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A WIDESPREAD SPECIES AT RISK
Ahardy inhabitant of the subalpine zone of
western North America, whitebark pine
(Pinus albicaulis) is a keystone tree species in
California’s subalpine forests, where it regu-
larly deﬁnes the upper treeline in the Sierra Nevada,
Cascade, Warner, and Klamath Mountains. Walking
portions of the John Muir Trail in the southern Sierra
Nevada, moving through extensive stands and mats of
whitebark, one might wonder why such an apparently
widespread and hardy species would be under consid-
eration for listing as a federally endangered species.
Though whitebark is not uncommon in California,
there is growing concern for its persistence, given
recent observations of increased mortality, which may
be exacerbated in coming decades due to the effects
of climate change (Millar et al. 2012, Moore et al.
2017, Meyer and North in press). It is such concerns,
in addition to dramatic and rapid declines throughout
much of its range, that have led to proposals for listing
this species under the federal Endangered Species Act
(USFWS 2011). Indeed, as we go to press, status infor-
mation related to listing is under review by the U.S.
Fish and Wildlife Service (USFWS).
SUBALPINE SENTINELS: UNDERSTANDING &
MANAGING WHITEBARK PINE IN CALIFORNIA
Michèle Slaton1, Marc Meyer2, Shana Gross3, Jonathan Nesmith4, Joan Dudney5,
Phillip van Mantgem6, Ramona Butz7
Figure 1. Whitebark pine cluster with basal sprouts on Table Mt.,
Bishop Creek, southern Sierra Nevada. [U.S. Forest Service]
“The slender lash-like sprays of the Dwarf Pine stream out in wavering ripples,
but the tallest and slenderest are far too unyielding to wave even in the heaviest gales.”
–John Muir refers to whitebark as Dwarf Pine in The Mountains of California, 1894
35VOL. 47, NO. 1, MAY 2019
1. USDA Forest Service, Paciﬁc Southwest Region, Remote Sensing Laboratory; firstname.lastname@example.org; author for correspondence
2. USDA Forest Service, Paciﬁc Southwest Region, Regional Ecology Program; email@example.com
3. USDA Forest Service, Paciﬁc Southwest Region, Regional Ecology Program; firstname.lastname@example.org
4. National Park Service Inventory & Monitoring Program, Sierra Nevada Network; email@example.com
5. Department of Environmental Science, Policy and Management, University of California, Berkeley; firstname.lastname@example.org
6. U.S. Geological Survey, Western Ecological Research Center; email@example.com
7. USDA Forest Service, Paciﬁc Southwest Region, Regional Ecology Program; firstname.lastname@example.org
In 2013, the U.S. Forest Service (USFS)
placed whitebark pine on its Sensitive Species
list in California. As a result, activities that could
potentially affect the species must be evaluated
under the National Environmental Policy Act.
Nonetheless, there are relatively few studies that
address the condition and health of whitebark
pine in California, as distinct from elsewhere in
western North America. Comprehensive man-
agement for whitebark pine was addressed in a
recent range-wide restoration strategy (Keane
et al. 2012), but this is largely focused on the
Rocky Mountains and Paciﬁc Northwest, due
to relatively high impacts in these regions from
threats such as mountain pine beetle outbreaks,
white pine blister rust, climate change, and ﬁre
exclusion (Keane et al. 2012, Keane et al. 2017).
So while these threats have caused precipitous
declines of whitebark outside California, the
southern Sierra population, for example, remains
relatively healthy (Nesmith et al. 2019).
This means there is a high degree of interest
among scientists, land managers, and stake-
holders in gaining a better understanding of
the potentially unique attributes of California’s
whitebark populations, which could serve a criti-
cal role in future management strategies. Even so,
California is the only region that does not cur-
rently have an active genetic restoration program
for whitebark. In other regions, these programs
often include the collection, breeding, and plant-
ing of stock resistant to the non-native invasive
pathogen, white pine blister rust. Current pros-
pects for the development of such a restoration
strategy and reforestation program in the state are
promising, but these efforts will require consider-
able effort, cost, and coordination (Maloney et al.
2012). Here we review the most recent work eval-
uating whitebark health and status in California,
and present the initial ﬁndings of a collaborative
effort to establish a baseline of stand structure
and health for continued monitoring.
A MAPPING CHALLENGE
At its highest elevations, whitebark often occurs in pure
or nearly pure stands, resulting in geographically isolated
stands on mountaintops. Most often found on windswept
alpine and subalpine slopes and ridges, whitebark can
either develop an upright stature or occur in krummholz
(German for “twisted wood”) cushions, or clumps, forming
sometimes impenetrable islands that may exceed an acre in
Figure 2. Distribution of whitebark in California, by geographic zone.
Surveys revealed much more limited distribution at the southern range
limit, with only two very small populations conﬁrmed south of the revised
boundary. Red arrow indicates highly isolated populations of Klamaths.
Inset courtesy Whitebark Pine Foundation. [http://whitebarkfound.org]
size. Its success at high elevations can be attributed in
part to tolerance of cold temperatures and adaptation
to a short growing season, as well as to its structural
ability to thrive near the ground surface, and thus
remain protected from winter winds and desiccation
under snow. Generally regarded as a disturbance-toler-
ant, early successional species, whitebark can be a ﬁrst
colonizer following a rockslide, avalanche, or stand-re-
placing ﬁre. Yet on the harshest sites at or near treeline,
it often forms “climax” communities where it is the
dominant species (Arno and Weaver 1990).
Lower, stands are typically co-dominated by moun-
tain hemlock, lodgepole pine, foxtail pine, western
white pine, limber pine, and red or white ﬁr (Tsuga
mertensiana, P. contorta ssp. murrayana, P. balfouriana,
P. monticola, P. ﬂexilis, Abies magniﬁca, A. concolor,
Although whitebark has a broad geographic range,
precise abundance and distribution information for
California is limited. In 2014, the USFS compiled an
updated map for whitebark-dominant stands, based
on the CalVeg dataset (USDA Forest Service 2013a),
2012 National Insect and Disease Risk Maps (Krist
et al. 2014), ﬁeld visits, high resolution imagery, and
aerial photography (Bokach 2014). Based on this
effort, we estimated that there are 150,558 hectares
(372,035 acres) of whitebark in stands greater than 0.4
hectares (one acre) in California.
Our more recent mapping and ground-truthing
efforts in 2018 indicate that map improvements are still
needed on over 20,000 acres, due to previous errors in
interpretation of aerial photography and other imagery
and also to the difﬁculty—even among experienced
botanists—of determining species identity in situ if
cones are absent. The two main look-alikes are western
white pine and limber pine (P. monticola and P. ﬂexilis,
respectively). You can distinguish western white pine in
the ﬁeld with a hand lens by noting the ﬁne serrations
on its needle-like leaves. But limber and whitebark
pine are virtually indistinguishable, especially when
young, before whitebark acquires its namesake color
and develops mature cones. Given that the distribution
of whitebark pine in California represents the south-
ernmost extent of the species (Arno and Hoff 1989),
and risk for populations occurring at their range edge
is elevated (Slaton 2015), continued study and map-
ping of these populations is needed to identify their
potentially unique genetic make-up and take potential
action, such as seed collection or restoration, to ensure
their continued persistence (Syring et al. 2016).
DEMOGRAPHY OF A SUBALPINE TREE
The seeds of whitebark pines are wingless and rarely
dispersed by wind. Instead they rely on dispersion
by squirrels or birds, primarily Clark’s nutcrack-
ers (Nucifraga columbiana) (Arno and Hoff 1989,
Tomback et al. 2001). These animals bury the seeds
in the soil in small caches; if not reclaimed, the seeds
may germinate and grow. Whitebark regeneration is
therefore found most often in clumps, a form which
can be accentuated by the tendency of lower branches
to become pressed horizontally against moist ground
from snow and then grow upright. Stems that do reach
tree size (greater than 7.5 centimeters in diameter at
breast height) are generally small compared to most
other conifers, with height and diameter averaging 7
meters (23 feet) and 20 centimeters, respectively, in
California (USFS, unpublished data).
Figure 3. (a) Whitebark pine cones [D. Pechurina], and (b) a cluster of seedlings and now empty seeds, cached in the soil by animals [USFS].
37VOL. 47, NO. 1, MAY 2019
Understanding the variability in stand structure and
reproductive patterns between geographic regions can
help to inform potential restoration strategies. For
example, the relatively low tree density in the Warner
Mountains, coupled with high proportions of conifers
other than whitebark (namely, white ﬁr) indicates that
the sun-loving whitebark trees may be more vulner-
able to being outcompeted by shade-tolerant species
than in other regions of California. Also, whitebark’s
low reproductive success—sexual or asexual—in the
Warner Mountains contrasts with the relatively high
densities of young seedlings on the eastern side of the
southern Sierra Nevada, perhaps indicating that suc-
cess of planting efforts may vary by biogeographic
Finally, whereas previous studies have found
increases in whitebark following disturbance in the
southern Sierra Nevada (Meyer et al. 2016), we did
not see this same correlation expressed at the scale of
geographic regions—e.g. high recruitment rates in the
Cascade and Klamath regions are coupled with rela-
tively low disturbance rates. Such variable relationships
emphasize how critical scale and ecological context are
to understanding stand dynamics and planning resto-
Given whitebark pine’s broad geographic extent,
consideration of genetic variation across regions is of
utmost importance in developing potential conserva-
tion actions (Coutts et al. 2016). Studies are currently
underway to assess regional genetic diversity and pos-
sible associations with climatic variables in central
and southern Sierra Nevada whitebark pine popu-
lations (Elizabeth Milano, personal communication).
In addition, we are ﬁnding whitebark stands at the
edge of the tree’s range in the southern Sierra Nevada
undergoing proportional increases in recruitment
of other conifer species, especially in the absence of
disturbances that would create canopy openings and
favor sun-loving whitebark (Slaton et al. in review).
We did not observe this, however, in the interior part
of its range. Thus, a revised southern distribution
map may provide critical information on these vul-
nerable population segments.
A RESILIENT, YET VULNERABLE TREE
The USFWS designated whitebark pine as a candi-
date for listing under the Endangered Species Act
in 2011 due to a suite of factors, including altered
ﬁre regimes; the introduced pathogen, white pine
Figure 4. Diversity in whitebark structure. (a) Tree islands
and clumps in southern Sierra Nevada, (b) upright trees,
killed by mountain pine beetle in Warner Mountains,
(c) extensive krummholz mat in the Cascades [USFS].
Figure 3. (a) Whitebark pine cones [D. Pechurina], and (b) a cluster of seedlings and now empty seeds, cached in the soil by animals [USFS].
blister rust (Cronartium ribicola); mountain pine
beetle (Dendroctonus ponderosae); and climate change
(Tomback and Achuff 2010, USFWS 2011). These
stressors have led to dramatic declines in whitebark
across much of its range in the Rocky Mountains
(Keane et al. 2012, Keane et al. 2017). Here we focus
on how these threats are likely to affect whitebark pop-
ulations in California in the future.
Changing ﬁre regimes
Fire plays an important role in maintaining the health
and resilience of whitebark pine forests throughout its
geographic range. Historically, ﬁres burned every 70
to 90 or more years in many upright (non-krumm-
holz) stands, although researchers have documented
shorter ﬁre return intervals in other high-elevation for-
ests (Murray and Siderius 2018, Meyer and North in
press). Fire effects are variable, with some stands burn-
ing primarily at low severity (i.e., non-lethal surface
ﬁres) because of sparse surface and canopy fuels, and
other stands burning at mixed severity (i.e., ﬁre effects
are highly variable over space and time) where trees
are denser and fuels are spatially contiguous (Keane et
al. 2012). Many areas in California are experiencing
rapid shifts in ﬁre severity, frequency, and extent, due
to factors including warming temperatures, past ﬁre
suppression, and increased human ignitions (Keeley
and Syphard 2016). We need more research and analy-
sis to understand the current and projected changes in
subalpine ﬁre regimes in California.
Blister rust is an invasive pathogen native to northeast-
ern Asia. It arrived in the United States around 1910
and spread through most of the range of whitebark
pine and related ﬁve-needle (or white) pines, reach-
ing the Sierra Nevada in 1968 (Kliejunas and Adams
2003). Whitebark is considered one of the most sus-
ceptible species of all the white pine hosts, including
western white pine and limber pine (Kinloch and
Within the Sierra Nevada, blister rust occurrence
and severity generally decline from north to south. For
example, in Lassen National Park, Jules et al. (2017)
found an average infection rate of 54% on whitebark
pine. Maloney et al. (2012) found that, on average,
35% of individual whitebark pine trees showed symp-
toms of infection in the Tahoe basin, while Nesmith
et al. (2019) and Dudney et al. (unpublished data)
estimate that less than 1% of individual trees in the
southern Sierra Nevada are infected. This trend is
likely due to a combination of factors, including the
relatively recent arrival of blister rust in the south, and
the Sierra’s relatively hot and dry climate. Although
infections are still relatively low in the southern Sierra
Nevada, Nesmith (2018a and 2018b) documents new
observations of blister rust in Yosemite, Sequoia, and
Kings Canyon National Parks.
Figure 5. Geographic diversity in impacts of disturbance agents in
whitebark ecosystems. Plot sample size indicated by n; data collected
2014-2018. Data combined from USFS and National Park Service
protocols, plot size 0.12 - 0.62 acre (0.05 – 0.25 hectare). Other
data sources indicate higher incidence of blister rust in central Sierra
(Maloney et al., 2012); note USFS reports incidence by stem, whereas
NPS reports by clump.
Figure 6. Variability in tree (> 7.5 centimeter diameter at breast height)
and seedling (< 5 years old) density by geographic zone. Asexual
regeneration is not accounted for here, although plots sampled in 2018
indicate highest basal sprout density in southern Sierra Nevada, and
lowest in Warner Mountains. Sample sizes as in Figure 5; statistical
analyses to be conducted following 2019 ﬁeld campaign.
39VOL. 47, NO. 1, MAY 2019
Mountain pine beetle
The mountain pine beetle is native to western North
America, including California, and is considered an
important agent of disturbance in maintaining struc-
tural and compositional diversity of conifer forests
(Weed et al. 2015). Recent warming trends have
allowed the beetle to complete its seasonal life cycle
at higher elevations, leading to increasingly common
infestations in whitebark pine (Logan and Powell
2001, Mock et al. 2007, Kauffmann et al. 2014).
It causes mortality by carving galleries through the
xylem and phloem, and can be especially aggressive in
drought-stressed trees. Although beetle outbreaks in
California have been much lower compared to most
other areas of its range, recent observations suggest
this trend is changing and beetle populations are
increasing (Millar et al. 2012, Meyer et al. 2016).
Our data collection in 196 plots across the state from
2014 through 2018 indicated that mountain pine
beetle is impacting 9% of whitebark pine trees. Many
trees with symptoms of past attack have survived, and
the chance of survival varies by region. For example,
statewide, roughly one-half of attacked trees died;
however, in the Warner Mountains, 100% of the
attacked trees appear to have died.
Studies are currently underway to understand the
impacts of warming temperatures, drought, and cli-
matic water deﬁcits on whitebark growth and sur-
vival in the Sierra Nevada. Dolance et al. (2013) has
presented evidence that warming temperatures may
increase recruitment and promote survival of small
trees, leading to shifting stand structure weighted
toward smaller, younger trees. However, tempera-
ture-induced increases in aridity may exacerbate phys-
iological stress and susceptibility to mountain pine
beetles (Logan et al. 2010, Millar et al. 2012, Moore et
al. 2017). In addition, low minimum temperatures are
known to control both beetle and blister rust spread
(Weed et al. 2013). Thus, rising temperatures may
facilitate an upward expansion of both blister rust and
beetles to higher elevations, creating concern for the
long-term outlook of whitebark pine.
Figure 7. Threats to whitebark pine: (a) Severe
mountain pine beetle attack at June Mt. Ski
Area, southern Sierra Nevada [B. Oblinger];
(b) mountain pine beetle galleries [USFS]; (c)
pitch produced by whitebark to expel mountain
pine beetles [USFS]; (d) white pine blister rust
aeciospores on whitebark. [USFS]
SCIENCE IN ACTION
Active forest management of California’s whitebark
stands has been exceedingly limited for several reasons.
First of all, the stands, which are mostly located in wil-
derness or roadless areas, are relatively inaccessible. And
secondly, they are found in more “natural” conditions
that appear relatively unaltered from historic reference
conditions, and therefore don’t need much active man-
agement for restoration (Meyer and North in press).
Nonetheless, prescribed ﬁre (wildland ﬁre managed for
resource objectives) has been identiﬁed as an import-
ant resource management tool elsewhere in the west-
ern U.S. for restoring whitebark pine forests that may
have experienced decades of ﬁre exclusion (Keane et al.
2012). In addition, USFS has recently implemented
several forest management projects in whitebark stands
within ski resorts in the Sierra Nevada. These projects
provide an opportunity for us to better understand the
effects of forest management treatments and mitiga-
tion measures on whitebark in California.
One example is the 2018 initiative by the USFS
Lake Tahoe Basin Management Unit and Heavenly
Mountain Resort to develop a proactive whitebark
management plan. The intent of this Whitebark Pine
Partnership Action Plan is to minimize impacts from
threats and to foster restoration by undertaking the
following actions: (1) restore stands and increase resil-
ience to stressors through mechanical thinning and
prescribed burning; (2) reduce white pine blister rust
where feasible by pruning and/or removing infected
trees; (3) promote stand regeneration through canopy
gap creation; and (4) collect viable seeds for genetic
testing and planting.
Another example is the emerging partnership of the
Inyo National Forest, Mammoth Mountain Ski Area,
National Fish and Wildlife Foundation, and CalTrout
to restore whitebark pine stands impacted by moun-
tain pine beetle and drought in the June Mountain ski
area. The project is designed to increase stand resil-
ience to future bark beetle attack and climate change;
promote and protect natural whitebark pine regener-
ation; reduce hazardous fuels associated with ampli-
ﬁed tree mortality (which decreases wildﬁre risk to the
nearby community of June Lake); and improve water-
shed function. In both examples, engaged partners will
monitor the effectiveness of the treatments and evalu-
ate long-term trends in ecosystem health.
Additional opportunities for restoration may exist
in whitebark stands found in several accessible spec-
tacular areas—with dramatic peaks and gorgeous
subalpine lakes—that attract large numbers of recre-
ational visitors. Potential impacts from recreational
activities, such as trail system use and camping, are not
well understood. So the Inyo National Forest recently
undertook an assessment of impacts to whitebark
pines in four major watersheds in the eastern Sierra
Nevada where paved roads, campgrounds, and trail-
heads occur in whitebark habitat. Recruitment in these
areas is extremely limited, and mature trees are affected
by soil compaction and by branch and stem cutting.
While these impacts occur in only a small portion of
the whitebark’s range, site accessibility and public visi-
bility make these areas excellent candidates for poten-
tial restoration and educational activities related to
whitebark pine health.
Figure 8. Results of a recreational impact study conducted by
the Inyo National Forest for six popular recreation areas with
campgrounds, trailheads, parking lots, and/or boat launches in
whitebark habitat. Photo taken at Saddlebag Campground, Tioga
Pass area. [USFS]
41VOL. 47, NO. 1, MAY 2019
Recognition of the variability in whitebark pine among
geographic regions has inspired our recent monitoring
efforts in California, which we hope will provide guid-
ance for appropriate restoration strategies. For exam-
ple, we are studying the beneﬁts of re-introducing ﬁre
in areas where it has apparently been long excluded
(e.g. Cascades and Klamaths), whereas in the Sierra
Nevada, we are identifying trees with genetic resistance
to white pine blister rust to promote resilience in those
As indicated above, there are relatively few long-
term monitoring datasets for whitebark populations
in California, due in part to the tree’s low timber val-
ues and limited accessibility to its remote habitat and
steep terrain. In addition, until recently, people have
believed that conditions for the whitebark were stable.
Just in the last decade, Millar et al. (2012) presented
long-term trends based on tree-ring chronologies and
USFS aerial detection surveys in which mappers esti-
mated the extent and type of disturbance, ﬁnding local-
ized, severe stand mortality in some portions of the
southeastern Sierra Nevada and Warner Mountains.
One of the very few examples of stand-level repeated
measurements of whitebark pine is from a long-term
U.S. Geological Survey study of a large (2.5 hectares)
forest plot in Yosemite National Park, in which all trees
have been censused annually since 1996. At this site,
annual counts show that the newly dead trees gener-
ally outnumber newly established trees, suggesting a
closer study of this site is needed (Das et al. 2013).
This demographic trend has been occurring despite
only recent and minor observations of white pine blis-
ter rust (Adrian Das, personal communication).
The National Park Service (NPS) Inventory &
Monitoring program is another recent source for long-
term monitoring data of whitebark pine. It began as
a regional monitoring effort of high elevation white
pines across several Paciﬁc West Region parks in 2011.
In Lassen, Yosemite, Sequoia, and Kings Canyon
national parks, the researchers have established 94 of
a planned 102 permanent plots (0.25 hectare) where
trees are being individually tracked and assessed every
three years (McKinney et al. 2012).
Implementing effective restoration requires an
understanding of the ecological context of the target
species (Keane et al. 2012). A broad-scale assessment
of whitebark condition in California was initiated in
2014 by the USFS, complementing the existing net-
work established on NPS lands. Such a monitoring
network that adequately represents all geographic
regions—regardless of land ownership—provides the
ability to inventory and monitor patterns of mortality
and regeneration, and to determine the rate and causes
of mortality. In addition, such a network can contrib-
ute to the development of restoration and adaptive
strategies and help identify where to prioritize manage-
ment actions. The USFS campaign was substantially
expanded in 2018 to 166 plots (0.08 hectare), and will
be completed in 2019, after which it will serve as a base-
line for future studies. Among the pressing questions
under investigation are:
1) Are there areas where regeneration is not keeping
up with mortality?
2) Where are stressors having the greatest impact, and
are the impacts expanding?
3) Are other high elevation conifers outcompeting
whitebark pine, and what role do disturbance
regimes play in that interaction?
4) Are there additional range distribution surprises,
similar to the revisions we found in the southern
Sierra Nevada? Isolated populations in the north-
ern Sierra Nevada and southern Great Basin in
California are ripe for exploration.
Until recently, stressors such as blister rust and moun-
tain pine beetle have had relatively small impacts in
California, compared to their impacts in other parts
of western North America, where they have largely
decimated whitebark pine populations. However, the
continued spread and intensiﬁcation of these stressors
and their interactions with a rapidly changing climate
may portend future whitebark declines in this region.
Clearly, the diversity of California’s whitebark stands,
with their many different ecological settings, and poten-
tially unique genetic composition, points to the need
for a strategy for monitoring, conservation, and resto-
ration that is tailored to each unique zone.
We thank Daria Pechurina, Ethan Bridgewater, Paul Slaton,
Kama Kennedy, Becky Estes, and Erin Ernst for sampling
assistance and data analysis. Erik Jules, Julie Evens, and
Ron Lanner provided comments that improved this man-
uscript. This work was supported in part by the USDA
Forest Service, Forest Health Protection Special Technology
Development Program, National Park Service, and the U.S.
Fish and Wildlife Service. Any use of trade, ﬁrm, or product
names is for descriptive purposes only and does not imply
endorsement by the U.S. Government.
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