www.sciencemag.org SCIENCE VOL 342 4 OCTOBER 2013 41

POLICYFORUM

 With projected climate change, we 

expect to face much more forest 

fi re in the coming decades. Policymakers are challenged not to categorize all 

fires as destructive to ecosystems simply 

because they have long fl ame lengths and kill 

most of the trees within the fi re boundary. Ecological context matters: In some ecosystems, 

high-severity regimes are appropriate, but climate change may modify these fi re regimes 

and ecosystems as well. Some undesirable 

impacts may be avoided or reduced through 

global strategies, as well as distinct strategies 

based on a forest’s historical fi re regime.

Fire regimes are commonly characterized 

by burn frequency and severity within a given 

area. Severity is often estimated as the proportion of overstory trees killed by fi re. In 

general, as frequency increases, fuels have 

less time to accumulate, reducing intensity 

and subsequent tree mortality. However, a 

great deal of variation occurs even within fi re 

regime types ( 1). The spatial scale and patchsize distribution of different severity classes 

are key in assessing whether fire regimes 

have changed over time and whether changes 

maintain or compromise forest ecosystems.

Globally, fi re frequency and severity vary 

among forest types. Essentially all fi res have 

high-severity effects, where most of the trees 

are killed, at some spatial scale and patch 

size. The critical issue is whether tree mortality patch sizes (and their temporal and spatial frequency) allow recovery of the same 

or similar vegetation types. If high-severity 

patch sizes are too large, microclimates and 

regeneration mechanisms (e.g., seed abundance and dispersal) can limit tree reestablishment (see the fi gure) . Large high-severity patches may produce vegetation type 

changes, especially in forests adapted to frequent, low- to moderate-severity fi re regimes 

or in forests that lack in situ propagule 

sources. Introduced species, such as nonnative grasses, also may alter forest fi re regimes 

and lead to changes in vegetation type ( 2). 

Changing fi re severity is at the heart of 

ecological debates about historically highfrequency, low- to moderate-severity fire 

regimes, such as ponderosa pine (Pinus ponderosa) and semiarid mixed-conifer forests. 

A central concern is whether high-severity patches in wildfi res are too large, which 

results in undesirable ecosystem changes 

(see the fi gure). Rising temperatures, related 

drought stresses, and increased fuel loads are 

driving high-severity patches to extraordinary sizes in some areas ( 3).

In contrast, forests adapted to low-frequency, high-severity regimes such as Rocky 

Managing Forests and Fire 

in Changing Climates

LAND USE

S. L. Stephens, 1 * J. K. Agee, 2 P. Z. Fulé, 3 M. P. North, 4 W. H. Romme, 5 T. W. Swetnam, 6

M. G. Turner 7 

Policy focused on fi re suppression only delays 

the inevitable.

*Corresponding author. E-mail: sstephens@berkeley.edu

1University of California, Berkeley, CA 94720, USA. 2University of Washington, Seattle, WA 98195, USA. 3Northern Arizona University, Flagstaff, AZ 86011, USA. 4U.S. 

Department of Agriculture Forest Service, Pacifi c Southwest 

Research Station, Davis, CA 95618, USA. 5Colorado State 

University, Fort Collins, CO 80523, USA. 6University of Arizona, Tucson, AZ 85721, USA. 7University of Wisconsin, 

Madison, WI 53706, USA.

Prefire forest High postfire resilience Low postfire resilience

High-frequency, low-severity fire Low-frequency, high-severity fire

Historical forest fi re regimes. (Top) Mixed-conifer forest in northern California 

with fuels accumulated from a century of fi re suppression (left), mature surviving and regenerating trees in an area that had been mechanically thinned to 

reduce fuels and residues either removed or burned (center, 10 years after the 

2002 Cone Fire), and an adjacent untreated area lacking live seed trees, now 

dominated by shrubs (right, 10 years after the Cone Fire). (Bottom) Lodgepole 

pine forests in Greater Yellowstone (left) regenerated abundantly from the canopy seedbank after the stand-replacing 1988 Yellowstone Fires (center, 15 years 

postfi re), but regeneration was greatly reduced in forests of comparable age and 

serotiny after the 2000 Glade Fire, which was followed by summer drought 1 year 

after the burn (right, 10 years postfi re). Forests are within 4 km of each other 

CREDIT: (TOP) MALCOLM NORTH; (BOTTOM) MONICA TURNER

at each site.

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42 4 OCTOBER 2013 VOL 342 SCIENCE www.sciencemag.org 

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Mountain lodgepole pine (Pinus contorta

var. latifolia) have evolved to regenerate after 

large, high-severity events. Seed banks stored 

in tree crowns survive even the highest-severity fi res and are released shortly after the fi re 

ends. If seeds germinate in open conditions 

conducive to relatively high growth rates, a 

new forest can become established in a few 

decades (see the fi gure). Other species can 

regenerate a new crown from one burned by 

fi re because of dormant buds.

Future Fire Under Changing Climates

With projected climate warming ( 4), forests around the globe will likely undergo 

major landscape-scale vegetation changes 

in coming decades. In some areas, plant productivity may decline to a point where fi re 

will become less frequent ( 5). In more productive areas, fi re regimes may shift from 

being mostly climate-controlled (top-down) 

to mostly fuel-controlled (bottom-up) ( 6). 

In both cases, slow vegetation change may 

be abruptly accelerated by a change in fi re 

regime driven by novel climatic conditions.

Increased frequency and size of large, 

severe forest fi res are expected in Australia, 

the Mediterranean Basin, Canada, Russia, 

and the United States ( 3, 7, 8). In the western 

United States, increased frequency and size 

of fi res is associated with increased temperatures, earlier spring snow melt, and longer 

fi re seasons ( 9)—mechanisms that are applicable to other regions of the world.

Trends and projections of climate and fi re 

responses suggest that new strategies to mitigate and adapt to increased fi re are needed 

to sustain forest landscapes. Identifying 

and implementing appropriate responses 

will not be easy because the complexity of 

local-to-regional dynamics makes uniform, 

simple, or unchanging policy and management strategies ineffective ( 10). It is especially diffi cult to motivate social response to 

environmental transitions that unfold slowly 

and are thus diffi cult to detect before it is 

too late ( 11).

We suggest strategies for forests of all fi re 

regimes: Landowners should follow “Firewise” guidelines (www.firewise.org/) for 

houses and other infrastructure. Increased 

development in fi re-prone landscapes has 

increased suppression costs, exacerbated 

risk to human safety and infrastructure, and 

reduced management options. People living 

in these forests must be prepared rather than 

relying solely on fire departments. Some 

places may be so hazardous that building should be prevented, discouraged, or 

removed (e.g., by regulation or insurance 

and/or tax incentives).

Fire managers should avoid trying to uniformly blacken wildfi re landscapes through 

burnout and mop-up operations, especially 

in burn interiors. As wildfire sizes have 

grown in recent decades, direct attack has 

been replaced with indirect attack, where 

fi re lines are placed some distance from the 

active fi re front, and then the area between is 

intentionally burned, often with high-severity 

fi re, to reduce fuel and create a wider fi re barrier. Unburned or partially burned patches are 

critical refugia that aid postfi re recovery in 

forests of all fi re regimes and should be conserved whenever possible.

Land managers could anticipate changes 

using models of species distribution and 

ecological processes and should consider 

using assisted migration ( 12). Dominant 

forest species may be unable to recover 

from fi res with large high-severity patches. 

Replacement ecosystems of shrublands or 

grasslands may provide some ecological 

benefi ts, but they offer very different habitats for wildlife and have reduced carbon 

storage relative to native forests.

We also suggest several distinct strategies based on a forest’s historical fi re regime. 

Mitigation in forests with historically highfrequency, low- to moderate-severity fire 

regimes: (i) Restore resilient forest structure similar to historical patterns that survived during past high-fi re periods (and those 

anticipated in the future) (see the figure). 

Fuel reduction and restoration treatments 

can increase resiliency by reducing density-dependent tree mortality ( 4) and excessive insect and/or disease problems and can 

increase spatial heterogeneity.

(ii) Fund forest restoration. We know how 

to treat forests to reduce fi re hazards, with generally positive or neutral ecological effects, 

although impacts to wildlife with large home 

ranges have not been fully assessed ( 13). Public acceptance of these treatments is increasing ( 14); the barrier is cost. Treatment rates 

are far below what is needed for landscape 

resilience ( 15). Because the federal government has no jurisdiction in development policies in the privately owned urban-wildland 

interface, state and local jurisdictions could 

pay for fi re suppression in the interface. This 

would enable a signifi cant increase in critical 

forest restoration funding and would probably reduce building in the interface.

Adaptation in forests with historically 

low-frequency, high-severity fire regimes:

(i) Expect changes in forest type and age 

across the landscape (see the fi gure). Some 

forest types will be relatively resilient to 

more frequent fi res, notably resprouting or 

seed-banking species. However, even these 

forests will likely exhibit substantial changes 

in landscape structure, such as shifts to a 

preponderance of young stands ( 16).

(ii) Some forests will change to nonforest 

vegetation after fi re. Spruce-fi r (Picea-Abies) 

and interior Douglas fi r (Pseudotsuga menziesii) forests may exhibit large changes in 

structure and species composition because 

they lack persistent seed banks or sprouting capability. Some areas may even shift to 

a nonforest state, especially if trees cannot 

reestablish in a warmer, drier climate. Such 

changes will not necessarily be catastrophic 

(e.g., a shift to nonforest could potentially 

increase water yield) and could be expected 

to reduce intensity of subsequent fi res. However, shifting from forest to nonforest would 

affect most ecosystem services. There are 

no clear guidelines for increasing the resilience of these forest types—unlike for forests 

adapted to high-frequency, low- to moderateseverity fi re regimes—other than minimizing 

additional stresses from excessive grazing, 

recreation, and salvage logging.

The annual cost of fire suppression is 

increasing and unsustainable; costs exceeded 

$2 billion in the United States in 2012. Fire 

policy that focuses on suppression only 

delays the inevitable, promising more dangerous and destructive future forest fi res. In 

contrast, land management agencies could 

identify large fi resheds (20,000 to 50,000 ha) 

where, under specifi ed weather conditions, 

managed wildfi re and large prescribed fi re 

are allowed to burn, sometimes after strategic 

mechanical fuel treatments ( 15). Acknowledging diversity in fi re ecology among forest types and preparing forests and people 

for larger and more frequent fi res could help 

reduce detrimental consequences.

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10.1126/science.1240294

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