United States Department of Agriculture

WHEN THE 

WELL RUNS 

Drought sets the stage for crowded forests to succumb to bark beetles.

Chris Fettig has been studying tree mortality around the globe for 

decades, but he was unprepared for the large-scale event that hit California’s 

central and southern Sierra Nevada in the last several years. 

Summary 

A record die-off of ponderosa pines in the

Sierra Nevada in 2016 led Chris Fettig, a

research entomologist with the U.S. Forest

Service’s Pacific Southwest Research Station,

to investigate the cause and what precipitated

the event. While bark beetles typically get the

blame, Fettig suspected something more must

have been at work for such devastation to occur.

Fettig knew bark beetles have long played a role

in ecosystem dynamics and focusing on them

alone would not lead to an effective approach

to managing forests as a valuable resource.

Drought and other factors can create a perfect

storm that leave trees vulnerable to massive

bark beetle outbreaks. Even without the bark

beetle, the severe drought of the last several

years may have damaged trees beyond their

ability to recover.

Future climate projections call for hotter, drier

conditions that will continue to stress tree

populations in the Sierra Nevada. However, land

management practices can promote forests

that are more resilient to both drought and bark

beetles. Managing for appropriate stand density

and species type can help forests adapt to

changing conditions.

I

t is now well known that bark beetles and 

drought have been taking a heavy toll in the 

Sierra Nevada, killing an estimated 129 million 

trees since 2010. In 2016 alone, 62 million trees 

died, representing more than a 100 percent increase 

in tree mortality across the state compared to levels 

observed in 2015. Millions of additional trees 

are weakened and expected to die in the coming 

months and years. 

“Quite simply, I was surprised by the magnitude,” 

said Fettig, a research entomologist with the U.S. 

Forest Service’s Pacifc Southwest Research Station 

in Davis, California. “Tis is an event that we have 

not seen in recorded history for the species of bark 

beetle responsible for most of the tree mortality 

observed.” 

“In addition to the size of the area impacted, what 

makes this event unique is how fast it has occurred, 

the size of the trees involved and the fact that large 

numbers of people live and recreate in these areas,” 

Fettig said. 

Understanding the dynamics of why such a large 

die-of of trees occurred can help land managers 

cope with changing climatic conditions and ensure 

that economically, socially and ecologically valuable 

forests continue to thrive into the future. 

Forest Pacific Southwest March 2018

Service Research Station
2 | When the Well Runs Dry 

A Crime of Opportunity 

Of the 550 species of bark beetles in North 

America, only about 15 cause large amounts of 

tree mortality, and only eight of those are active in 

California. Fettig had a hunch about the culprit in 

this most recent die-of. 

Te western pine beetle is known to target 

ponderosa pines, the species of tree that has 

sufered the most. But for defnitive proof, Fettig 

set to work in the feld to examine trees and fnd 

out what exactly was responsible for causing the 

high levels of tree mortality observed. 

a distinctive fngerprint in the form of “gallery 

patterns” or etchings on the inner bark or phloem. 

When Fettig peeled back the bark of dead 

and dying trees to examine their phloem, the 

fngerprints were unmistakable. Te western pine 

beetles had launched a massive attack against 

ponderosa pines in the Sierra Nevada. But bark 

beetles have been a part of the natural ecosystem 

for millennia, so how did things spiral out of 

control so quickly? 

Healthy trees emit sap (resin or pitch) into the 

An adult bark beetle bores into a tree where it holes that bark beetles create in the tree bark. 

lays its eggs. Te larvae then hatch and Tis sticky resin can control the attackers by 

the young beetles spend most of their drowning them. Terefore, for such a huge 

lives in the tree before emerging as kill to occur, there had to be something else at 

mature beetles ready to repeat the work. Something that set the stage for the 

cycle. During its life cycle, each bark destruction. It looked suspiciously like a 

beetle species in California leaves crime of opportunity. 
Forest Service, Pacific Southwest Research Station | 3 

1929 Tree density conditions

2008 Tree density conditions

Too Many ‘Straws’

In The Ground 

To understand the relationship 

between drought and bark 

beetles, Fettig and his colleagues 

poured over hundreds of past 

research papers. Teir fndings 

were summarized in the article 

“Observed and Anticipated 

Impacts of Drought on Forest 

Insects and Diseases in the 

United States,” published in the 

May 2016 issue of Forest Ecology 

and Management. 

Fettig knew that severe bark 

beetle outbreaks occur when 

trees are stressed. Tis can be 

caused by a number of factors, 

such as drought, warmer 

temperatures, air pollution, or 

overcrowded forests that create 

competition for resources. 

When forest density is high, 

competition for water is 

amplifed, and with too many 

“straws” trying to pull 

moisture from the ground, 

trees are unable to get the 

water they need to 

maintain their 

vigor and repel beetles. Trees in 

less dense stands are generally 

healthier and better able to repel 

beetles and survive attacks. 

Thirsty Trees Are

Vulnerable Trees 

Research also repeatedly showed 

how drought conditions cause 

individual trees to react in a way 

that creates an opportunity for 

pests. Trees that aren’t getting 

enough water close their stomata, 

the tiny openings in leaves and 

needles through which plants 

breathe, to minimize loss of 

moisture. Closing the stomata 

also reduces the uptake of carbon 

dioxide, which in turn decreases 

photosynthesis, slowly starving 

the trees. Tis leaves them weak 

and less efective at defending 

themselves from bark beetles and 

other pests. 

Intense drought also afects the 

ability to transport water, a tree’s 

lifeblood, throughout the plant. 

With insufcient precipitation, 

bubbles form in the 

xylem, or vascular tissue, 

creating a condition 

known as cavitation. Tis can 

prevent the distribution of much 

needed water up the trunk and 

out to limbs, again, weakening 

the tree and its defenses. Tis 

condition also afects the ability 

to deploy terpene-rich resins that 

trees use to “pitch out” invaders. 

After several years of severe 

drought, the winter of 2016-

2017 yielded record precipitation 

in much of California. Tat 

might sound like good news, 

but it won’t necessarily give 

damaged trees relief. “Cavitation 

can permanently damage trees, 

meaning heavily cavitated trees 

may still die despite this winter’s 

precipitation,” Fettig said. 

However, Fettig expects to see 

a decline in most bark beetle 

numbers in the Sierra Nevada in 

2018 and associated reductions 

in levels of tree mortality. Te 

exception is in the most heavily 

impacted areas, where most 

of the susceptible hosts have 

already been killed. Tere, little 

or no change is expected to be 

observed. 
4 | When the Well Runs Dry 

Managing For Resilience 

Fettig’s research also focuses on increasing resilience 

to future droughts and bark beetle infestations 

through land management that better matches 

stand densities to water availability. In April 2007, 

he and his colleagues published “Te Efectiveness 

of Vegetation Management Practices for Prevention 

and Control of Bark Beetle Infestations in 

Coniferous Forests of the Western and Southern 

United States.” Teir fndings include the following: 

• Bark beetles are a natural component of forest 

ecosystems. Eradication is neither possible nor 

desirable, and periodic outbreaks will occur as 

long as susceptible forests and favorable climatic 

conditions exist. Eforts to prevent undesirable 

levels of bark beetle-caused tree mortality must 

focus on changing stand susceptibility through 

reductions in tree competition, disrupting 

beetles’ ability to fnd hosts and reducing the 

fecundity, ftness and survivorship of target bark 

beetle species. 

• Eforts to prevent undesirable levels of bark 

beetle-caused tree mortality at the landscape 

level must account for the spatial distribution of 

both forest types and stand ages. In many areas, 

treatments should be implemented to increase 

heterogeneity. 

• Management to reduce stand or landscape-level 

susceptibility to bark beetles must address factors 

related to tree density. 

Te large loss of trees in 2016 from bark beetles 

and drought highlights the indirect, but profound 

efects that climate can have on an ecosystem. And 

while foresters and land managers cannot control 

the amount of rain that will fall in the future, they 

can take steps that will help forests emerge with 

a better chance of survival while continuing to 

constitute a valuable resource for the region and the 

nation. 

Key Findings 

• Drought stresses trees in a number of ways, 

making them less able to resist pests like bark 

beetles. 

• In times of drought, trees close the tiny 

openings, called stomata, in their leaves or 

needles to conserve moisture. But this reduces 

the trees’ carbon dioxide uptake and their 

ability to photosynthesize, thereby reducing 

tree health and vigor. 

• Cavitation of the internal vascular structures 

trees use to transport water throughout 

the plant can be permanently damaged 

by drought. Insufcient moisture also 

compromises the trees’ ability to produce 

resin and “pitch out” bark beetles. 

Land Management

Implications 

• Meteorological forecasts point to the southern 

Sierras transitioning to a climate that will be 

drier, making it increasingly important to 

manage forests for drought and resistance to 

pests. 

• Te most efective way to help trees resist 

bark beetles in times of drought is to manage 

stand densities prior to drought to reduce 

competition for vital resources. Less dense 

stands better refect the historical state of 

forests. 

• Actively managing forests through the use of 

mechanical thinning and prescribed burning 

can help reduce tree density and create a 

forest that is more resilient to bark beetle 

outbreaks. 

• Land managers may have to rethink which 

species are appropriate for their area in light 

of changing climatic conditions. For example, 

ponderosa pines might not be a good ft for 

lower elevations in the future as conditions 

may become too hot and dry to sustain them. 
Forest Service, Pacific Southwest Research Station | 5 

 

 

Scientist Profile

Chris Fettig is a research

entomologist with the Pacific

Southwest Research Station.

He studies the roles and

impacts of insects in forest

ecosystems, as well as how

to mitigate their effects.

He received his Ph.D. in

forest entomology from the

University of Georgia in

Athens. 

Bark beetle being expelled by a 

healthy tree

Questions & Answers

Could we have averted or limited this large-scale

tree mortality event? 

Mechanisms contributing to bark beetle outbreaks are complex and 

include density-dependent and density-independent factors. However, 

two conditions must be met for an outbreak to occur: 1) Tere 

must be several years of weather conducive to beetle survival and 

population growth; and 2) there must be an abundance of susceptible 

host trees. 

Most experts agree that the use of prescribed fre and mechanical 

thinning treatments to decrease competition among trees would 

have reduced the scale and severity of tree mortality observed in 

the central and southern Sierra Nevada. However, we must also 

acknowledge the unusual level of drought stress observed in these 

forests, which reduces both the efectiveness of these treatments, as 

well as the ability of trees to repel bark beetle attack. 

Have bark beetles adapted to changes in the climate? 

In recent decades, billions of conifers have been killed by bark beetles 

in forests ranging from Alaska to Mexico, and several outbreaks 

are regarded as the largest and most severe in recorded history. For 

each of these records, we see a trigger that can be linked to climate 

change, whether expressed directly through the bark beetle species 

(e.g., increases in temperature that increase the number of generations 

produced) or indirectly through the host (e.g., the drought’s efect on 

ponderosa pine in the central and southern Sierra Nevada). 

With the drought over, at least for this year, how long

before trees are healthy enough to repel beetles? 

For some bark beetles, we expect to see an immediate decline in their 

population. Others are capable of causing widespread tree mortality 

for several years after drought conditions have been alleviated. Te 

western pine beetle, the species responsible for causing most of 

the tree mortality in the central and southern Sierra Nevada, fts 

somewhere in the middle. Given that the western pine beetle has 

multiple generations per year and initiates fight at the beginning of 

the growing season (i.e., before trees have time to respond to increases 

in growing space associated with tree mortality), we expect some 

elevated levels of tree mortality to continue in 2018, and then to 

begin to decline. However, in the most heavily impacted areas, most 

of the susceptible trees have already been killed, so we don’t anticipate 

changes in those tree mortality levels. 
6 | When the Well Runs Dry 

What’s Next?

Following the recent die-of of ponderosa pines in 

the Sierra Nevada, Fettig and his colleagues at the 

Pacifc Southwest Research Station have established 

research plots to determine which species of trees 

will emerge as new dominant or co-dominant 

species within the forest and what the tree species 

mix will look like. For example, as a thick layer 

of forest litter and dead pine needles carpet the 

ground, it could block ponderosa pine seeds from 

coming into contact with the soil and germinating. 

As some trees die and stands thin, the increase in 

sunlight reaching the forest foor also can afect 

what types of plants thrive or could result in the 

proliferation of weeds. So far, oaks and cedars seem 

to be the tree species least afected by this mortality 

event, but it remains to be seen what consequences 

that might have. 

Fettig is currently studying test plots to determine 

the optimal combination of species and stand 

densities for optimum health in a hotter, drier 

environment. Related work by Pacifc Southwest 

Research Station scientists is looking at how forest 

fuels are afected over time by dead and dying 

trees, and how this might contribute to fre risk and 

the danger of large-scale wildfres. 

For Further Reading 

• Anderegg, W.L. et al. 2015. Tree mortality from 

drought, insects, and their interactions in a 

changing climate. New Phytologists Journal.

Vol. 208, Issue 3, pp. 674–683.

• Bentz, B.J. et al. 2010. Climate change and 

bark beetles of the western United States and 

Canada: Direct and indirect effects. BioScience. Vol. 60, No. 8, pp. 602–613.

• Fettig, C.J. et al. 2007. The effectiveness of

vegetation management practices for prevention and control of bark beetle infestations in 

coniferous forests of the western and southern 

United States. Forest Ecology and Management Vol. 238, pp. 24−53.

• Fettig, C.J. et al. 2013. Changing climates, 

changing forests: A Western North American 

perspective. Journal of Forestry. Vol. 111, No.

3, pp. 214-228.

• Fettig, C.J. 2016. Native bark beetles and

wood borers in Mediterranean forests of 

California. In Lieutier, F. and T.D. Paine, eds.

Insects and Diseases of Mediterranean Forest

Systems. Springer International Publishing,

Switzerland, pp. 499–528.

• Kolb, T.E. et al. 2016. Observed and anticipated impacts of drought on forest insects and

diseases in the United States. Forests Ecology 

and Management. 380: 321–334.

Web Resources 

• http://bit.ly/SierraEcoRestoration

Learn more about the Pacific 

Southwest Research Station at 

https://www.fs.fed.us/psw or 

scan the QR code to the left with 

your smart device. 

Credits

Dry soil texture (front cover) 

by hotblack at morguefile.com 

(0001379338996) 

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