How does tree mortality caused by drought and insects affect forests accustomed to frequent fire?

Research results were published January 17, 2018

This week a group of nine scientists and researchers published the results of their work considering how unusually high tree mortality affects wildfires in California’s Sierra Nevada forests that over thousands of years have adapted to frequent fire. They point out that fire suppression-caused forest densification has increased competition among trees for water and other resources, destabilizing many frequent fire forests by making them prone to mortality from other agents such as bark beetles.

Authors

Scott L. Stephens, Brandon M. Collins, Christopher J. Fettig, Mark A. Finney, Chad M. Hoffman, Eric E. Knapp, Malcolm P. North, Hugh Safford, Rebecca B. Wayman

The abstract and conclusions are below. The entire paper can be accessed at BioScience. The illustrations are from the document.


Abstract

Massive tree mortality has occurred rapidly in frequent-fire-adapted forests of the Sierra Nevada, California. This mortality is a product of acute drought compounded by the long-established removal of a key ecosystem process: frequent, low- to moderate-intensity fire. The recent tree mortality has many implications for the future of these forests and the ecological goods and services they provide to society. Future wildfire hazard following this mortality can be generally characterized by decreased crown fire potential and increased surface fire intensity in the short to intermediate term. The scale of present tree mortality is so large that greater potential for “mass fire” exists in the coming decades, driven by the amount and continuity of dry, combustible, large woody material that could produce large, severe fires. For long-term adaptation to climate change, we highlight the importance of moving beyond triage of dead and dying trees to making “green” (live) forests more resilient.

Fire Responses Post Drought Beetle
A conceptual diagram showing fuel load and expected fire behavior in a mixed-conifer forest prior to and following a major bark-beetle-caused tree-mortality episode, with either (a) no follow-up-fuels treatment or (b) periodic prescribed fire to consume fuels. Surface-fire intensity is expected to roughly follow surface fuel load, whereas crown-fire potential is regulated by the amount of surface fuel (necessary to heat and dry live fuels to the point of combustion), as well as crown bulk density.

 

Forest Responses Severe Drought
Forest responses following a severe drought (1999–2002) in the Sierra de San Pedro Mártir (SSPM), Baja California, Mexico (a, drought and bark-beetle-caused tree mortality followed by wildfire; b, drought- and bark-beetle-caused tree mortality only) and in the southern California mountains (SCM), California, United States (c, drought- and bark-beetle-caused tree mortality at larger scales; d, drought and bark-beetle-caused tree mortality at stand scale. Note no wildfire in either SCM area). The SSPM and SCM photos were taken in 2004 and 2003, respectively. The SSPM site experienced a wildfire immediately following the multiyear drought (picture from 2003), with the photos capturing effects of both drought- and wildfire-related tree mortality. Pictures (a), (b), and (d) from SLS, (c) from G. Barley.

[…]

Conclusions

Unprecedented Sierra Nevada tree mortality has rapidly occurred after a severe drought with effects compounded by forest densification from decades of fire suppression. In the central and southern Sierra Nevada some areas have experienced more than 90% tree mortality, producing extensive landscapes of standing dead trees. This differs from mortality resulting from stand-replacing wildfire because bark beetles do not reduce surface fuels or jumpstart succession of shade-intolerant, fire-resistant pines. Forest managers have been struggling to determine whether these new postmortality conditions will increase wildfire intensity and/or severity, what the near- and long-term effects on forest communities will be, and what the appropriate intervention measures are.

In the first decade, wildfire severity in bark beetle killed frequent fire (FF) forests may be little affected over current conditions. Other than a brief increase during the “red phase” when most dead needles are still on recently killed trees, the reduction in canopy fuels is counterbalanced by an increase in surface fuels (figure 2). However, these are no grounds for complacency because current conditions in the majority of mixed-conifer and yellow pine forests in California already consist of unnaturally high surface fuel loads and corresponding elevated fire hazards (figure 2; Lydersen et al. 2014, Stephens et al. 2015).

The more troubling projection is how extensive loading of large-sized woody fuels in future decades may contribute to dangerous mass fires beyond the predictive capacity of current fire models. These fires can generate their own wind and weather conditions and create extensive spotting, making fire behavior and its impact on structures and public safety difficult to manage and predict. In addition, such intense fires could prevent forests from becoming re-established. Lacking the legacy of live trees that historic FF would have left (Stephens et al. 2008), large unburned areas of dead trees may also produce unusual forest succession patterns. These patterns will likely favor shade-tolerant and hardwood tree regeneration, limited shrub growth, and accumulating large woody fuels that would likely kill regenerating forests when wildfire inevitably occurs. The scale of contiguous tree mortality entrenches the homogeneity produced by fire suppression, reducing the fine-scale heterogeneity of forest conditions that contributes to resilience and biodiversity. Management could enhance adaptation to climate-change-induced stress if it focused more of its resources on creating spatially and temporally variable patterns in green FF forests that are better aligned with local moisture availability and fire patterns (North et al. 2009).

Many of our FF forests have failed to receive the very management that could increase resilience to disturbances exacerbated by climate change, such as the application of prescribed fire and mechanical restoration treatments (Stephens et al. 2016). Recent tree mortality raises serious questions about our willingness to address the underlying causes. If our society doesn’t like the outcomes from recent fires and extensive drought-induced tree mortality in FF forests, then we collectively need to move beyond the status quo. Working to increase the pace and scale of beneficial fire and mechanical treatments rather than focusing on continued fire suppression would be an important step forward.

California reservoirs still suffering from drought

Above: the status of the large reservoirs in California as of February 16, 2016, showing the current water levels and the historic average for the date. California Department of Water Resources.

In spite of significant rain over parts of California over the last six months all but one of 12 large reservoirs in the state are still storing water at levels below the historic average for the date. Folsom Lake has 117 percent of average while the other 11 have from 30 to 80 percent.

Precipitation predicted for Thursday in the Sierra Nevada Mountains should help a little, with some areas above 7,000 feet receiving a foot or more of snow.

snow Sierras 2-18-2016
Snow prediction Sierra Nevada Mountains, 0400 Feb. 17 through 1600 Feb. 18.

The photo below shows the extreme northern end of Trinity Lake on August 9, 2014 when it held about 40 percent of average. On February 16 of this year it was at 43 percent.

Trinity Lake drought
North end of Trinity Lake in northern California, August 9, 2014. Photo by Bill Gabbert.

It remains to be seen how the winter weather will affect the 2016 wildfire season. It is a factor of course, but more significant is the weather DURING the fire season. If it is hot, dry, and windy, there will be major fires.

Wildfire potential through March, 2016

On December 1 the Predictive Services section at the National Interagency Fire Center issued their Wildland Fire Potential Outlook for December, 2015 through March, 2016. The data represents the cumulative forecasts of the ten Geographic Area Predictive Services Units and the National Predictive Services Unit.

If their forecasts are accurate, it looks like a continuation of pretty benign conditions across the United States this winter.

Here are the highlights from their outlook.

December
December wildfire 2015 Outlook

  • Significant fire potential is normal across the majority of the U.S.
  • Below normal significant wildland fire potential will persist across most of the Southeastern U.S. and Puerto Rico.

January
January 2016 wildfire Outlook

  • Significant fire potential is normal across the majority of the U.S.
  • Below normal significant wildland fire potential will persist across most of the southeastern U.S. and Puerto Rico.

February-March
February-March 2016 wildfire Outlook

  • An area of above normal significant fire potential will develop across the central interior portion of the eastern U.S. Above normal potential will also affect the Hawaiian Islands.
  • Below normal significant wildland fire potential will persist across most of the Southeastern U.S. and Puerto Rico.
  • Significant fire potential is normal across the majority of the U.S.

And as a bonus — the Drought Monitor:

Drought Monitor December 1, 2015

Drought Monitor Change

Bonus #2, Percent of Normal Precipitation:

Precip percent of normal California

How much precipitation is needed to end the drought in western states?

precipitation end drought 3 months

The three-year drought in the western United States and especially in California became more obvious this year as wildfires were influenced by low moisture in live vegetation, and in some areas once-healthy trees began to show drought-induced stress.

The current El Niño is expected to influence weather patterns during the coming winter and forecasters predict higher than normal precipitation across the southern portions of the United States, including southern California.

The map above illustrates how much precipitation is needed over a three-month period to end or ameliorate the current drought. Most of northern California will need from 6 to 12 inches according to NOAA.

Drought Monitor 10-27-2015

 

NOAA’s disclaimer about the map at the top of the article:

This [map] only tells you how much precipitation a location needs to get the Palmer Hydrological Drought Index (PHDI) to a certain value based on the model’s equations. It does not tell you how much precipitation is needed to refill a reservoir, restore groundwater to normal, or bring an ecosystem back to normality. It also does not incorporate snowpack into its calculations, and mountain snowpack is a crucial part of hydrology in the U.S. West.

New York Times, on California’s drought and its effect on wildfires

NY Time fire article
NY Times article article about California’s drought and the effect on wildfires. (click to enlarge)

The New York Times has a visually stunning article about the drought in California and its effect on wildfires. It was published about four days before record-setting rain in the south part of the state caused flooding and a bridge washout on a heavily-travelled Interstate Highway. This one rain event, however, will not turn around the drought, or have any lasting effect on the trees and brush that have already died due to lack of water.

The article was written by Haeyoun Park, Damien Cave, and Wilson Andrews. The photos are by Zackary Canepari. If you enjoy seeing awesome photos, especially of wildfires, check it out — preferably on a computer with a large monitor.

Thanks and a tip of the hat go out to Roy.

CAL FIRE says vegetation conditions are the worst on record

Redding, CA sunset CAL FIRE engines.
Sunset in Redding, California enhanced by smoke from the Eiler Fire, August 10, 2014. (Click to see a larger version.) Photo by Bill Gabbert.

Below is an excerpt from an article at Capital Public Radio:

…CAL FIRE says the timing of this year’s rains and four years of drought will combine to make fire conditions in 2015 the worst on record.

“We measure the fuel moisture content of all of the vegetation -the brush and the trees and we track that over the course of time and compare it month to month each year,” says Ken Pimlott, Director of CAL FIRE. “And we put it through formulas and determine how much energy and how much heat it will put out when it’s burning. And we have seen -we saw it last year and we will see it again this year- we’ll be reaching records for potential heat output for times of the year that would normally not be burning in those conditions.

CAL FIRE Director Ken Pimlott urged homeowners to clear space and conserve water.

“We don’t have water to water lawns and unnecessary landscaping. So, what that means is, is you need to  remove that vegetation as it dries. We don’t want your dry lawn and your dry brush to contribute to more of the fire hazard. So, stop watering your lawn and remove it.”

Thanks and a tip of the hat got out to Barbara.