Content uploaded by Thomas William Swetnam
Author content
All content in this area was uploaded by Thomas William Swetnam
Content may be subject to copyright.
1418
Ecological Applications,
12(5), 2002, pp. 1418–1433
q
2002 by the Ecological Society of America
ECOLOGICAL RESTORATION OF SOUTHWESTERN PONDEROSA PINE
ECOSYSTEMS: A BROAD PERSPECTIVE
C
RAIG
D. A
LLEN
,
1,10
M
ELISSA
S
AVAGE
,
2
D
ONALD
A. F
ALK
,
3
K
IERAN
F. S
UCKLING
,
4
T
HOMAS
W. S
WETNAM
,
5
T
ODD
S
CHULKE
,
4
P
ETER
B. S
TACEY
,
6
P
ENELOPE
M
ORGAN
,
7
M
ARTOS
H
OFFMAN
,
8
AND
J
ON
T. K
LINGEL
9
1
U.S. Geological Survey, Jemez Mountains Field Station, HCR 1, Box 1, #15, Los Alamos, New Mexico 87544 USA
2
Department of Geography, University of California, Los Angeles, California 90095-1524 USA
3
Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona 85721 USA
4
Center for Biological Diversity, Tucson, Arizona 85702 USA
5
Laboratory of Tree-Ring Research, University of Arizona, Tucson, Arizona 85721 USA
6
Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131 USA
7
College of Natural Resources, University of Idaho, Moscow, Idaho 83844 USA
8
Southwest Forest Alliance, Flagstaff, Arizona 86002 USA
9
New Mexico Department of Game and Fish, P.O. Box 25112, Santa Fe, New Mexico 87504 USA
Abstract.
The purpose of this paper is to promote a broad and flexible perspective on
ecological restoration of Southwestern (U.S.) ponderosa pine forests. Ponderosa pine forests
in the region have been radically altered by Euro-American land uses, including livestock
grazing, fire suppression, and logging. Dense thickets of young trees now abound, old-
growth and biodiversity have declined, and human and ecological communities are in-
creasingly vulnerable to destructive crown fires. A consensus has emerged that it is urgent
to restore more natural conditions to these forests. Efforts to restore Southwestern forests
will require extensive projects employing varying combinations of young-tree thinning and
reintroduction of low-intensity fires. Treatments must be flexible enough to recognize and
accommodate: high levels of natural heterogeneity; dynamic ecosystems; wildlife and other
biodiversity considerations; scientific uncertainty; and the challenges of on-the-ground im-
plementation. Ecological restoration should reset ecosystem trends toward an envelope of
‘‘natural variability,’’ including the reestablishment of natural processes. Reconstructed
historic reference conditions are best used as general guides rather than rigid restoration
prescriptions. In the long term, the best way to align forest conditions to track ongoing
climate changes is to restore fire, which naturally correlates with current climate. Some
stands need substantial structural manipulation (thinning) before fire can safely be reintro-
duced. In other areas, such as large wilderness and roadless areas, fire alone may suffice
as the main tool of ecological restoration, recreating the natural interaction of structure and
process. Impatience, overreaction to crown fire risks, extractive economics, or hubris could
lead to widespread application of highly intrusive treatments that may further damage forest
ecosystems. Investments in research and monitoring of restoration treatments are essential
to refine restoration methods. We support the development and implementation of a diverse
range of scientifically viable restoration approaches in these forests, suggest principles for
ecologically sound restoration that immediately reduce crown fire risk and incrementally
return natural variability and resilience to Southwestern forests, and present ecological
perspectives on several forest restoration approaches.
Key words: anthropogenic change; ecological restoration; ecosystem management; fire sup-
pression effects; forest restoration programs; ponderosa pine forests; reference conditions; South-
western United States, natural range of variation.
I
NTRODUCTION
The
Pinus ponderosa
(ponderosa pine) forests of the
American Southwest (Arizona, New Mexico, and ad-
joining portions of Utah and Colorado) have experi-
enced major changes in ecological structure, compo-
sition, and process because of recent human activities
(Fig. 1). Over a century of livestock grazing, fire sup-
pression, logging, road construction, predator control,
and exotic-species introductions have altered most
Manuscript received 15 June 2001; revised and accepted 29
October 2001; final version received 2 January 2002.
10
E-mail: craig allen@usgs.gov
Southwestern ponderosa pine forests from conditions
that had prevailed for thousands of years (Covington
and Moore 1994, Swetnam et al. 1999). A critical
change has been a dramatically increased vulnerability
of these forests to large, destructive crown fires that
threaten both human and ecological communities.
A general consensus has emerged that it is urgent to
restore more natural conditions to these forests, but
substantial debate persists about how to best achieve
this goal (Nijhuis 1999, Covington 2000, Kloor 2000,
Jenkins 2001). The purpose of this paper is to promote
a broad and flexible perspective on ecological resto-
ration of Southwestern ponderosa pine forests. We sup-
October 2002 1419
RESTORATION OF PONDEROSA PINE ECOSYSTEMS
F
IG
. 1. Ponderosa pine forests of the American Southwest. (Top) Open ponderosa pine forest representing ‘‘typical’’ pre-
1900 conditions, with grassy understory and surface fire activity. (Bottom) Altered ponderosa pine stand in need of restoration,
showing changes in both stand structure and species composition. The dense midstory of mixed conifer trees provides ladder
fuels that favor crown fire development.
port the development and implementation of a diverse
range of scientifically viable restoration approaches in
these forests that address the critical issues of forest
heterogeneity, scientific uncertainty, and effects on
wildlife. In addition, we suggest principles for ecolog-
ically sound restoration approaches. It is not our in-
tention to emphasize a critique of any particular model,
but we do present ecological perspectives on several
alternative forest restoration approaches.
B
ACKGROUND
Southwestern ponderosa pine ecosystems were
shaped through time by stochastic and deterministic
processes, including frequent surface fires, episodic re-
generation, insect infestations, and regional climate
events such as droughts (Dahm and Geils 1997, Allen
and Breshears 1998, Kaufmann et al. 1998, Swetnam
and Betancourt 1998). These processes contributed to
heterogeneous forest spatial patterns at local and land-
1420
CRAIG D. ALLEN ET AL.
Ecological Applications
Vol. 12, No. 5
F
IG
. 2. View looking northwest at the Cerro Grande Fire on the afternoon of 10 May 2000 as it burned toward Los
Alamos, New Mexico (USA). Similar large, stand-replacing fires are becoming increasingly common in Southwestern pon-
derosa pine forests.
scape scales (Cooper 1960, White 1985), with pattern
shifts through time within a natural range of variability
(Swetnam et al. 1999).
Since European settlement in the middle to late
1800s, pervasive changes have tended to homogenize
ponderosa pine forest patterns in the Southwest. Large
trees have decreased in number due to logging. Historic
livestock grazing and fire suppression have promoted
the development of unnaturally dense stands of sup-
pressed young trees. This condition now threatens the
remaining large trees through competition and by fu-
eling increasingly extensive crown fires (Covington
and Moore 1994, Covington et al. 1994) as in ponde-
rosa pine forests of other regions (Agee 1993, Everett
et al. 1997, Smith and Arno 1999). In some stands
species compositions have shifted toward less fire-re-
sistant trees such as
Abies concolor
(white fir),
Pseu-
dotsuga menziesii
(Douglas-fir), and
Juniperus
(juni-
per) species. These changed conditions now affect mil-
lions of hectares of ponderosa pine in the Southwest
(U.S. General Accounting Office 1999).
Alteration of stand structures and species composi-
tions has in turn altered natural processes in South-
western forests. Understory grasses and forbs have de-
creased in abundance and diversity (Covington and
Moore 1994, Bogan et al. 1998), replaced by deep mats
of slowly decomposing pine needles. As a result, nu-
trient cycling dynamics have been disrupted (White
1994) and overall biodiversity levels decreased (Allen
1998). Old-growth ponderosa pine forests have become
rare (Harrington and Sackett 1992, Noss et al. 1995)
and meadows have shrunk due to tree encroachment
(Swetnam et al. 1999). Some vertebrate animal species,
such as the Northern Goshawk (Reynolds et al. 1992,
U.S. Fish and Wildlife Service 1998; but see Kennedy
1997), are thought to have declined in abundance due
to habitat alterations. Hydrologic cycles have been
modified in more densely forested watersheds, likely
decreasing total streamflows, peak flows, and base
flows (Ffolliott et al. 1989). An increase in number,
size, and severity of stand-replacing fires (Dahm and
Geils 1997, Swetnam and Betancourt 1998, Hardy et
al. 1999) threatens both human and ecological com-
munities (Moir and Dieterich 1988, U.S. General Ac-
counting Office 1999). The aftermath of such fires in-
cludes short-term amplification of erosion and flooding
(Agee 1993, White 1996, Robichaud et al. 2000).
Landscape scars created by total canopy destruction
may persist as grasslands or shrublands for decades to
centuries because ponderosa pine seed production and
recruitment is erratic, and the relatively heavy, wing-
less seeds cannot disperse far from surviving, mature
trees. For example, large portions of the 1950 A1 fire
near Flagstaff, Arizona, USA, remain as grassland, and
the 1953 Circle Cross Fire in the Sacramento Moun-
tains, New Mexico, USA, contains much persistent
shrubland (M. Savage,
unpublished data
). If the current
trajectories of anthropogenically driven change contin-
ue, serious ecological damage to ponderosa pine eco-
systems will accumulate (Covington et al. 1994, Noss
et al. 1995).
These worrisome trends have long been evident to
some forest scientists and ecologists (Weaver 1951,
Cooper 1960). Only recently, however, has a broad sci-
entific, social, and political consensus emerged that res-
toration of ecological sustainability in Southwestern
ponderosa pine forests is necessary and urgent (Cov-
ington and Moore 1994, Covington et al. 1994, 1997,
Suckling 1996, Nijhuis 1999). This social and political
consensus has developed rapidly in response to recent
major wildfire seasons, such as 2000, when 3
3
10
6
ha
burned nationwide (Fig. 2). Although much of this
burned area was in non-forested landscapes, or in high-
elevation forest types that are adapted to high-intensity
October 2002 1421
RESTORATION OF PONDEROSA PINE ECOSYSTEMS
fires (Morrison et al. 2000), ecologically worrisome
crown fires in ponderosa pine and mixed-coniferforests
were common, and are a chief focus of concern. Much
of this concern also stems from the fact that urban
encroachment into these pine-dominant forests is ex-
tensive and increasing.
Ecological restoration efforts in the United States
have recently been proposed for millions of hectares
of public lands by federal, state, and local government
agencies (e.g., USDA Forest Service 2000, USDA and
USDI 2000
a
,
b
, USDI 2000, Western Governors’ As-
sociation et al. 2001, Marston et al. 2001, Matthews
2001). These restoration proposals generally seek to
thin forests with combinations of tree harvesting and
prescribed burning to increase resilience to natural dis-
turbance events such as fires, insects, and regional
drought, and thereby reduce the risk of catastrophicfire
events.
Vigorous public and scientific debates have devel-
oped over the relative risks and trade-offs of different
approaches to restore forests in the Southwest (Suck-
ling 1996, Nijhuis 1999, Covington 2000, Kloor 2000,
MacNeil 2000, Jenkins 2001), as in other regions
(Brown 2000, Marston et al. 2001). Silvicultural ap-
proaches focused on tree harvest have been criticized
based on concerns that short-term economics rather
than ecological sustainability or decreasing fire hazard
are the real underlying justifications of treatments. Al-
though prescribed fire programs have been underway
for several decades, the scale and intensity of these
restoration efforts have been inadequate to reverse the
overall trends of degradation in Southwestern pine for-
ests. Concerns about excessive smoke and the risks of
prescribed burning (highlighted by the Cerro Grande
Fire of 2000) have also constrained public support for
the use of fire alone as a restoration treatment.
AB
ROAD
R
ESTORATION
P
ERSPECTIVE
In this paper we offer a broad perspective on eco-
logical restoration of Southwestern United States pon-
derosa pine forests. We propose not so much a contrast
from current models as an expanded view that encom-
passes and supports a diverse range of scientifically
viable restoration approaches. This includes the cur-
rent, most widely debated and applied Southwestern
ponderosa pine model, which involves a relatively pre-
cise restoration of presettlement stand structures
(Moore et al. 1999). There is a clear need for reference-
based approaches that integrate structure, composition,
and ecosystem processes. Such restoration models need
to be adequately flexible to address the diverse eco-
logical and social conditions in Southwestern forests.
We believe that the practice of ponderosa pine resto-
ration will be best served by a variety of approaches
across a broad range of forest settings.
Ecological restoration aims to enhance the resilience
and sustainability of forests through treatments that
incrementally return the ecosystem to a state that is
within an historic range of conditions, known as the
‘‘natural range of variability’’ (Landres et al. 1999).
The Society for Ecological Restoration (Tucson, Ari-
zona, USA) offers the following definition of ‘‘
ecol-
ogial restoration
: ‘the process of assisting the recovery
and management of ecological integrity. Ecological in-
tegrity includes a critical range of variability in bio-
diversity, ecological processes and structures, regional
and historic context, and sustainable cultural practic-
es.’ ’’
11
Several key issues suggest a need for a broad
perspective on ecological restoration of Southwestern
forests: high levels of natural heterogeneity across for-
est landscapes; current and future conditions that may
fall outside the natural range of variability; wildlife
habitat and other biodiversity considerations; and our
imperfect understanding of these complex systems. We
focus here on ecological considerations in ponderosa
pine forest restoration, acknowledging that the success
of restoration programs also requires political, finan-
cial, and social support. Our intent is not to exclude
such dimensions, but to clarify the scientific basis for
forest restoration programs.
The importance of heterogeneity in time and space
Ecological restoration requires an understanding of,
and respect for, the variability of ponderosa pine eco-
systems across the Southwest. These forests are created
by dynamic interactions among natural processes (such
as fire) and forest structure (such as tree density and
canopy openings), across variable landscapes. Regen-
eration and disturbance patterns interact with long- and
short-term climate fluctuations and human activities to
influence fire frequency, intensity, size, seasonality,
and severity (Baisan and Swetnam 1997, Swetnam and
Betancourt 1998, Kaye and Swetnam 1999, Grissino-
Mayer and Swetnam 2000). Forest patterns and pro-
cesses often exhibit lagged interactions with climate
fluctuations, along with substantial inertia in the face
of climate change. Consequently, forests often exist in
disequilibrium with current climate.
Variability in structure and process means that pat-
terns of stand density, species composition, and dis-
turbance regimes differ significantly across landscapes
and throughout the region. Ponderosa pine grows
across a 1500-m elevational gradient in many mountain
ranges (e.g., Allen and Breshears 1998), with at least
21 different ponderosa pine ‘‘habitat types’’ recognized
across diverse landscape conditions in the Southwest
(Alexander and Ronco 1987). Documented pre-1900
mean fire-return intervals varied from about 4 to 36 yr
in Southwestern ponderosa pine forests (Swetnam and
Baisan 1996). Post-suppression species compositions
have been markedly affected by increased recruitment
of fire-sensitive
Pinus edulis
(pin˜on) and juniper spe-
cies at low elevations and mixed-conifer species at
higher elevation (Fig. 1). Mature tree densities (
.
30
11
URL:
^
http://www.ser.org
&
1422
CRAIG D. ALLEN ET AL.
Ecological Applications
Vol. 12, No. 5
F
IG
. 3. Schematic representation of the restoration concept. If the natural range of variability is seen as a multidimensional
‘‘envelope’’ of ecological conditions, then the goal of restoration is to move an altered ecosystem back toward its pre-
disruption envelope (the darkest region), and to allow natural processes over time to dynamically reestablish a range of
natural structural conditions.
cm [12 inches] in diameter at breast height) measured
ca. 1900 AD in Southwestern pine ecosystems ranged
between 19 and 126 trees/ha (8–51 trees/acre) (Wool-
sey 1911), whereas today densities of all live stems
often exceed 2470 trees/ha (1000 trees/acre) (Allen
1998; D. Falk,
unpublished data
).
This variability in ponderosa pine forests affects
management approaches, as recognized by Pearson
(1950:13): ‘‘Foresters are constantly reminded that
since conditions change from one place to another,
management cannot be uniform.’’ Ecological restora-
tion should recognize and retain the natural heteroge-
neity characteristic of presettlement forests (White and
Walker 1997). Given the heterogeneity of Southwestern
ponderosa pine forests, identifying general ecological
reference conditions for restoration goals can be dif-
ficult, if not arbitrary.
Use of the natural-range-of-variability
concept for restoration
The concept of ‘‘natural range of variability’’ has
some theoretical flaws and practical limitations (Lan-
dres et al. 1999, Millar and Wolfenden 1999, Swetnam
et al. 1999), but nevertheless has proven useful as a
framework for evaluating the current status of ecosys-
tems relative to past conditions, and for identifying
ecologically justifiable restoration goals (Kaufmann et
al. 1994, Morgan et al. 1994, White and Walker 1997,
Landres et al. 1999). Southwestern forests experienced
some level of human influence for thousands of years,
but human-caused changes in ponderosa pine after
1850 far exceeded the influence of indigenous people
(Covington et al. 1994, Kaye and Swetnam 1999, Allen
2002). In particular, the cessation of the historical re-
gime of frequent, low-intensity fires has pushed struc-
tural characteristics of many forests well outside the
natural range of variability that existed over the past
millennium (Swetnam et al. 1999).
Ponderosa pine restoration treatments should be de-
signed to reestablish trends in forest processes, partic-
ularly fire, leading to natural ranges of variation in
composition and structure (Landres et al. 1999, Moore
et al. 1999, Stephenson 1999). If the natural range of
variability is seen as an envelope of conditions, then
the goal of restoration is to move an altered ecosystem
toward its pre-disruption envelope, and to allow, or use,
natural processes over time to dynamically reestablish
diverse natural structures (Fig. 3).
In the long term, the best way to align forest con-
ditions to track ongoing climate changes is to restore
fire, which naturally correlates with current climate.
Some stands need substantial structural manipulation
before fire can safely be reintroduced, but in many
cases fire can then do the preponderance of the work
of ecological restoration, recreating the natural inter-
action of structure and process.
The successful reintroduction of frequent surface fire
in the Gila Wilderness, New Mexico, USA, exemplifies
how incremental fire treatments can achieve substantial
restoration of ponderosa pine forests. Detailed quan-
October 2002 1423
RESTORATION OF PONDEROSA PINE ECOSYSTEMS
titative studies of before-and-after forest densities are
lacking, but qualitative observations (including repeat
photographs) by knowledgeable fire managers (Webb
and Henderson 1985; P. Boucher and R. D. Moody,
unpublished report
[1996] to the Gila National Forest
[Silver City, New Mexico, USA]) and fire scientists
(Rollins 2000; T. W. Swetnam,
personal observations
)
suggest that repeated natural fire use (as many as four
events since 1975 in some areas) have successfully
opened up ponderosa pine stands, and helped to rees-
tablish grassy understories. A key to the Gila’s success
was a sufficiently large area where smoke and threats
of occasional flare-ups during the burning season
(sometimes months) do not threaten urban areas. An-
other key to this success was that managers werepatient
and incremental in implementing the burning program;
most of the large fires were initially allowed to burn
only during the relatively cool and wet season after the
start of the Southwest monsoons in July or August.
Although such large, roadless areas are uncommon, the
Gila’s example shows that mechnical thinning is not
always a prerequisite for forest restoration treatments
with fire.
Restoration aims to eventually move forests back to
within the natural range of variability. However, this
need not necessarily be accomplished in the first treat-
ment or tied to a particular moment in time or past
forest structure. Initial treatments can be designed to
markedly decrease crown fire risks at stand and land-
scape scales by decreasing the continuity of hazardous
fuel conditions. Conservative initial treatments would
be the minimum necessary to reduce vulnerability to
stand-replacement fire to an acceptable level. We pro-
pose that conservative, incremental treatments involv-
ing both mechanical thinning and fire over time periods
of years to decades are ecologically appropriate and
advisable for many Southwestern forests.
There is a clear need to begin widespread restoration
treatments now to reduce the threat of catastrophic fire.
Such treatments should use a variety of prescriptions
and treatments to address trade-offs and produce di-
verse restoration outcomes. For example, restoration
back to presettlement tree densities through intensive
thinnings greatly reduces risk of crown fire spread in
areas of concentrated treatment (Fule´ et al. 2001), but
may generate more slash surface fuels initially than
lighter thinnings. Large slash loads can exacerbate the
short-term fire hazard unless mitigated (Graham et al.
1999), which can be expensive and/or have substantial
environmental effects (e.g., machine crushing slash
into the ground surface). Less intensive treatments, suf-
ficient to reduce crown fire risk and strategically placed
to interrupt continuous fuels, may be able to more
quickly address a larger proportion of the regional for-
est. While practical and economic considerations, such
as direct risk to human communities, may drive treat-
ment strategies in some situations, long-term ecolog-
ical goals should also guide treatment actions if eco-
logical restoration is desired. Incrementally thinning
ponderosa forests over a period of years using varied
combinations and intensities of chainsaws and fire may
effectively reduce fire risks while maintaining ecolog-
ical integrity.
Biodiversity considerations
Southwestern ponderosa pine forests provide habitat
for at least 250 species of vertebrate animals (Patton
and Severson 1989, New Mexico Department of Game
and Fish 2000), as well as many plants. When resto-
ration treatments modify the physical and biotic en-
vironment, plants and animals are affected in various
ways (Rieman and Clayton 1997, Oliver et al. 1998;
W. W. Covington, A. Waltz, P. Fule´ and G. Verkamp,
unpublished report
to U.S. Bureau of Land Manage-
ment, Arizona Strip District). Of particular concern is
the potential for restoration actions to reduce the via-
bility of metapopulations of sensitive species through
habitat alteration and fragmentation (Stacey et al. 1996,
U.S. Fish and Wildlife Service 1998, Holthausen et al.
1999). This includes invertebrates and soil organisms
that are critical to ecosystem function.
An incremental approach to restoration provides op-
portunities for the adjustment of future treatments
based on wildlife responses, so as not to foreclose fu-
ture options. Restoration projects should strive to min-
imize or spatially constrain adverse impacts to rare,
sensitive, and declining species. Care should be taken
to ensure that adequate habitat is maintained for sen-
sitive species during the phase between initialtreatment
and final restoration.
Not all wildlife habitat elements are equally vulner-
able or irreplaceable. For example, large oak trees grow
slowly and are especially valuable habitat for a wide
array of wildlife (Rosenstock 1998). Some areas should
be left untreated to serve as habitat refuges during the
stress of treatments, until new landscape conditions are
known to provide adequate habitat.
Native plants and animals are adapted to the natu-
rally high levels of heterogeneity in Southwestern pon-
derosa pine ecosystems, and some species are depen-
dent upon diverse habitats for their survival (e.g.,
Reynolds et al. 1992, U.S. Fish and Wildlife Service
1995, Dodd et al. 1998). Thus, biodiversity consider-
ations reinforce the need to avoid creating uniform
stand and landscape conditions. A diverse landscape
with patches of variable tree densities, including some
areas of relatively high density, should be developed
to accommodate species with different habitat adap-
tations. Retaining some dead, deformed, and diseased
trees, and some clumps of large trees with interlocking
crowns, will maintain structural complexity and im-
portant food and nesting habitat (Bennetts et al. 1996,
Bull et al. 1997). Such trees are important elements of
genetic diversity in their own right as well (Millar and
Libby 1989, Rehfeldt 1991, Ledig 1992).
Restoration activities often involve biodiversity
1424
CRAIG D. ALLEN ET AL.
Ecological Applications
Vol. 12, No. 5
trade-offs, particularly in the short term and at the local
scale. For example, snags and downed logs provide
essential habitat for many birds and mammals (Bull et
al. 1997), but these woody structures are susceptible
to destruction in restoration treatments, especially from
fire (Tiedemann et al. 2000). However, restorationtreat-
ments also provide opportunities to increase habitat
heterogeneity and biodiversity. For example, where
large snags are scarce because of past management
activities, new ones will be created through fire mor-
tality and other disturbances as forests are restored
(Pearson 1950). The restoration of more diverse habitat
structures and of natural processes such as surface fire
and associated nutrient cycling should help many na-
tive species (cf. Johnson and Wauer 1996; W. W. Cov-
ington, A. Waltz, P. Fule´, and G. Verkamp,
unpublished
report
to U.S. Bureau of Land Management, Arizona
Strip District). The increased vigor and diversity of
native understory plant species after restoration will
provide important habitat conditions for many kinds of
animals (e.g., Waltz and Covington 1999) and hope-
fully increase resistance to ongoing and future inva-
sions by exotic plants (Crawford et al. 2001).
Scientific uncertainty and the limitations of historical
reference conditions
Ecological restoration must recognize the limits of
scientific knowledge. While a large body of research
exists on ponderosa pine forests, we are still limited
in our understanding of ecosystem function. Historical
fire frequency is often the most reliable element of our
reconstructions, although it too has limitations (Swet-
nam et al. 1999). We know less about past distributions
of fire size, severity, and spatial pattern than we do
about frequency (Morgan et al. 2001). Swetnam and
Baisan (1996) and Baker and Ehle (2001) discuss un-
certainties and biases associated with the sampling de-
sign and data analyses of fire-scar studies.
Uncertainties in the reconstruction of forest stand
composition and spatial structure result from missing
evidence, such as logs and stumps removed by fire,
logging, and decay (M. M. Moore, D. W. Huffman, W.
W. Covington, J. E. Crouse, P. Z. Fule´, and W. H. Moir,
unpublished report
to USDA Forest Service, Rocky
Mountain Station, Flagstaff, Arizona). Reconstruction
of the density and location of large trees is far more
reliable than of small-diameter stems and seedlings that
decompose rapidly or burn even in surface fires (Ste-
phenson 1987). Reconstructed overstory-tree densities
are thus best used conservatively as minimum values
for establishing quantitative restoration targets for
stand density, rather than as maxima. Knowledge of
other ecological conditions in the past, such as wildlife
population dynamics, may be highly uncertain because
available methods preclude precise reconstructions.
Uncertainty will also exist because society is unlikely
to invest in the development of detailed reconstructions
of past stand structures and fire histories for every res-
toration site, suggesting the need for site-specific flex-
ibility to develop restoration targets based upon an in-
ferred natural range of variability in stand conditions.
A sense of urgency in reducing the threat of destruc-
tive fires should be balanced with patience in accom-
plishing restoration, along with major commitments to
thoughtful experimentation and monitoring to ensure
we learn as rapidly as possible. Restoration science is
young, and it is difficult to predict the results of our
actions. Application of a broad restoration perspective,
implemented through a variety of alternative treat-
ments, will be more effective than any single restora-
tion treatment.
P
RINCIPLES FOR
E
COLOGICAL
R
ESTORATION OF
S
OUTHWESTERN
P
ONDEROSA
P
INE
F
ORESTS
Here we outline 16 broad principles for restoration
of Southwestern United States ponderosa pine forest
ecosystems. Application of these principles will foster
implementation of a diverse range of ecologically jus-
tifiable restoration projects. While focused on ponde-
rosa pine forests, these principles also apply more gen-
erally to other forest types in the Southwest, such as
some pin˜on–juniper and mixed-conifer forests.
1)
Reduce the threat of crown fire
.—A key resto-
ration priority must be the rapid reduction of the wide-
spread risk of unnatural crown fires, both within stands
and across landscapes. The initial treatment on any site
should be substantial enough to decrease forest vul-
nerability to stand-replacing fire.
2)
Prioritize and strategically target treatment ar-
eas
.—Key considerations for prioritizing restoration
treatment areas are degree of crown fire risk, proximity
to human developments and important watersheds, pro-
tection of old-growth forests and habitats of sensitive
species, and strategic positioning to break up land-
scape-scale continuity of hazardous fuels (Weather-
spoon and Skinner 1996, Agee et al. 2000, Finney
2001). For example, a landscape of alternating north-
and south-facing ridges could have restoration treat-
ments focused on the south-aspect sites to reduce the
continuity of hazardous fuels at the landscape scale,
and thereby restore more natural conditions at both site
and landscape scales.
3)
Develop site-specific reference conditions
.—Site-
specific historical ecological data can provide infor-
mation on the natural range of variability for key forest
attributes, such as tree age structure and fire regimes,
that furnish
local
‘‘reference conditions’’ for restora-
tion design. A variety of constraints, however, prevent
the development of historical information on every
hectare of land needing restoration. General goals
should be to restore ecological integrity and function-
ing, rather than precise stand structural conditions as
they existed at a particular point of time in the past.
4)
Implement multiple conservative interventions
.—
Incremental restoration through multiple treatments is
a conservative approach to achieving desired changes.
October 2002 1425
RESTORATION OF PONDEROSA PINE ECOSYSTEMS
Restoration treatments should strive to use the least
disruptive techniques, and balance intensity and exten-
siveness of treatments. In many areas, conservative ini-
tial treatments would be the minimum necessary to
adequately reduce the threat of unnatural crown fire.
For example, a conservative strategy would be the
placement of treatment areas to interrupt fuel conti-
nuities and thereby reduce overall landscape risk of
extensive crown fires, despite treating only a small frac-
tion of the landscape (Agee et al. 2000). Fires ignited
by lightning or by people under carefully prescribed
conditions may be sufficient to reestablish natural con-
ditions in many locations (e.g., in the Gila Wilderness
of New Mexico [USA]). In the extensive areas where
fire alone cannot safely reduce tree densities and haz-
ardous ladder fuels, mechanical thinning of trees will
be needed before the introduction of prescribed fire.
Patient, effective treatments will provide more options
for the future than aggressive attempts to restore 120
yr of change at once. In certain areas, however, such
as some urban–wildland interfaces, trade-offs with im-
minent crown fire risks require consideration of rapid,
heavy thinning of most small-diameter trees.
5)
Utilize existing forest structure
.—Restoration ef-
forts should incorporate and build upon valuable ex-
isting forest structures such as large trees and groups
of trees of any size with interlocking crowns. These
features are important for some wildlife species, such
as Abert’s squirrels and goshawks, and should not be
removed completely just to recreate specific historical
tree locations. Since evidence of long-term stability of
precise tree locations is lacking, the selection of
‘‘leave’’ trees and tree clusters in restoration treatments
can be based on the
contemporary
spatial distribution
of trees, rather than pre-1900 tree positions. Historical
forest structure conditions can be restored more quickly
by maximizing use of existing forest structure. Leaving
some relatively dense within-stand patches of trees
need not compromise efforts to reduce landscape-scale
crown fire risk.
Gus Pearson, the patriarch of ponderosa pine silvi-
culture in the Southwest, recognized the need to be
flexible and use existing stand structures. Pearson
(1950:29) stated that the use of theoretical tree-density
distributions as guides for silvicultural treatments ‘‘are
for application in principle rather than in letter, because
the stands must be taken as found and remedies must
be sought in modification rather than reconstruction.’’
This is also true for ecological restoration.
6)
Restore ecosystem composition
.—Where fire sup-
pression has allowed fire-sensitive tree species like ju-
nipers or white fir to become abundant in historical
ponderosa pine forests, treatments should set a trend
toward restoring dominance of the more fire-resistant
ponderosa pines. However, mechanical removal of all
fire-sensitive invaders is inappropriate, given the het-
erogeneous and dynamic nature of these forests. Res-
toration of fire may eventually restore locally appro-
priate forest tree composition and structure (Miller and
Urban 2000).
Missing compositional elements, such as herbaceous
understories or extirpated vertebrates and invertebrates,
also require restoration attention. The forest understo-
ry, including shrubs, grasses, and forbs, is an important
ecosystem component that directly affects tree regen-
eration patterns, fire behavior, watershed functioning,
wildlife habitat, and overall patterns of biodiversity.
Similarly, soil organisms are vital elements that can
influence community composition and dynamics (Hole
1981). A robust understory provides a restraint on tree
regeneration and is essential for carrying surface fires.
The establishment and maintenance of more natural
patterns of understory vegetation diversity and abun-
dance are integral to ecological restoration. Understo-
ries should be protected from overgrazing during res-
toration to allow full recovery of herbaceous biodi-
versity and biomass, and of associated ecosystem com-
ponents and processes such as pollinators and surface
fires.
The implementation of restoration treatments re-
quires special care to protect soils and watersheds (Jur-
gensen et al. 1997, Rieman and Clayton 1997). Mini-
mizing mechanical disturbance of soils and avoiding
the construction of new roads will minimize sedimen-
tation, disruption of surface runoff, and other detri-
mental ecosystem effects (Trombulak and Frissell
2000). Minimizing ground disturbance also will reduce
impacts to the numerous archeological sites found in
many Southwestern forests.
7)
Retain trees of significant size or age
.—Large and
old trees, especially those established before ecosystem
disruption by Euro-American settlement, are rare, im-
portant, and difficult to replace. Their size and struc-
tural complexity provide critical wildlife habitat by
contributing crown cover, influencing understory veg-
etation patterns, and providing future snags. Ecological
restoration should protect the largest and oldest trees
from cutting and crown fires, focusing treatments on
excess numbers of small young trees. Given wide-
spread agreement on this point, it is generally advisable
to retain ponderosa trees larger than 41 cm (16 inches)
dbh and all trees with old-growth morphology regard-
less of size (i.e., yellow bark, large drooping limbs,
twisted trunks, flattened tops). Despite the heteroge-
neity of forest site and stand conditions in the South-
west, cutting of larger trees will seldom be ecologically
warranted as ‘‘restoration’’ treatments at this time due
to their relative scarcity. Following this guideline
would significantly reduce hazards of stand-replacing
fires in most cases and also favor the development of
future old-growth forest conditions (Moir and Dieterich
1988, Harrington and Sackett 1992). Public concern
about forest manipulation would also be reduced by
ensuring that ‘‘large’’ trees are not being targeted.
Some ponderosa pine forests contain extremely old
trees and dead wood remnants that may be small but
1426
CRAIG D. ALLEN ET AL.
Ecological Applications
Vol. 12, No. 5
are important because they contain unique and rare
scientific information in their growth rings (Grissino-
Mayer et al. 1997). Such trees have become increas-
ingly scarce in the late 20th century, and the initial
reintroduction of fire often consumes these tree-ring
resources. Restoration programs should identify, in-
ventory, sample, and preserve them where possible.
8)
Consider demographic processes
.—The under-
lying processes of natural tree regeneration and mor-
tality should be incorporated in restoration design.
Southwestern conifer regeneration occurs in episodic,
often region-wide pulses, linked to wet–warm climate
conditions and reduced fire occurrence (Savage et al.
1996, Swetnam and Betancourt 1998, Mast et al. 1999).
Periods with major regeneration pulses in the South-
west during the 20th century include the 1910s–1920s
and 1978–1998 (Savage et al. 1996, Swetnam and Be-
tancourt 1998). Some of this regeneration would have
survived under natural conditions. Restoration efforts
should retain a proportion of these cohorts, toward the
goal of maximizing options for ecosystem resiliency.
9)
Integrate process and structure
.—Ecological sus-
tainability requires the restoration of process as well
as structure (Stephenson 1999). Natural disturbance
processes, including fire, insect outbreaks, and
droughts, are irreplaceable shapers of the forest. In par-
ticular, fire regimes and stand structures interact and
must be restored in an integrated way; mechanical thin-
ning alone will not reestablish necessary natural dis-
turbance regimes. At the same time, fire alone may be
too imprecise or unsafe in many settings, so a com-
bination of treatments may often be the safest and most
certain restoration approach.
Perhaps the single best indicator of whether a pro-
posed approach should be considered as ‘‘ecological
restoration’’ of a ponderosa pine forest is to evaluate
whether the treatment will successfully restore surface
fire as a keystone process. Approaches that do not in-
clude an explicit and long-term commitment to restore
frequent surface fire fail to merit the adjective ‘‘eco-
logical.’’
10)
Control and avoid using exotic species
.—Seed-
ing of exotic grasses and forbs should be prohibited as
ecologically incompatible with good restoration. Once
established, exotic species can be extremely difficult
or impossible to remove. Even seeding with native spe-
cies from commercial sources risks the near certainty
of exotic weed contaminants and the establishment of
non-local genotypes at the expense of locally evolved
and adapted genotypes. The common use of annual
cereal crops such as rye and barley that typically de-
cline to low levels within a matter of years still leaves
persistently altered ecosystems through ‘‘the ghost of
competition past,’’ as their initial flush of growth tends
to monopolize the soil resources, reduce the success of
native plants (Barclay 2000), and alter long-term suc-
cessional outcomes.
If enhancement of herbaceous vegetation is needed,
using locally hand-collected wild seeds or transplanting
individuals from nearby areas into treatments to serve
as seed sources is slower and more expensive, but eco-
logically safer. In general, it is ecologically desirable
to avoid seeding, and to instead allow native herba-
ceous vegetation to recover incrementally through nat-
ural processes of dispersal and establishment after res-
toration treatments.
The widespread practice of seeding exotic grasses to
‘‘rehabilitate’’ watersheds after severe fires could be
largely eliminated by preemptive restoration treatments
that reduce fire severity and foster recovery of native
herbaceous ground cover before crown fires occur. Res-
toration treatments should also routinely incorporate
early actions to control the establishment and spread
of aggressive exotics that can be expected from res-
toration-related site disturbance (Crawford et al. 2001).
Control actions should include active detection efforts
to identify infestations at an early phase, and treatments
ranging from hand-pulling to the careful spot appli-
cation of biodegradable herbicides.
11)
Foster regional heterogeneity
.—The Southwest
is a region of complex topography, hydrology, and
soils. As a result, biological communities vary at local,
landscape, and regional scales, and so should restora-
tion efforts. Ecological restoration should also incor-
porate the natural variability of disturbance regimes
across heterogeneous landscapes. Heterogeneityshould
be fostered in planning and implementing ecological
restoration at all spatial scales, including within and
between stands, and across landscape and regional
scales.
12)
Protect sensitive communities
.—Certain ecolog-
ical communities embedded within ponderosa pine for-
ests, such as some riparian areas, could be adversely
affected by on-site prescribed burning or mechanical
thinning. Restoration efforts should protect these and
other rare or sensitive habitats, which are often hotspots
of biological diversity, particularly those that are de-
clining in abundance and quality in the region.
13)
Assess cumulative effects
.—It is important to
consider and plan for the cumulative effects of resto-
ration work, since these efforts will take place syn-
chronously throughout large areas in the Southwest.
Restoration projects will also occur within a regional
context of other human actions such as timber sales,
private land developments, roads, and livestock graz-
ing. These land uses have varied impacts at all scales,
and often uncertain interactions with restoration efforts
must be considered cumulatively.
14)
Protect from overgrazing
.—Grass, forb, and
shrub understories are essential to plant and animal
diversity and soil stability. Robust understories are also
necessary to restore natural fire regimes and to limit
excessive pine seedling establishment (Rummell 1951,
Madany and West 1983). Where possible, defer live-
stock grazing after initial surface fire treatment until
the herbaceous layer has fully recovered (Belsky and
October 2002 1427
RESTORATION OF PONDEROSA PINE ECOSYSTEMS
T
ABLE
1. Variables that should be considered in the development of an ecological restoration
plan. Each restoration project will have a unique plan, based on locally appropriate choices
for each variable, reflecting local priorities and the diversity of ponderosa pine forests in the
region.
Post-treatment structure: age, size, density
Stem density, by size class
Basal area (m
2
/ha), by size class
Crown cover (%)
Snags and down logs (no./ha, mass/ha), by size class
Thinning size cap (i.e., maximum-diameter tree removed)
Old tree protection measures (e.g., removal of basal fine fuels and ladder fuels; removal of
competing understory stems)
Composition
Retention of fire-sensitive or shade-tolerant native tree species
Existing canopy trees (%)
Understory recruitment (%)
Control or elimination of non-native understory species
Spatial configuration
Distribution of mature trees, random to clustered
Reliance on presettlement cluster locations for post-treatment configuration
Use of existing tree clusters in post-treatment configuration
Retention of heterogeneous (spatially variable) structure and density at stand and land-
scape scales
Ecosystem processes and disturbances
Restoration of natural or prescribed fire regime
Projected post-treatment distribution of fire intervals (e.g., mean and range)
Post-treatment livestock grazing regime
Overall restoration plan
Site-specificity of treatment prescription
Time span over which thinning treatments are implemented
Number of restoration-treatment entries
Long-term monitoring commitment
Monitoring components (e.g., plants, animals, soil, water)
Level of detail of measurements
Frequency of measurements
Duration of measurements
Adaptive management loop in decision process
Blumenthal 1997). Prevention of overgrazing must be
an integral part of ponderosa pine forest restoration.
15)
Establish monitoring and research programs
.—
Given the uncertainties about effects of restoration
treatments, well-designed monitoring, research, and
documentation programs are essential if we are to learn
from, and evaluate the success or failure of, ongoing
restoration efforts. Wildlife and understory plant pop-
ulations can be monitored as indicators of ecological
change and restoration outcomes. Monitoring programs
must be put in place before treatments begin, and must
evaluate responses of key ecosystem components and
processes at multiple scales (Covington and Moore
1994, Covington et al. 1997). Even better, when pos-
sible, restoration projects should be set up as experi-
ments with replicates and controls to test alternative
hypotheses (Covington et al. 1997). One benefit of the
perspective proposed here is that it will test the eco-
logical effects of restoration efforts across a wide range
of treatments and prior forest conditions. The locations
and prescriptions for all restoration treatments should
be archived in a geographic information system (GIS),
so that land managers and researchers now and into the
future have access to site-specific records of restoration
treatments.
16)
Implement adaptive management
.—Ecological
restoration is an incremental process that may take a
century or two to fully achieve. It requires a long-term
management commitment, especially to maintain nat-
ural fire regimes and to protect and develop old-growth
stands. Restoration will be most successful where land
managers learn from treatment experiences and adap-
tively adjust their approaches through time (Holling
1978). Extended social and fiscal support will be need-
ed to sustain such long-term restoration programs.
Conceptually, each of the restoration principles out-
lined above can be thought of as describing one or more
axes of variation. For example, intensity of thinning
treatment, heterogeneity within the treatment area,
post-treatment species composition, experimental unit
size, fidelity to location of existing tree clusters, and
post-thinning fire frequency constitute design variables
for restoration (Table 1). Thus, the principles describe
a set of key dimensions in the design of restoration
treatments, and any particular project will choose a
unique set of values for the variables. These principles
1428
CRAIG D. ALLEN ET AL.
Ecological Applications
Vol. 12, No. 5
support implementation of a diverse range of ecological
restoration projects.
E
COLOGICAL
P
ERSPECTIVES ON
S
EVERAL
F
OREST
R
ESTORATION
A
PPROACHES
As debate over restoration of Southwestern United
States forests has intensified (Njjhuis 1999, Jenkins
2001), a variety of philosophies and approaches are
being presented that differ in their core perspectives
and probable on-the-ground outcomes. These ap-
proaches include emphases on: (a) fire-risk reduction,
(b) economics, (c) natural regulation, and (d) structure-
oriented restoration. Here we provide some reflections
from an ecological perspective on these varying res-
toration approaches.
Fire risk reduction
Concerns over catastrophic crown fire impacts to hu-
man communities and ecosystems, along with the cur-
rent availability of substantial funding to reduce haz-
ardous forest fuel conditions (see the National Fire Plan
[USDA and USDI 2000
a
]), have the potential to drive
ecologically insensitive projects, which are not really
restorations. If the primary objective of a project is
solely to reduce crown fire risk (which may be an ap-
propriate societal goal in some areas), even ‘‘success-
ful’’ treatments may further damage forests rather than
restoring them. When ecological principles are applied,
however, fire risk reduction can be an essential part of
‘‘ecological restoration’’ of ponderosa pine forests
(Fule´ et al. 2001).
Economics
Concerns exist that legitimate and desirable econom-
ic goals, such as sustaining rural economies and min-
imizing subsidy costs, could also lead to ecologically
insensitive thinning or logging projects masquerading
as forest ‘‘restoration’’ (Hanson, 2000, Jenkins 2001,
National Forest Protection Alliance 2001). Projects fo-
cused on economic outcomes do not qualify as ‘‘eco-
logical restoration’’ unless the treatments are based on
ecological principles.
However, if ecological principles are used then eco-
nomic utilization of forest products may enhance forest
restoration. For example, restoration treatments will
occur sooner and over larger areas if some costs can
be recovered through the sale of wood from small-
diameter stems. Restoration can be an opportunity for
the re-engagement of local communities in the creation
of more sustainable forest-based economies (e.g., see
the Community Forest Restoration Act of 2000).
Natural regulation
One perspective on Western forests believes that: (1)
stand-replacing fires do not cause severe ecological
damage because crown fires are natural; and (2) the
forests will heal themselves best if we take a hands-
off approach and allow natural processes to function
with minimum interference (Wuerthner 1999).
Extensive stand-replacing fires naturally occurred in
many western forest types (Agee 1993), but not in
Southwestern ponderosa pine forests (Swetnam and
Baisan 1996). Recent fire history studies in the Black
Hills of South Dakota (USA) (Shinneman and Baker
1997) and in the Front Range of Colorado (USA)
(Brown et al. 1999) suggest that ponderosa pine forests
in those regions may have sustained crown fires of
unknown size in the presettlement era. Although no
such evidence has been reported for Southwestern pon-
derosa pine forests, it is possible that localized crown
fires were not unknown in this forest type before 1900.
Certainly, fire effects are always variable, and even
modern crown fires do not have uniformly devastating
effects across all burned landscapes. However, the in-
creasingly large crown fires of recent years in ponde-
rosa pine forests are clearly resulting in extreme hy-
drological, geomorphic, and ecological responses, in-
cluding amplified flooding (Veenhuis 2002), acceler-
ated soil erosion and stream channel changes (White
1996, Robichaud et al. 2000), loss of old-growth forest
(e.g., the Cerro Grande Fire affected much old-growth
forest, including large portions of a
;
15 mile
2
contig-
uous patch where essentially all trees were killed [C.
Allen,
personal observations
]), increasing abundance
of invasive exotics (Crawford et al. 2001), and con-
version of forests to different vegetation types (M. Sav-
age,
unpublished data
). The current state of ecological
and historical knowledge of Southwestern ponderosa
pine forests provides compelling justifications to ac-
tively manage to restore more resilient and sustainable
conditions (Covington and Moore 1994, Swetnam et
al. 1999).
Presettlement structure restoration
The most substantial scientific work to address the
need for restoration of Southwestern ponderosa pine
forests and to determine the ecosystem effects of treat-
ments has been made by Covington and co-workers at
the Ecological Restoration Institute of Northern Ari-
zona University (Kaye and Hart 1998; W. W. Coving-
ton, A. Waltz, P. Fule´, and G. Verkamp,
unpublished
report
to U.S. Bureau of Land Management, Arizona
Strip District). Their efforts have focused on attempts
to reconstruct and reestablish specific stand reference
conditions that existed just prior to the date of cessation
of the natural fire regime (Moore et al. 1999). The goal
has been to replicate tree densities and spatial patterns
as accurately as possible for sites where presettlement
stand structures have been reconstructed.Conceptually,
this approach takes a broad view of ‘‘reference con-
ditions,’’ and incorporates the restoration of surface fire
as a key process. However, there are some ecological
issues concerning on-the-ground implementation of
this approach.
The choice of a specific moment in time as the initial
October 2002 1429
RESTORATION OF PONDEROSA PINE ECOSYSTEMS
restoration target—for example the date of the last
widespread fire in the late 1800s (Moore et al. 1999)—
is potentially problematic. Any particular moment is
unique in the long-term history of an ecosystem, and
forests often exist in disequilibrium with current cli-
mate to some degree (Millar and Woolfenden 1999).
Moreover, an increasing body of evidence indicates that
late 20th century and current climate is unprecedented
on a time scale of at least 1000 yr (Mann et al. 1998,
Crowley 2000).
Restoration of structural characteristics of a forest
back to the date of the last widespread fire in the late
19th century implies that tree recruitment processes of
the 20th century were entirely unnatural. In fact, post-
settlement tree regeneration pulses would have con-
tributed important structural elements to contemporary
forests, even if tree-thinning surface fires had contin-
ued. It is difficult to estimate what proportion of these
cohorts, much less which individual trees, would have
survived to maturity (Mast et al. 1999). It may be un-
wise to automatically choose a century-old date as a
regional target condition for current forests, as the rep-
lication of plant densities and spatial arrangements that
existed at any particular date in the past may compro-
mise ecosystem resilience in future decades and cen-
turies. For example, this structure-oriented approach
can result in the aggressive removal of too many trees
during the initial entry, which may seriously constrain
ecosystem response and management options. Given
their relatively slow growth rates, existing trees rep-
resent a form of biological capital that should be con-
served. Multiple, incremental treatments, using com-
binations of fire and thinning, may be more conser-
vative and ecologically justifiable than the immediate
restoration of a particular structural element, such as
the ca. 1880 spatial distribution of overstory tree stems.
Given the great heterogeneity of Southwestern pon-
derosa pine forests, local data on fire history and pre-
settlement stand structures are required to implement
this precise structural restoration approach. Collecting
such data is expensive and requires the persistence of
presettlement forest ‘‘evidences’’ (Fule´ et al. 1997),
which have been lost in some locations due to logging,
harvesting of fuelwood, fire, or decay. As a result, it
is not practical, cost effective, or in some cases even
possible to reconstruct detailed structural reference
conditions for every restoration project area. Moreover,
some important data (such as density and spatial dis-
tribution of small trees) cannot be reconstructed from
remnant historical evidence.
A structure-oriented approach to forest restoration is
being proposed for wide implementation in the South-
west because of its methodological clarity, grounding
in quantifiable conditions, and scientific and political
support (Covington et al. 1997, Moore et al. 1999, Jen-
kins 2001). We consider historic structure to be one
ecologically valid reference criterion for forest resto-
ration. There are, however, theoretical and practical
concerns over regional application of a structure-ori-
ented approach (see Morgan et al. [1994], Kaufmann
et al. [1994], Landres et al. [1999], Wagner et al.
[2000], and Southwest Forest Alliance [2001] for re-
views and discussions). We contend that a broader con-
ceptual basis, as outlined in this paper, provides a more
comprehensive framework, and one that reflects the
diverse ecological conditions that exist in Southwestern
forests. We agree with Covington (2000:136) that there
is no ‘‘‘one size fits all’ approach to restoring the eco-
logical integrity of ponderosa pine forests.’’ A diversity
of restoration approaches should be applied in the
Southwest.
C
ONCLUSIONS
A primary goal of ecological restoration should be
to enhance resilience and sustainability of ecosystems.
Incremental adjustment of ecosystems back within an
envelope of natural range of variability should achieve
this goal (Covington and Moore 1994, Holling and
Meffe 1996, Stephenson 1999). A successful restora-
tion is one that sets ecological trends in the right di-
rection. In Southwestern ponderosa pine ecosystems
this means reducing tree density and ladder fuels along
with associated crown fire risk, protecting large trees,
restoring surface fires, and increasing herbaceous
ground cover and overall biodiversity levels. A con-
servative restoration program would incorporate nat-
ural variability by avoiding uniform treatments across
extensive areas. Spatial heterogeneity is critical to for-
est biodiversity. Existing forest structures, such as tree
groups and large trees, should not be removed simply
to recreate historical tree spatial patterns. While his-
torical reference conditions are useful for identifying
the types, magnitudes, and causes of ecosystem change,
such reconstructions are best used as general guides
rather than as precise and rigid prescriptions (Landres
et al. 1999).
Theories of restoration ecology and the practice of
ecological restoration are evolving rapidly. We propose
not so much a departure from current models as a
broader framework that encompasses a diversity of ap-
proaches. Restoration science cannot proceed without
empirical tests and experimentation across a broad
range of conditions, and restoration practice needs to
incorporate new insights derived from the scientific and
practical creativity of many individuals and institu-
tions. We need steady progress in restoration science
in order to sustain the social, political, and financial
support required for an effective regional restoration
program. Some restoration experiences, however, will
probably prove humbling, as our knowledge and meth-
ods will always be imperfect.
Risks and trade-offs are inherent in wildland man-
agement. For example, a controversial trade-off in-
volves consideration of merchantable timber harvest as
part of restoration. Commercial activities can help fund
restoration of less hazardous forest fuel conditions
1430
CRAIG D. ALLEN ET AL.
Ecological Applications
Vol. 12, No. 5
more quickly, but raise the risk of extractive economic
imperatives dominating decision making about ‘‘res-
toration’’ activities.
We need to begin large-scale restoration of ponde-
rosa pine ecosystems now. The present vulnerability of
these forest ecosystems requires that we temper our
need for more complete information with an urgency
created by the current risk of crown fires. Even res-
toration failures at this point can provide information
useful in refining future work, as long as we simulta-
neously invest in research and monitoring. At the same
time, we must be cautious in our application of res-
toration treatments. We must manage for surprise,
whether from climate variability, human influences, or
the synergies among known and unknown factors. It
would be prudent to incorporate buffers for uncertainty
in restoration work. Impatience, extractive economics,
or hubris could lead to widespread application of un-
duly intrusive treatments that could further damage for-
est ecosystems. The broad perspective outlined here
supports vigorous but conservative approaches for im-
mediately reducing crown fire risk and restoring natural
variability and long-term resilience to ponderosa pine
forest ecosystems in the American Southwest.
A
CKNOWLEDGMENTS
We appreciate the effortsofcolleagues at the GrandCanyon
Forest Partnership and the Ecological Restoration Instituteof
Northern Arizona University to share their restoration ex-
periences and perspectives, especially Pete Fule´, Wally Cov-
ington, and Brad Ack. Carl Edminster, Bill Block, Jon Keeley,
Peter Landres, Dave Parsons, Phil Weatherspoon, and an
anonymous reviewer provided helpful comments on earlier
versions of this paper. This material is based upon work sup-
ported by the USDA Forest Service, U.S. Geological Survey,
Wilburforce Foundation, Laird Norton Foundation, New
Mexico Community Foundation, and the National Science
Foundation under Grant number SBR-969410.
L
ITERATURE
C
ITED
Agee, J. K. 1993. Fire ecology of Pacific Northwest forests.
Island Press, Covelo, California, USA.
Agee, J. K., B. Bahro, M. A. Finney, P. N. Omi, D. B. Sapsis,
C. N. Skinner, J. W. van Wagtendonk, and C. P. Weather-
spoon. 2000. The use of shaded fuelbreaks in landscape
fire management. Forest Ecology and Management 127:55–
66.
Alexander, R. R., and F. Ronco, Jr. 1987. Classification of
the forest vegetation on the national forests of Arizona and
New Mexico. USDA Forest Service Research Note RM-
469.
Allen, C. D. 1998. A ponderosa pine natural area reveals its
secrets. Pages 551–552
in
M. J. Mac, P. A. Opler, C. E.
Puckett Haecker, and P. D. Doran, editors. Status and trends
of the nation’s biological resources. Two volumes. U.S.
Department of Interior, U.S. Geological Survey, Reston,
Virginia, USA.
Allen, C. D. 2002. Lots of lightning and plenty of people:
an ecological history of fire in the American Southwest.
Pages 143–183
in
T. R. Vale, editor. Western wilderness:
fire, native peoples, and the natural landscape. Island Press,
Covelo, California, USA.
Allen, C. D., and D. D. Breshears. 1998. Drought-induced
shift of a forest/woodland ecotone: rapid landscape re-
sponse to climate variation. Proceedings of the National
Academy of Sciences (USA) 95:14839–14842.
Baisan, C. H., and T. W. Swetnam. 1997. Interactions of fire
regimes and land-use history in the central Rio Grande
Valley. USDA Forest Service Research PaperRM-RP-330.
Baker, W. L., and D. Ehle. 2001. Uncertainty in surface-fire
history: the case of ponderosa pine forests in the Western
United States. Canadian Journal of Forest Research 31:
1205–1226.
Barclay, A. D. 2000. Effects of seeding with Rye grass (
Lol-
ium multiflorum
) on vegetation recovery following fire in
a ponderosa pine (
Pinus ponderosa
) forest. Thesis. Uni-
versity of Arizona, Tucson, Arizona, USA.
Belsky, A. J., and D. M. Blumenthal. 1997. Effects of live-
stock grazing on stand dynamics and soils in upland forests
of the Interior West. Conservation Biology 11:315–327.
Bennetts, R. E., G. C. White, F. G. Hawksworth, and S. E.
Severs. 1996. The influence of dwarf mistletoe on bird
communities in Colorado ponderosa pine forests. Ecolog-
ical Applications 6:899–909.
Bogan, M. A., C. D. Allen, E. H. Muldavin, S. P. Platania,
J. N. Stuart, G. H. Farley, P. Melhop, and J. Belnap. 1998.
Southwest. Pages 543–592
in
M. J. Mac, P. A. Opler, C.
E. Puckett Haecker, and P. D. Doran, editors. Status and
trends of the nation’s biological resources. Two volumes.
U.S. Department of Interior, U.S. Geological Survey, Res-
ton, Virginia, USA.
Brown, P. M., M. R. Kaufmann, and W. D. Shepperd. 1999.
Long-term, landscape patterns of past fire events in a mon-
tane ponderosa pine forest of central Colorado. Landscape
Ecology 14:513–532.
Brown, R. 2000. Thinning, fire, and forest restoration: a sci-
ence-based approach for national forests of the interior
Northwest. Defenders of Wildlife, Washington, D.C., USA.
Bull, E. L., C. G. Parks, and T. R. Torgersen. 1997. Trees
and logs important to wildlife in the Interior Columbia
River Basin. USDA Forest Service General Technical Re-
port PNW-GTR-391.
Community Forest Restoration Act. 30 October 2000. Public
Law 106-393, Statutes at Large 114:1607–1628.
Cooper, C. F. 1960. Changes in vegetation, structure, and
growth of southwestern pine forests since white settlement.
Ecological Monographs 30:129–164.
Covington, W. W. 2000. Helping western forests heal: the
prognosis is poor for United States forest ecosystems. Na-
ture 408:135–136.
Covington, W. W., R. L. Everett, R. Steele, L. L. Irwin, T.
A. Daer, and A. N. D. Auclair. 1994. Historical and antic-
ipated changes in forest ecosystems of the Inland West of
the United States. Journal of Sustainable Forestry 2:13–63.
Covington, W. W., P. Z. Fule´, M. M. Moore, S. C. Hart, T.
E. Kolb, J. N. Mast, S. S. Sackett, and M. R. Wagner. 1997.
Restoring ecosystem health in ponderosa pine forests of
the Southwest. Journal of Forestry 95:23–29.
Covington, W. W., and M. M. Moore. 1994. Southwestern
ponderosa pine forest structure: changes since Euro-Amer-
ican settlement. Journal of Forestry 92:39–47.
Crawford, J. S., C.-H. A. Wahren, S. Kyle, and W. H. Moir.
2001. Responses of exotic plant species responses to fires
in
Pinus ponderosa
forests in northern Arizona. Journal of
Vegetation Science 12:261–268.
Crowley, T. J. 2000. Causes of climate change over the past
1000 years. Science 289:270–277.
Dahm, C. W., and B. W. Geils, techical editors. 1997. An
assessment of forest ecosystem health in the Southwest.
USDA Forest Service General Technical Report RM-GTR-
295.
Dodd, N. L., S. S. Rosenstock, C. R. Miller, and R. E.
Schweinsburg. 1998. Tassel-eared squirrel population dy-
namics in Arizona: index techniques and relationships to
October 2002 1431
RESTORATION OF PONDEROSA PINE ECOSYSTEMS
habitat condition. Technical Report 27. Arizona Game and
Fish Department, Phoenix, Arizona, USA.
Everett, R., D. Schellhaas, D. Spurbeck, P. Ohlson, D. Keen-
um, and T. Anderson. 1997. Structure of Northern Spotted
Owl nest stands and their historical conditions on the east-
ern slope of the Pacific Northwest Cascades, USA. Forest
Ecology and Management 94:1–14.
Ffolliott, P. F., G. S. Gottfried, and M. B. Baker, Jr. 1989.
Water yield from forest snowpack management: research
findings in Arizona and New Mexico. Water Resources Re-
search 25:1999–2007.
Finney, M. A. 2001. Design of regular landscape fuel treat-
ment patterns for modifying fire growth and behavior. For-
est Science 47:219–228.
Fule´, P. Z., W. W. Covington, and M. M. Moore. 1997. De-
termining reference conditions for ecosystem management
of southwestern ponderosa pine. Ecological Applications
7:895–908.
Fule´, P. Z., A. E. M. Waltz, W. W. Covington, and T. A.
Heinlein. 2001. Measuring forest restoration effectiveness
in reducing hazardous fuels. Journal of Forestry 99(11):
24–29.
Graham, R. T., A. E. Harvey, T. B. Jain, and J. R. Tonn. 1999.
The effects of thinning and similar stand treatments on fire
behavior in western forests. USDA Forest Service General
Technical Report PNW-GTR-463.
Grissino-Mayer, H. D., and T. W. Swetnam. 2000. Century-
scale climate forcing of fire regimes in the American South-
west. The Holocene 10:207–214.
Grissino-Mayer, H. D., T. W. Swetnam, and R. K. Adams.
1997. The rare old-aged conifers of El Malpais: their role
in understanding climate change in the American South-
west. Pages 155–161
in
K. Mabery, compiler. Natural his-
tory of El Malpais National Monument. Bulletin 156. New
Mexico Bureau of Mines and Mineral Resources, Socorro,
New Mexico, USA.
Hanson, C. 2000. Chopping down the forest won’t save it.
[Letter to the Editor.] New York Times, National edition,
19 May 2000, Section A: p. 27 (col. 1).
Hardy, C. C., D. L. Bunnell, J. P. Menakis, K. M. Schmidt,
D. G. Long, D. G. Simmerman, and C. M. Johnston. 1999.
Coarse-scale spatial data for wildland fire and fuel man-
agement. USDA Forest Service, Rocky Mountain Research
Station, Fire Sciences Laboratory, Missoula, Montana,
USA. [Online, URL:
^
http://www.fs.fed.us/fire/fuelman
&
.]
Harrington, M. G., and S. S. Sackett. 1992. Past and present
fire influences on southwestern ponderosa pine old growth.
Pages 44–50
in
M. R. Kaufmann, W. H. Moir and R. L.
Bassett, editors. Old-growth forests in the Southwest and
Rocky Mountain regions. USDA Forest Service General
Technical Report RM-213.
Hole, F. D. 1981. Effects of animals on soil. Geoderma 25:
75–112.
Holling, C. S. 1978. Adaptive environmental assessment and
management. John Wiley and Sons, New York, New York,
USA.
Holling, C. S., and G. K. Meffe. 1996. Command and control
and the pathology of natural resource management. Con-
servation Biology 10:328–337.
Holthausen, R. S., M. G. Raphael, F. B. Samson, D. Ebert,
R. Hiebert, and K. Menasco. 1999. Population viability in
ecosystem management. Pages 135–156
in
R. C. Szaro et
al., editors. Ecological sewardship–a common referencefor
ecosystem management. Volume II. Elsevier Science, Ox-
ford, UK.
Jenkins, M. 2001. ‘‘Making forests safe again won’t be a
walk in the park.’’ High Country News, 7 May, p. 10.
Johnson, T. H., and R. H. Wauer. 1996. Avifaunal response
to the 1977 La Mesa fire. Pages 70–94
in
C. D. Allen,
technical editor. Fire effects in Southwestern forests. Pro-
ceedings of the Second La Mesa Fire Symposium. USDA
Forest Service General Technical Report RM-GTR-286.
Jurgensen, M. F., A. E. Harvey, R. T. Graham, D. S. Page-
Dumroese, J. R. Tonn, M. G. Larsen, and T. B. Jain. 1997.
Impacts of timber harvesting on soil organic matter, nitro-
gen, productivity, and health of inland Northwest forests.
Forest Science 43:234–251.
Kaufmann, M. R., R. T. Graham, D. A. Boyce, Jr., W. H.
Moir, L. Perry, R. T. Reynolds, R. L. Bassett, P. Mehlhop,
C. B. Edminster, W. M. Block, and P. S. Corn. 1994. An
ecological basis for ecosystem management. USDA Forest
Service General Technical Report RM-246.
Kaufmann, M. R., L. S. Huckaby, C. M. Regan, and J. Popp.
1998. Forest reference conditions for ecosystem manage-
ment in the Sacramento Mountains, New Mexico. USDA
Forest Service General Technical Report RMRS-GTR-19.
Kaye, J. P., and S. C. Hart. 1998. Ecological restoration alters
nitrogen transformations in a ponderosa pine-bunchgrass
ecosystem. Ecological Applications 8:1052–1060.
Kaye, M. W., and T. W. Swetnam. 1999. An assessment of
fire, climate, and Apache history in the Sacramento Moun-
tains, New Mexico, USA. Physical Geography 20:305–330.
Kennedy, P. L. 1997. The Northern Goshawk (
Accipter gen-
tilis atricapillus
): Is there evidence of a population decline?
Journal of Raptor Research 31:95–106.
Kloor, K. 2000. Returning America’s forests to their ‘‘nat-
ural’’ roots. Science 287:573–575.
Landres, P., P. Morgan, and F. J. Swanson. 1999. Overview
of the use of natural variability concepts in managing eco-
logical systems. Ecological Applications. 9:1179–1188.
Ledig, F. T. 1992. Human impacts on genetic diversity in
forest ecosystems. Oikos 63:87–108.
MacNeil, J. S. 2000. Forest fire plan kindles debate. Science
289:1448–1449.
Madany, M. H., and N. E. West. 1983. Livestock grazing–
fire regime interactions within montane forests of Zion Na-
tional Park. Ecology 64:661–667.
Mann, M. E., R. Bradley, and M. K. Hughes. 1998. Northern
hemisphere temperatures during the past millennium: in-
ferences, uncertainties, and limitations. Geophysical Re-
search Letters 26:759–762.
Marston, E. L. Criley, A. Brower, S. Kennedy, J. Downing,
H. Nolde, C. Smith, and H. Swartz. 2001. ‘‘Restoring the
range of light’’ [a series of articles]. High Country News,
27 August, pp. 1, 8–14.
Mast, J. N., P. Z. Fule´, M. M. Moore, W. W. Covington, and
A. E. M. Waltz. 1999. Restoration of presettlement age
structure of an Arizona ponderosa pine forest. Ecological
Applications 9:228–239.
Matthews, M. 2001. ‘‘Back into the woods: The West goes
to work cleaning up its forests.’’ High Country News, 7
May, pp. 1, 8–11.
Millar, C. I., and W. J. Libby. 1989. Restoration: Disneyland
or a native ecosystem? A question of genetics. Fremontia
17:3–10.
Millar, C. I., and W. B. Woolfenden. 1999. Therole of climate
change in interpreting historical variability. Ecological Ap-
plications 9:1207–1216.
Miller, C., and D. L. Urban. 2000. Modeling the effects of
fire management alternatives on Sierra Nevada mixed-co-
nifer forests. Ecological Applications 10:85–94.
Moir, W. H., and J. H. Dieterich. 1988. Old-growth ponderosa
pine from succession in pine–bunchgrass forests in Arizona
and New Mexico. Natural Areas Journal 8:17–24.
Moore, M. M., W. W. Covington, and P. Z. Fule´. 1999. Ref-
erence conditions and ecological restoration: a Southwest-
ern ponderosa pine perspective. Ecological Applications 9:
1266–1277.
Morgan, P., G. H. Aplet, J. B. Haufler, H. C. Humphries, M.
M. Moore, and W. D. Wilson. 1994. Historical range of
1432
CRAIG D. ALLEN ET AL.
Ecological Applications
Vol. 12, No. 5
variability: a useful tool for evaluating ecosystem change.
Journal of Sustainable Forestry 2:87–111.
Morgan, P., C. Hardy, T. W. Swetnam, M. G. Rollins, and D.
G. Long. 2001. Mapping fire regimes across time and
space: understanding coarse and fine-scale patterns. Inter-
national Journal of Wildland Fire 10:329–342.
Morrison, P. H., J. W. Karl, K. J. Harma, L. Swope, T. K.
Allen, and P. Becwar. 2000. Assessment of summer 2000
wildfires: landscape history, current condition, and own-
ership. Pacific Biodiversity Institute, Winthrop, Washing-
ton, USA. [Online, URL:
^
http://www.pacificbio.org
&
.]
National Forest Protection Alliance. 2001. Restoration and Our
National Forests. [Online, URL:
^
http://www.forestadvocate.
org
&
.]
New Mexico Department of Game and Fish. 2000. Biota
information system of New Mexico. Electronic database,
Version January 2000. New Mexico Department of Game
and Fish, Santa Fe, New Mexico, USA. [Available only on
CD-ROM].
Nijhuis, M. 1999. ‘‘Flagstaff searches for its forests’ future.’’
High Country News, 1 March, pp. 8–12.
Noss, R. F., E. T. LaRoe, III, and J. M. Scott. 1995. Endan-
gered ecosystems of the United States: a preliminary as-
sessment of loss and degradation. National Biological Ser-
vice Biological Report 28. U.S. Department of the Interior,
Washington, D.C., USA.
Oliver, C. D., A. Osawa, and A. Camp. 1998. Forestdynamics
and resulting animal and plant population changes at the
stand and landscape levels. Journal of Sustainable Forestry
6:281–312.
Patton, D. R., and K. E. Severson. 1989. WILDHARE: a
wildlife habitat relationships data model for southwestern
ponderosa pine. Pages 268–276
in
A. Techle, W. W. Cov-
ington, and R. H. Hamre, technical coordinators. Multi-
resource management of ponderosa pine forests. USDA
Forest Service General Technical Report RM-185.
Pearson, G. A. 1950. Management of ponderosa pine in the
Southwest. USDA Forest Service, AgriculturalMonograph
no. 6. U.S. Government Printing Office, Washington, D.C.,
USA.
Rehfeldt, G. E. 1991. Models of genetic variation for
Pinus
ponderosa
in the inland Northwest (U.S.A.): applications
in gene resource management. Canadian Journal of Forest
Research 21:1491–1500.
Reynolds, R. T., R. T. Graham, M. H. Reiser, R. L. Bassett,
P. L. Kennedy, D. A. Boyce, Jr., G. Goodwin, R. Smith,
and E. L. Fisher. 1992. Management recommendations for
the Northern Goshawk in the Southwestern United States.
USDA Forest Service General Technical Report RM-217.
Rieman, B., and J. Clayton. 1997. Wildlife and native fish:
issues of forest health of sensitive species. Fisheries22(11):
6–15.
Robichaud, P. R., J. L. Beyers, and D. G. Neary. 2000. Eval-
uating the effectiveness of postfire rehabilitation treat-
ments. USDA Forest Service General Technical Report
RMRS-GTR-63.
Rollins, M. G. 2000. Twentieth century landscape fire pat-
terns in the Gila/Aldo Leopold Wilderness Areas, New
Mexico, and Selway-Bitterroot Wilderness Area, Idaho/
Montana. Dissertation. University of Arizona, Tucson, Ar-
izona, USA.
Rosenstock, S. S. 1998. Influence of Gambel oak on breeding
birds in ponderosa pine forests of northern Arizona. Condor
100:485–492.
Rummell, R. S. 1951. Some effects of livestock grazing on
ponderosa pine forest and range in central Washington.
Ecology 32:594–607.
Savage, M., P. M. Brown, and J. Feddema. 1996. The role
of climate in a pine forest regeneration pulse in the south-
western United States. Ecoscience 3:310–318.
Shinneman, D. J., and W. L. Baker. 1997. Nonequilibrium
dynamics between catastrophic disturbances and old-
growth forests in ponderosa pine landscapes of the Black
Hills. Conservation Biology 11:1276–1288.
Smith, H. Y., and S. F. Arno, editors. 1999. Eighty years of
change in a managed ponderosa pine forest. USDA Forest
Service General Technical Report RMRS-GTR-23.
Southwest Forest Alliance. 2001. Why the Flagstaff preset-
tlement restoration model should not be applied to public
forest lands. Southwest Forest Alliance, Flagstaff, Arizona,
USA. [Online, URL:
^
http://www.swfa.org
&
.]
Stacey,P. B., V. A. Johnson, and M. L. Taper. 1996. Migration
within metapopulations: the impact upon local population
dynamics. Pages 267–291
in
I. Hanski and M. Gilpin, ed-
itors. Metapopulation biology: ecology, genetics and evo-
lution. Academic Press, New York, New York, USA.
Stephenson, N. L. 1987. Use of tree aggregations in forest
ecology and management. Environmental Management 11:
1–5.
Stephenson, N. L. 1999. Reference conditions for giant se-
quoia forest restoration: structure, process, and precision.
Ecological Applications 9:1253–1265.
Suckling, K. 1996. Forests forever! A plan to restore eco-
logical and economic integrity to the Southwest’s National
Forests and forest dependent communities. Southwest For-
est Alliance, Flagstaff, Arizona, USA.
Swetnam, T. W., C. D. Allen, and J. L. Betancourt. 1999.
Applied historical ecology: using the past to manage for
the future. Ecological Applications 9:1189–1206.
Swetnam, T. W., and C. H. Baisan. 1996. Historical fire re-
gime patterns in the Southwestern United States since AD
1700. Pages 11–32
in
C. D. Allen, technical editor. Fire
effects in Southwestern forests. Proceedings of the Second
La Mesa Fire Symposium. USDA Forest Service General
Technical Report RM-GTR-286.
Swetnam, T. W., and J. L. Betancourt. 1998. Mesoscale dis-
turbance and ecological response to decadal climatic var-
iability in the American Southwest. Journal of Climate 11:
3128–3147.
Tiedemann, A. R., J. O. Klemmedson, and E. L. Bull. 2000.
Solution of forest health problems with prescribed fire: Are
forest productivity and wildlife at risk? Forest Ecology and
Management 127:1–18.
Trombulak, S. C., and C. A. Frissell. 2000. Review of eco-
logical effects of roads on terrestrial and aquatic commu-
nities. Conservation Biology 14:18–30.
USDA Forest Service. 2000. Protecting people and sustain-
ing resources in fire-adapted ecosystems: a cohesive strat-
egy. USDA Forest Service, Washington, D.C., USA. [Also
online, URL:
^
http://www.fireplan.gov
&
.]
USDA and USDI [USDA Forest Service, and U.S. Depart-
ment of the Interior]. 2000
a
. National fire plan: managing
the impact of wildfires on communities and the environ-
ment. A report to the President in response to the wildfires
of 2000. [Online, URL:
^
http://www.fireplan.gov
&
.]
USDA and USDI [USDA Forest Service and USDI Bureau
of Land Management]. 2000
b
. Interior Columbia Basin
Final Environmental Impact Statement. Interior Columbia
Basin Ecosystem Management Project, Boise, Idaho, USA.
[Online:
^
http//www.icbemp.gov/
&
.]
USDI [United States Department of the Interior]. 2000. Re-
ducing the risks and consequences of catastrophic wildland
fires on DOI lands. Washington, D.C.
U.S. Fish and Wildlife Service. 1995. Recovery plan for the
Mexican Spotted Owl (
Strix occidentalis lucida
). U.S. Fish
and Wildlife Service, Albuquerque, New Mexico, USA.
U.S. Fish and Wildlife Service. 1998. Northern goshawk
status review. U.S. Fish and Wildlife Service, Portland,
Oregon, USA.
U.S. General Accounting Office. 1999. Western nationalfor-
October 2002 1433
RESTORATION OF PONDEROSA PINE ECOSYSTEMS
ests: a cohesive strategy is needed to address catastrophic
wildfire threats. GAO/RCED-99-65. U.S. Government
Printing Office.
Veenhuis, J. 2002. Effects of wildfire on the hydrology of
Capulin and Rito de los Frijoles Canyons, Bandelier Na-
tional Monument, New Mexico, Water Resources Investi-
gations Report. U.S. Geological Survey,Albuquerque,New
Mexico, USA,
in press
.
Wagner, M. R., W. M. Block, B. W. Geils, and K. F. Wenger.
2000. Restoration ecology: A new forest management par-
adigm or another merit badge for foresters? Journal of For-
estry 98:22–27.
Waltz, A. F. M., and W. W. Covington. 1999. Butterfly rich-
ness and abundance increase in restored ponderosa pine
ecosystem (Arizona). Ecological Restoration 17:244–246.
Weatherspoon, C. P., and C. N. Skinner. 1996. Landscape-
level strategies for forest fuel mangement. Chapter 56
in
Sierra Nevada ecosystem project. Final report to Congress.
Volume II. Assessments and scientific basis for manage-
ment options. Wildland Resources Center Report number
39. Centers for Water and Wildland Resources, University
of California, Davis, California, USA.
Weaver, H. 1951. Fire as an ecological factor in Southwestern
ponderosa pine forests. Journal of Forestry 49:93–98.
Webb, D. R., and R. L. Henderson. 1985. Gila Wilderness
prescribed fire program. Pages 413–414
in
J. E. Lotan, B.
M. Kilgore, W. C. Fischer, and R. W. Mutch, technical
coordinators. Proceedings—Symposium and Workshop on
Wilderness Fire. USDA Forest Service General Technical
Report INT-182.
Western Governors’ Association, U.S. Forest Service, and
U.S. Department of the Interior. 2001. A collaborative
approach for reducing wildland fire risks to communities
and the environment: 10-year comprehensive strategy, Au-
gust 2001. [Online, URL:
^
http://www.fireplan.gov
&
.]
White, A. S. 1985. Presettlement regeneration patterns in a
southwestern ponderosa pine stand. Ecology 66:589–594.
White, C. S. 1994. Monoterpenes: their effects on ecosystem
nutrient cycling. Journal of Chemical Ecology 20:1381–
1406.
White, P. S., and J. L. Walker. 1997. Approximating nature’s
variation: selecting and using reference information in res-
toration ecology. Restoration Ecology 5:338–349.
White, W. D. 1996. Geomorphic responses of six headwater
basins fifteen years after the La Mesa fire. Pages 95–113
in
C. D. Allen, technical editor. Fire effects inSouthwestern
forests: Proceedings of the Second La Mesa Fire Sympo-
sium. U.S. Forest Service General Technical Report RM-
GTR-286.
Woolsey, T. S., Jr. 1911. Western yellow pine in Arizona and
New Mexico. U.S. Forest Service Bulletin 101. U.S. Gov-
ernment Printing Office, Washington, D.C., USA.
Wuerthner, G. 1999. ‘‘Friendly fire.’’ Green Roots—the
Newsletter of Forest Guardians, Summer 1999, no. 6, p. 5.
