Scientists: Weakened by past management, Montana forests face effects of climate change

The 2017 Montana Climate Assessment includes an in-depth report on the effects of climate change on the state’s forests. (Bitterroot Mountains by Philip Higuera/Montana Climate Assessment)

Past management practices have reduced the genetic diversity of Montana’s forests, limiting their ability to adapt to climate change.

The public and private scientists who released the 2017 Montana Climate Assessment last week said “genetic diversity largely determines a species’ ability to survive extreme events and adapt to changing conditions.”

That’s why historically, “the high genetic diversity of many tree species allowed forests to tolerate a wide range of environmental conditions and adapt to shifts in climate,” according to the first-of-its-kind report.

Now come the significant temperature increases documented worldwide in recent decades. In Montana, that warming exceeds the national and global averages – with temperatures here increasing by 2-3 degrees since 1950 and projected to warm as much as 9.8 degrees by the end of the century. (Read related story.)

And the future changes could come even more rapidly and significantly, according to the 32 federal, state, academic and special-interest scientists who contributed to the climate assessment.

That’s because atmospheric carbon dioxide is at its highest level in 3 billion years, and is projected to continue rising. That, in turn, will drive climate change.

“Although the magnitude of potential climate change may be comparable to variability experienced in the past, the rate of that change is anticipated to be significantly greater, with substantial implications for Montana’s forests,” according to the scientists.

Released by the joint University of Montana-Montana State University Institute on Ecosystems, the statewide climate assessment’s chapter on forests emphasizes that humans can – and have had – an influence over how the state’s forests adapt to a changing world.

“Human actions have undoubtedly altered forest genetics, at least in part through silvicultural practices,” the scientists said. “Because physical characteristics (tree height or basal diameter) are used to select trees for harvest, silvicultural practices can alter forest genetics, which are the bases for these physical differences.

“Selective harvesting may have substantially altered the presence of rare genetic characteristics, which are often those needed by a species to adapt to climate change.”

It’s true in Montana, but also globally, said the researchers who wrote the projections of coming changes in the state’s forests. They include Alisa A. WadeAshley P. BallantyneAndrew J. Larson, and W. Matt Jolly.

The scientists added this ominous footnote: “However, it is unclear whether species with even high levels of genetic and physical diversity can adapt fast enough to the rapid and extreme shifts in climate that are projected over the next century.”

Climate change is having a direct effect on Montana’s forests – warmer, wetter conditions improving forest productivity in some stands and warmer, drier conditions increasing tree mortality in others.

“Shifts in temperature and precipitation can have both positive and negative direct effects on forest establishment and regeneration, growth and productivity, and mortality,” the report said. “Overall, net impacts are likely to be negative, particularly in water-limited areas.”

“The speed and magnitude of climate change may mean that increased forest mortality and contractions in forest distributions will outpace any gains in forest growth and productivity over the long run, leading to a net loss of forested area in Montana,” the scientists said. “However, range shift responses will be highly dependent on species and region.”

Wildfires already are starting to increase in incidence and severity, in part because of climate change. (inciweb)

The indirect effects are even more serious, for tree species and human beings alike, according to the assessment. Montanans already see the start of these changes on the mountainsides around them.

“The indirect effects of climate change on forests, such as changing wildfire and beetle outbreak severity, are already having a large impact on the health of Montana’s forests and in some instances, these impacts are easier to predict,” the scientists wrote.

Here are the key effects, as outlined in the Montana Climate Assessment:

  • An increase in fire risk (i.e., probability of occurrence)—including an increase in size and possible frequency and/or severity (i.e., tree mortality)—is expected in the coming century as a result of a) prolonged fire seasons due to increased temperatures, and b) increased fuel loads from past fire suppression. Spatial patterns of fire activity will be complex and dependent on disturbance history and current stand condition. Fire risk may increase in all forests; fire severity may increase the most in lower elevation forests.
  • Rising temperatures are likely to increase bark beetle survival, but climate-induced changes to other insects and forest pathogens are more varied and less certain.
  • There may be a reduction in the amount of carbon stored in forests. Rising temperatures and increased atmospheric CO2can increase forest productivity and thus the carbon stored in organic matter. However, warmer temperatures can also reduce soil carbon through increased decomposition rates. Overall, increased tree mortality from increased forest disturbance may cause a reduction in forest carbon storage.

“Across Montana, conditions that lead to high fire risk are becoming more common: Seasonal maximum temperatures are increasing, snowmelt is occurring earlier, minimum relative humidities are decreasing, and fuels are becoming drier,” the scientists concluded. “Combined, these factors have led to the fire season lengthening globally between 1979 and 2013. In addition, across the western U.S., fuel loads and tree densities have increased as a result of fire suppression practices beginning in the 1920s, as well as other land uses, such as timber harvest and grazing. As a result, it is clear that climate change, combined with greater fuel loads, has increased Western fire activity over the past 30 years. …

“Although fire modeling is complex and models specific to Montana climate divisions are unavailable, recent studies suggest likely trends for the state. McKenzie and Littell project that water balance deficits will increase, likely leading to increased area burned. Jolly et al. project that warmer summers and reduced moisture will also continue to lengthen fire seasons.”

Scientists both in Montana and nationally expect to see increasing insect epidemics because of global warming. (U.S. Forest Service)

Climate change also means an increase in pine beetles and resulting outbreaks and mortality, according to the report.

The beetles are currently on the wane in western Montana forests because of a reduction in susceptible host trees after years of epidemic-level infestations. But climate change will bring the invasion back, stronger than before.

“We project that rising winter temperatures will result in increased mountain pine beetle populations,” the authors said. “Those increases will result from fewer cold snaps, and hence substantially decreased likelihood of seasonal mountain pine beetle die-off.”

“It appears … that a warming climate will increase insect-related forest mortality, depending on the presence of susceptible host trees,” they continued. “Already, warming temperatures have expanded the range of beetles, and the largest recorded bark beetle epidemic in western forests has occurred in the past 15 years. Higher temperatures, if large enough, lead to more severe droughts as water is more rapidly and completely evaporated from soils and streams, which may, in turn, make forests more susceptible to western spruce budworm outbreaks.”

The assessment emphasizes that land managers can help forests make the transition to a warmer climate, although it offers no guarantees.

To do nothing, though, will result in a radically altered landscape, the scientists said.

“Managers should consider multiple scenarios of potential climate shifts and contemplate a suite of adaptation strategies,” they wrote. “We recommend a bet-hedging approach, understanding the range of potential options and their possible consequences, and selecting among those that provide the most likely benefit given future uncertainty.”

Quoting a 2012 study, the research team lists four management options in a era of climate change:

  1. Promote resistance. Enhance the ability of species of system to resist forces of climate change.
  2. Increase resilience. Enhance the capacity of system to absorb impact without substantial changes to processes and functionality.
  3. Enable ecosystems to respond. Assist a system’s transition to an altered state that is adaptive to a changed climate while minimizing disruptive outcomes.
  4. Re-align highly altered ecosystems. Use restoration techniques to allow a system’s function to continue through changing climate conditions.

Read the full report on the effect of climate change on Montana’s forests here. The entire 2017 Montana Climate Change Assessment is available here.