition for water in a thick, overcrowded forest.
Ponderosa pines take the long view, closing up the pores in their needles and sharply decreasing their use of water. This helps them survive at the cost of reduced growth rates, according to the University of Washington researchers publishing in the journal Global Change Biology.
Quaking aspen, on the other hand, changes its physiology to continue growing on less water. The aspen toughen the xylem vessels that move water from root to leaf and thicken their leaves. This enables the aspen to continue growing.
The researchers suggested the Ponderosa pines, which live for 600 to 800 years, have evolved to “hunker down” during a drought. This enables them to survive repeated droughts in their long lifetimes.
The aspen, on the other hand, only live for 100 years or so – although their underground root mass many last much longer. The aspen try to sprint past the drought. That works for a short drought, but has made them vulnerable to the kinds of decades-long megadroughts that hit the Western US starting in about 2000.
Tree thickets compound the impact of drought. Densities in the ponderosa pine forest have risen from about 50 per acre to more like 800 per acre in the past century, thanks to grazing, logging and fire suppression.