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United States Department of Agriculture 

Cre ating More Res ilient 

FORESTS 

Through Active Manag ement 

Despite their stately nature and presumable permanence on the landscape, forests are actually 

always in a state of fux. Growing and dying, one species succeeding another, with the 

occasional disturbance, such as a wildfre or insect infestation, to wipe the slate clean and allow 

the process to start anew. And beyond these successional dynamics, forests change in structure, 

composition and ecological function over longer times as climates naturally vary. 

Summary 

Drought, combined with higher temperatures, 

g

ss 

es 

is contributing to “megadisturbances” that 

threaten western forests and require a 

new approach to silviculture management. 

Malcolm North, Connie Millar and Jessica 

Wright, researchers with the U.S. Forest 

Service’s Pacific Southwest Research

Station, have examined how changing 

climate conditions will likely transform 

the forests of the future and make 

recommendations on how to achieve 

resiliency. 

Reduced density and diversification of

species has shown promise in creating 

resistance to traditional and new threats. 

More detailed recommendations by North 

call for mimicking the effects of naturally 

occurring and regular fires. Southern-facin

slopes benefit from wider spacing of larger

trees. Cool, moist areas that experience le 

fire intensity can support denser growth.

Managing individual stands for ladder fuels 

must be combined with regional approach 

that include varying stand density and the 

presence of open spaces. 

Healthy forests typically are able to accommodate 

the minor, as well as the occasionally abrupt 

and more severe, disturbance and over time adapt 

to naturally changing climates in diverse ways. 

Ecologists ascribe this ability as “resilience.” However, 

when the magnitude of the disturbance, the frequency 

of the occurrence, pace of change, or a number of 

other extraneous factors make it difcult for a forest 

to adapt, then ecosystems can change quite drastically. 

Connie Millar and Malcolm North, researchers with 

the U.S. Forest Service’s Pacifc Southwest Research 

Station, have been studying how disturbances and 

climatic conditions have shaped Californian and 

Great Basin forest ecosystems from both a historical 

and pre-historical perspective. Teir research is 

giving land managers insights on how to successfully 

shift forest structure and species composition to 

create landscapes that are better able to survive 

today’s disturbances, as well as weather tomorrow’s 

uncertainties. 

How Much Stress Can a Forest Take? 

Te evolving understanding of forest dynamics has 

led to greater recognition that fres, insects and 

diseases are essential parts of a normal, functioning 

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2 | Creating More Resilient Forests Through Active Management 

ecosystem and are, in fact, 

necessary for forest health. 

And while drought, too, has 

always existed as part of the 

cyclical nature of weather 

patterns, researchers have dubbed 

the new combination of heat 

and drought “global change 

droughts” or “hotter droughts.” 

Tis new breed of hotter drought 

intensifes stress on trees, making 

them more vulnerable to other 

factors, such as insects and 

diseases. Higher temperatures 

also increase water loss through 

evaporation, as well as reduce 

snowpacks, which play a vital 

role in storing water that sustains 

landscapes through drier seasons. 

Millar, a senior research 

ecologist, co-authored a research 

paper in 2015 titled “Temperate 

Forest Health in an Era of 

Emerging Megadisturbances.” 

In it, she discusses how much 

disturbance a forest can tolerate, 

what the long-term efects might 

be, and how to minimize losses. 

“Megadisturbances,” defned 

in this case as the combination 

of extreme drought and other 

stressors, can drive abrupt tree 

mortality, according to Millar. 

She and her co-author predict 

that some forests will continue 

to rebound from these stresses. 

Over the long term, however, 

most temperate forests are likely 

to adjust and adapt Te efects 

will range from minor changes 

in forest density, age and species 

composition, to more signifcant 

changes that include major 

transformations to vegetation 

types that can better survive 

in the new environmental 

conditions. 

As land managers try to cope, 

Millar describes a typical cycle 

where foresters frst try to 
Forest Service, Pacifc Southwest Research Station | 3 

maintain tracts that refect the 

current norm. Tis can lead to 

some success, but if changes in 

climate and stressors are part of a 

longer-term trend, the forests will 

ultimately be rendered unstable 

and not adapted to current 

conditions. Once a threshold is 

exceeded, substantial mortality 

can occur and recovery can be 

slow or a new forest type can 

take hold. 

“We have to understand that 

historic conditions may not be 

appropriate now as a guide,” 

Millar said. “If we want to use Researchers at the Pacifc a general technical report 

historic conditions as a guide, we Southwest Research Station authored by Malcolm North 

must fnd a time in the past that are conducting studies in the and his colleagues at the Pacifc 

is most like where we’re headed Stanislaus-Tuolumne, Sagehen Southwest Research Station, 

in the future.” and Teakettle Experimental ofers solid guidelines. 

Forests to better understand Millar’s work in paleoecology, “Simply restoring the forests to how to create a forest structure a feld of study that examines their previous conditions is not that can be sustained over time, ecological shifts across millennia, the right approach,” said North, is resilient to disturbances, and recommends management a research forest ecologist. “Given supports wildlife and recreation. practices that infuence the the reality of a changing climate transition to a more resilient While those fndings are not yet and recurring drought, we need forest. Resource managers must published, one thing is becoming a new management strategy, anticipate the evolution of the clear: Land managers need to be one that goes beyond short- forest and even facilitate it by ready to evolve their approach term fuel treatment objectives taking measures to reduce the to woodland maintenance and and incorporates long-term probability of sudden die-back, for reforestation in response to ecological restoration and habitat example, by thinning forests to emerging conditions. improvement. We are conducting reduce competition for resources. Active Forest research now that will better 

Te goal is a more gradual Management Required inform those strategies and transition. Tis sort of managed practices.” change prevents the total collapse For foresters who are ready 

of the ecosystem and builds the to proactively manage for the Reforestation approaches where 

evolved forest back up to a point changing climate, “An Ecosystem new trees are planted close 

where the forest, albeit changed, Management Strategy for together with future plans for 

continues to fourish. Sierran Mixed Conifer Forests,” thinning, while once the norm, 
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4 | Creating More Resilient Forests Through Active Management 

may not always be the most 

efective or expedient method, 

according to North. 

Instead, North asserts that 

complexity – or heterogeneity 

– is the forest’s friend and ally, 

improving trees’ ability to 

respond to stress and support 

wildlife. Complexity includes 

varying stand density, including 

openings of various sizes, a mix 

of tree sizes and ages, and the 

presence of shrubs and meadows. 

Given enough space, sunlight, 

nutrients and water, trees have 

the capacity to grow and thrive. 

Tey become stressed when 

lacking any of these elements, 

which make them more 

susceptible to attack by insects 

and diseases, as well as drought 

or wildfre. 

Similarly, because insect and 

disease populations tend to 

target one type or species of tree, 

diversity will also help reduce the 

possibility of one insect or disease 

killing all the trees in a region. 

Ramiro Rojas, a deputy regional 

silviculturist with the Forest 

Service’s Pacifc Southwest 

Region, has seen North’s 

recommendations put into 

practice and reports good results. 

“Where we are able to reduce 

density and create heterogeneity, 

those stands seem to have far 

more survivors, both large and 

small trees,” Rojas said. “Where 

there’s more density, they don’t 

do as well.” 

Rojas emphasized that the Pacifc 

Southwest Research Station’s 

previously mentioned general 

technical report provides a basic 

framework, not a one-size-fts-all 

solution. Foresters who choose 

to implement North and his 

colleagues’ recommendations 

must do so based on their 

unique, local circumstances, 

Rojas said. For example, 

sometimes there is a tension 

between creating forest tracts 

that can survive drought, fre and 

pests, and maintaining a habitat 

suitable for important species, 

like the spotted owl or fsher, 

which require more density. 

Other factors afect the success 

of implementation of the 

recommendations within the 

report, Rojas noted. For example, 

the quality of the soil, elevation 

of the forest, size of the trees 

and the localized intensity of the 

drought all come into play. 

Moving Trees Uphill 

If forests are going to be 

replanted, which species and 

which seed sources make the 

most sense under projected 

climate scenarios? Jessica Wright, 

a research geneticist with the 

research station, is studying 

whether shifting to trees from 

warmer climates leads to greater 

survivorship in the face of 

stressors. 

“Te traditional approach was 

to plant seeds from the same 

seed zone when replanting,” 

Wright said. “But now it is 

being considered appropriate 
Forest Service, Pacifc Southwest Research Station | 5 

to move seeds uphill 500 feet 

to compensate for the warmer 

temperatures we are seeing.” 

Wright pointed to an online 

resource called the Seedlot 

Selection Tool (https:// 

seedlotselectiontool.org/sst/) for 

help in replanting that is suited 

for changes in climate. 

Te Seedlot Selection Tool is a 

collaboration between the U.S. 

Forest Service, Oregon State 

University and the Conservation 

Biology Institute. It is a webbased mapping application 

designed to help natural resource 

managers match seedlots with 

planting sites based on climatic 

information. It allows the Mimicking Nature — The Role of Fire 

user to control many input North and his co-authors recommend creating landscape diversity

parameters, such as selecting in the Sierra Nevada by mimicking the forest conditions that would 

particular climate variables (i.e., be the result of regular, natural fires.

temperature and precipitation), Following this model, cool or moist areas, where historically fire

and climate change scenarios. would have burned less frequently or at lower severity, would 

Te generated maps include the have higher density and greater canopy cover. In contrast, upper, 

southern-facing slopes, which receive more sunlight, would have seed zones, allowing the user low densities of large fire-resistant trees. to compare the results of the 

When planting trees, then, North’s research suggests introducing tool with traditional planting a diverse mix of trees planted at varying densities, based on their approaches. position on a slope, or the steepness of the slope, as well as the 

“We just planted more than availability of water. 

1,000 pine trees as part of a For individual groupings of trees, known as stands, vertical diversity 

post-wildfre reforestation project can be achieved by organizing groups of trees based on their 

canopy height, or strata, according to North. For example, a group using trees from warmer and of intermediate-sized trees that could serve as ladder fuels might drier locations,” Wright said. be thinned or removed if they are growing under taller trees. The “We are monitoring to see how same-size trees in a discrete group, however, might be left alone if

they are perform. Te idea is the group does not present a ladder fuel hazard for taller trees. 

to answer the question: Do This strategy would produce within-stand vertical differentiation, as these trees from warmer places well as discrete tree clusters, which would contribute to horizontal 

perform better here in this time heterogeneity. 

of changing climate?” 
6 | Creating More Resilient Forests Through Active Management 

Te results from this study will be compared to 

the predictions made by the Seedlot Selection 

Tool to determine how well it predicted plant 

performance, and particularly which climate 

variables were most closely associated with plant 

survivorship. 

Conclusion: The Time to Act 

Is Now 

Te current environmental conditions and 

projections that call for higher temperatures, 

drought and related disturbances will require a 

more active and thoughtful approach to forest 

management if we are to adequately maintain 

the resources forests provide. Monitoring and 

ongoing research will continue to shed light 

on the efectiveness and best applications of 

these eforts, but the important thing is to start 

implementing the most promising of those 

practices now. 

“Tere has to be a sense of urgency,” Rojas said. 

“We cannot lose hundreds of thousands of acres 

of forest every decade.” 

What’s Next? 

Malcolm North and several of his colleagues 

at the Pacifc Southwest Research Station are 

currently examining how extensive tree mortality 

from a decade-long drought in California might 

afect wildfre and the future development of the 

forest as it responds to and re-establishes in areas 

where most of the large overstory trees have died. 

Studies suggest that managed fre is probably 

the best, most economical means of thinning 

and restoring forests over the large expanses of 

afected forests, but such broad use isn’t always 

feasible. North is working with scientists and 

land managers to address fre implementation 

issues, such as air quality, liability, risk aversion 

and cost. 

Key Findings 

• The emergence of “hotter droughts” are creating 

previously unseen conditions that are likely to 

change forests as we know them in the Sierra 

Nevada. 

• Hotter droughts, combined with other 

disturbances, can cause abrupt tree mortality. 

Some forests can recover, but other regions 

could transition to a landscape with far fewer 

trees, unless steps are taken to intervene. 

• Historical norms may no longer serve as a guide 

for coping with tree mortality, and foresters must 

find ways to manage for new climate projections.

• Managing the transition of forests to new norms 

can prevent a total collapse of an ecosystem. 

• Mimicking the role of fire in shaping forests can

create a more resilient ecosystem. 

Land Management Implications 

• Trying to maintain current forest structures may 

be futile in the face of long-term climate changes

and the related stressors of insects, drought and 

diseases. 

• Shaping forests that are best suited to survive 

means anticipating what climate change will 

bring. 

• The Pacific Southwest Research Station’s

general technical report “An Ecosystem 

Management Strategy for Sierran Mixed Conifer 

Forests” (GTR-220) provides guidelines on

how to structure forests to create resiliency, but 

individual conditions must be taken into account. 

• The Seedlot Selection Tool (https:// 

seedlotselectiontool.org/sst/), used in

conjunction with current seed zones, can help in

selecting trees for replanting that are suited for 

projected changes in climate.
 

Forest Service, Pacifc Southwest Research Station | 7 

For Further Reading Scientist Profiles

Malcolm North is a research 

forest ecologist with the 

U.S. Forest Service’s Pacific

Southwest Research Station.

His work includes research 

examining different management 

practices and the effect of 

forest structure, composition and function. His lab 

focuses on forest and fire ecology of Sierra Nevada

mixed-conifer forest. He has worked at several

experimental forests, examining new silvicultural 

practices and their ability to increase forest 

resilience to fire and drought.

Connie Millar is a senior 

research ecologist with the 

U.S. Forest Service’s Pacific

Southwest Research Station.

Her research team focuses on 

the role of historic and ongoing 

climate change in high-elevation

forest ecosystems, including the Eastern Sierra 

Nevada. She is also involved in the integration of 

science with policy, and is particularly interested 

in interpreting and communicating the current 

research on climate change and its ecological 

effects in conservation and restoration. 

Jessica Wright is a research 

geneticist with the U.S. Forest 

Service’s Pacific Southwest

Research Station. She uses

genetic tools to understand 

population responses to threats 

including climate change, 

pathogens and stress in forest ecosystems. 

Her current research intends to understand the 

genetics underlying tree responses to climate and 

how populations might be impacted by a changing 

climate. 

• North, M.; Managing Sierra Nevada

Forests, 2012, Gen. Tech. Rep. PSWGTR-237. U.S. Department of Agriculture,

Forest Service, Pacific Southwest

Research Station

• North, M.; Lydersen, J.; Knapp, E.; Collins,

B.; 2013. Quantifying spatial patterns of

tree groups and gaps in mixed-conifer

forests: Reference conditions and longterm changes following fire suppression

and logging, Forest Ecology and

Management, 304: 370-382

• North, M.; Stine, P.; O’Hara, K.; Zielinski,

W.; Stephens, S. 2009. An Ecosystem

Management Strategy for Sierran Mixed

Conifer Forests, Gen. Tech. Rep. PSWGTR-220. U.S. Department of Agriculture,

Forest Service, Pacific Southwest

Research Station

• Millar, C; Stephenson, N. 2015. Temperate

Forest Health in an Era of Emerging

Megadisturbance, Science, 304: 823-826.

• Achieving Long-Term Forest Health and

Resilience in California, http://www.

fire.ca.gov/treetaskforce/downloads/

FHRWG%20White%20Paper.pdf

Web Resources 

• Stanislaus-Tuolumne Experimental Forest

https://www.fs.fed.us/psw/ef/stanislaus_

tuolumne/

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. 

USDA is an equal opportunity provider, employer and lender. 
8 | Creating More Resilient Forests Through Active Management