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Subalpine sentinels: understanding and managing whitebark pine in California



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Ahardy inhabitant of the subalpine zone of
western North America, whitebark pine
(Pinus albicaulis) is a keystone tree species in
Californias 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).
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;; author for correspondence
2. USDA Forest Service, Pacific Southwest Region, Regional Ecology Program;
3. USDA Forest Service, Pacific Southwest Region, Regional Ecology Program;
4. National Park Service Inventory & Monitoring Program, Sierra Nevada Network;
5. Department of Environmental Science, Policy and Management, University of California, Berkeley;
6. U.S. Geological Survey, Western Ecological Research Center;
7. USDA Forest Service, Pacific Southwest Region, Regional Ecology Program;
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 Californias
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.
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. []
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).
The seeds of whitebark pines are wingless and rarely
dispersed by wind. Instead they rely on dispersion
by squirrels or birds, primarily Clarks 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].
(a) (b)
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-
ration activities.
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.
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
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
Dupper 2002).
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 Sierras 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.
Climate change
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]
(a) (b)
(c) (d)
Active forest management of Californias 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 dont 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 Californias 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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... Whitebark pine was assigned a listing priority number (LPN) of 2, meaning the threats are of high magnitude and are imminent. The U.S. Forest Service listed whitebark pine on its Sensitive Species list in California in 2013 (Slaton et al. 2019b). Table 1 summarizes the various listing entities and current treatment and status of whitebark pine. ...
... Analyzing the whitebark pine genome allows for further study of population genetics, resistance genes, and other topics. California is the only region that does not currently have an active genetic restoration program for whitebark (Slaton et al. 2019b) so our local understanding is still quite limited. ...
... This is significantly more than occurrences reported by , who indicate about 25% of 15 compiled plots sampled in the Warner mountains (part of the Modoc region) showed fire impacts with no impacts in the Cascade-Klamath range (n=26). also compiled fire impact data across the Sierra Nevada (n= 189) with impacts as low as 1-2% in the central Sierra and up to 15% in the southern Sierras (Slaton et al. 2019b), thus there appears to be some variation based on sampling within the various regions. The 2014 Whites Fire occurred in high elevations of the Russian Wilderness in the western Klamath National Forest, burning small stands of whitebark pine in the subalpine forests along the ridge north of Russian Peak above Russian Creek (see Figure 5). ...
Technical Report
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In the state of California, the habitat and ecology of whitebark pine (Pinus albicaulis) is unique due to the variety of ecological settings where this important tree species is found. The most extensive stands occur in high-elevation, open ridges and slopes of the central and southern portions of the Sierra Nevada mountain range. To the north, as the Sierra Nevada range transitions to the Cascade Range in Lassen County, whitebark pine occurs on volcanic summits from Lassen Volcanic National Park to Mount Shasta (the two largest stands in the Cascades) as well as on other summits of high elevation. Near the border of California to Nevada and Oregon, whitebark pine inhabits high elevations of the Great Basin into the Warner Mountains. Lastly, but importantly, are isolated stands of whitebark in the Klamath Mountains – these sky islands are scattered across the diverse geological landscape of northwest California. Because of this vast diversity in landscape and in scale, this status report explores the specific biology, ecology, distribution, of and threats to whitebark pine within the state of California. In collaboration with US Forest Service, the California Native Plant Society Vegetation Program compiled existing ground-based datasets and references from agency staff, researchers, and others across California with a focus on the presence and impacts to whitebark pine. All current data are stored in a geodatabase for sharing with USFS and partners. We created GIS map displays of whitebark pine’s range/extent and pest/pathogen occurrences (mountain pine beetle and white pine blister rust) across four regions and 12 national forests within California.
Whitebark pine (Pinus albicaulis Engelm.) is a keystone high‐elevation tree species occurring across much of western North America, yet it has been listed as threatened under the US Endangered Species Act due to rapid population declines and extensive ongoing pressures from white pine blister rust (Cronartium ribicola), mountain pine beetle (Dendroctonus ponderosae), and increasing temperature and aridity associated with climate change. Past research has shown that whitebark pine mortality is more likely in hotter and drier sites, but no broad‐extent analyses spanning multiple ecoregions have directly quantified the relationship between mortality probability and its environmental drivers at a fine spatial scale. To address this gap, we used spatially continuous high‐resolution (30 m) maps of mortality in California produced by a remote sensing‐based change detection algorithm, and we developed generalized additive models to explain this mortality variability using relevant biophysical variables. We found that maximum daily temperature was strongly positively associated with mortality intensity and that mortality was also generally more intense in sites with greater solar exposure (e.g., south‐facing slopes). Our results support the interpretation that as climate warms and aridity increases, populations in “trailing edge” (i.e., hot, dry) sites are most vulnerable. The environmental relationships we quantify will aid in identifying the locations and environments with the greatest risk of future mortality as well as those likely to serve as refugia.
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This assessment uses historical observations and datasets, as well as studies conducted in contemporary reference landscapes (i.e., those with active fire regimes and minimal management impacts) to define the natural range of variation (NRV) for red fir (Abies magnifica) and subalpine forests in northwestern California and southwestern Oregon.
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The Inventory & Monitoring Division of the U.S. National Park Service conducts long-term monitoring to provide park managers information on the status and trends in biological and environmental attributes including white pines. White pines are foundational species in many subalpine ecosystems and are currently experiencing population declines. Here we present results on the status of whitebark and foxtail pine in the southern Sierra Nevada of California, an area understudied relative to other parts of their ranges. We selected random plot locations in Yosemite, Sequoia, and Kings Canyon national parks using an equal probability spatially-balanced approach. Tree- and plot-level data were collected on forest structure, composition, demography, cone production, crown mortality, and incidence of white pine blister rust and mountain pine beetle. We measured 7899 whitebark pine, 1112 foxtail pine, and 6085 other trees from 2012–2017. All factors for both species were spatially highly variable. Whitebark pine occurred in nearly-pure krummholz stands at or near treeline and as a minor component of mixed species forests. Ovulate cones were observed on 25% of whitebark pine and 69% of foxtail pine. Whitebark pine seedlings were recorded in 58% of plots, and foxtail pine seedlings in only 21% of plots. Crown mortality (8% in whitebark, 6% in foxtail) was low and significantly higher in 2017 compared to previous years. Less than 1% of whitebark and zero foxtail pine were infected with white pine blister rust and <1% of whitebark and foxtail pine displayed symptoms of mountain pine beetle attack. High elevation white pines in the southern Sierra Nevada are healthy compared to other portions of their range where population declines are significant and well documented. However, increasing white pine blister rust and mountain pine beetle occurrence, coupled with climate change projections, portend future declines for these species, underscoring the need for broad-scale collaborative monitoring.
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Whitebark pine (Pinus albicaulis Engelm.) is a foundation species of high elevation forest ecosystems in the Cascade Mountain Range of Oregon, Washington, and British Columbia. We examined fire evidence on 55 fire history sites located in the Cascade Range. To estimate dates of historic fires we analyzed 57 partial cross-sections from fire-scarred trees plus 700 increment cores. The resulting 101 fire events indicate fire has been a widespread component of Cascadian whitebark pine stands. Results are site specific and vary considerably. Whitebark pine stands appear to burn in a variety of severities and frequencies. Sites where fire intervals were detected ranged from 9 to 314 years, with a median of 49 years, and averaging 67 years. Fire intervals shortened significantly with higher latitudes. In assessing the most recent fire event at each site, overall, 56 percent burned as stand replacing events. In the 20th century, the number of fires diminished significantly. Due to conservation imperatives, re-introducing fire should be undertaken with extreme care to avoid substantial mortality of this endangered species.
Technical Report
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Whitebark pine and foxtail pine serve foundational roles in the subalpine zone of the Sierra Nevada. They provide the dominant structure in tree-line forests and regulate key ecosystem processes and community dynamics. Climate change models suggest that there will be changes in temperature regimes and in the timing and magnitude of precipitation within the current distribution of these species, and these changes may alter the species’ distributional limits. Other stressors include the non-native pathogen white pine blister rust and mountain pine beetle, which have played a role in the decline of whitebark pine throughout much of its range. The National Park Service is monitoring status and trends of these species. This report provides complementary information in the form of habitat suitability models to predict climate change impacts on the future distribution of these species within Sierra Nevada national parks. We used maximum entropy modeling to build habitat suitability models by relating species occurrence to environmental variables. Species occurrence was available from 328 locations for whitebark pine and 244 for foxtail pine across the species’ distributions within the parks. We constructed current climate surfaces for modeling by interpolating data from weather stations. Climate surfaces included mean, minimum, and maximum temperature and total precipitation for January, April, July, and October. We downscaled five general circulation models for the 2050s and the 2090s from ~125 km2 to 1 km2 under both an optimistic and an extreme climate scenario to bracket potential climatic change and its influence on projected suitable habitat. To describe anticipated changes in the distribution of suitable habitat, we compared, for each species, climate scenario, and time period, the current models with future models in terms of proportional change in habitat size, elevation distribution, model center points, and where habitat is predicted to expand or contract. The current whitebark pine model compared favorably with the distribution of species occurrence samples across park landscapes. The total amount of suitable habitat for whitebark pine in the parks was projected to be nearly equivalent in the 2050s under scenario B1 as under the more extreme scenario, A2, but with increases over the current model of 20% to 22% across the study area. In Yosemite National Park (YOSE), the amount of suitable habitat decreased 16% to 28% for the 2050s and increased in Sequoia & Kings Canyon National Parks (SEKI) by 44% to 49%. For the 2090s, the amount of whitebark pine habitat was predicted to remain unchanged, on average, in YOSE and increase in SEKI by an average of 14%, again with little difference between scenarios. However, the distribution of habitat by elevation was similar for whitebark pine among models in both YOSE and SEKI and under both climate scenarios, while model center points indicated slight geographic shifts to the south under both scenarios in both YOSE and SEKI. Models indicated bioclimatically suitable habitat in the current model will decrease for whitebark pine at a rate of 17% to 20% for the 2050s and 20% to 23% for the 2090s across the study area. These results contrast with changes in habitat abundance from modeling results in the Greater Yellowstone Area but echo the increase in suitable habitat predicted for the full range of whitebark pine. viii The distribution of suitable foxtail pine habitat in the current climate-topography model also compared favorably with the current distribution reflected in occurrence samples. The future distribution of suitable habitat for foxtail pine was projected to be similar in SEKI under the two climate scenarios, and it was not projected to extend north into YOSE. Compared with the current model, the amount of habitat was projected to decrease, with a greater decrease for the 2090s than for the 2050s and more under the extreme scenario than under the optimistic scenario. Model center points reflected geographic shifts in suitable habitat, largely to the north. Currently suitable habitat for foxtail pine is predicted to be lost at rates of 25% to 35% compared with the current model. Loss is similar between scenarios for the 2050s but greater under A2 than under B1 for the 2090s. Overall, models indicated that suitable habitats for whitebark and foxtail pine are more likely to shift geographically within the parks by 2100 rather than decline precipitously. This implies park managers might focus conservation efforts on stressors other than climate change, working toward species resilience in the face of threats from introduced disease and elevated native insect damage. More specifically, further understanding of the incidence and severity of white pine blister rust and other stressors in high elevation white pines would help assess vulnerability from threats other than climate change.
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Plant population responses are key to understanding the effects of threats such as climate change and invasions. However, we lack demographic data for most species, and the data we have are often geographically aggregated. We determined to what extent existing data can be extrapolated to predict population performance across larger sets of species and spatial areas. We used 550 matrix models, across 210 species, sourced from the COMPADRE Plant Matrix Database, to model how climate, geographic proximity and phylogeny predicted population performance. Models including only geographic proximity and phylogeny explained 5-40% of the variation in four key metrics of population performance. However, there was poor extrapolation between species and extrapolation was limited to geographic scales smaller than those at which landscape scale threats typically occur. Thus, demographic information should only be extrapolated with caution. Capturing demography at scales relevant to landscape level threats will require more geographically extensive sampling.
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Climate and weather have long been noted as playing key roles in wildfire activity, and global warming is expected to exacerbate fire impacts on natural and urban ecosystems. Predicting future fire regimes requires an understanding of how temperature and precipitation interact to control fire activity. Inevitably this requires historical analyses that relate annual burning to climate variation. Fuel structure plays a critical role in determining which climatic parameters are most influential on fire activity, and here, by focusing on the diversity of ecosystems in California, we illustrate some principles that need to be recognized in predicting future fire regimes. Spatial scale of analysis is important in that large heterogeneous landscapes may not fully capture accurate relationships between climate and fires. Within climatically homogeneous subregions, montane forested landscapes show strong relationships between annual fluctuations in temperature and precipitation with area burned; however, this is strongly seasonal dependent; e.g., winter temperatures have very little or no effect but spring and summer temperatures are critical. Climate models that predict future seasonal temperature changes are needed to improve fire regime projections. Climate does not appear to be a major determinant of fire activity on all landscapes. Lower elevations and lower latitudes show little or no increase in fire activity with hotter and drier conditions. On these landscapes climate is not usually limiting to fires but these vegetation types are ignition-limited. Moreover, because they are closely juxtaposed with human habitations, fire regimes are more strongly controlled by other direct anthropogenic impacts. Predicting future fire regimes is not rocket science; it is far more complicated than that. Climate change is not relevant to some landscapes, but where climate is relevant, the relationship will change due to direct climate effects on vegetation trajectories, as well as by feedback processes of fire effects on vegetation distribution, plus policy changes in how we manage ecosystems.
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Whitebark pine (Pinus albicaulis) inhabits an expansive range in western North America, and it is a keystone species of subalpine environments. Whitebark is susceptible to multiple threats – climate change, white pine blister rust, mountain pine beetle, and fire exclusion – and it is suffering significant mortality range-wide, prompting the tree to be listed as ‘globally endangered’ by the International Union for Conservation of Nature and ‘endangered’ by the Canadian government. Conservation collections (in situ and ex situ) are being initiated to preserve the genetic legacy of the species. Reliable, transferrable, and highly variable genetic markers are essential for quantifying the genetic profiles of seed collections relative to natural stands, and ensuring the completeness of conservation collections. We evaluated the use of hybridization-based target capture to enrich specific genomic regions from the 27 GB genome of whitebark pine, and to evaluate genetic variation across loci, trees, and geography. Probes were designed to capture 7,849 distinct genes, and screening was performed on 48 trees. Despite the inclusion of repetitive elements in the probe pool, the resulting dataset provided information on 4,452 genes and 32% of targeted positions (528,873 bp), and we were able to identify 12,390 segregating sites from 47 trees. Variations reveal strong geographic trends in heterozygosity and allelic richness, with trees from the southern Cascade and Sierra Range showing the greatest distinctiveness and differentiation. Our results show that even under non-optimal conditions (low enrichment efficiency; inclusion of repetitive elements in baits), targeted enrichment produces high quality, codominant genotypes from large genomes. The resulting data can be readily integrated into management and gene conservation activities for whitebark pine, and have the potential to be applied to other members of 5-needle pine group (Pinus subsect. Quinquefolia) due to their limited genetic divergence. © 2016 Syring, Tennessen, Jennings, Wegrzyn, Scelfo-Dalbey and Cronn.
Technical Report
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The mission of the National Park Service is ―to conserve unimpaired the natural and cultural resources and values of the national park system for the enjoyment of this and future generations‖ (National Park Service 1999). To uphold this goal, the Director of the NPS approved the Natural Resource Challenge to encourage national parks to focus on the preservation of the nation‘s natural heritage through science, natural resource inventories, and expanded resource monitoring (National Park Service 1999). Through the Challenge, 270 parks in the national park system were organized into 32 inventory and monitoring networks. All Inventory and Monitoring (I&M) networks within the National Park Service have identified high priority park vital signs, indicators of ecosystem health, which represent a broad suite of ecological phenomena operating across multiple temporal and spatial scales. Our intent has been to develop a balanced and integrated suite of vital signs that meets the needs of current park management, and that also will accommodate unanticipated environmental conditions and management questions in the future. Three Pacific West Region I&M networks, the Klamath Network (KLMN), Sierra Nevada Network (SIEN), and the Upper Columbia Basin Network (UCBN) have identified a common vital sign: high elevation white pine species. These tree species are vulnerable to invasive pathogens as well as other stressors such as climate-change induced drought, and have been recognized as high priority vital signs for five parks: Crater Lake National Park (CRLA) and Lassen Volcanic National Park (LAVO) in KLMN, Sequoia-Kings Canyon National Park (SEKI) and Yosemite National Park (YOSE) in SIEN, and Craters of the Moon National Monument and Preserve (CRMO) in UCBN. Currently, populations of these white pine species and their respective communities are in better ecological condition within the participating network parks compared to populations elsewhere in the Cascades and Rocky Mountains (e.g., blister rust infection and mountain pine beetle infestation rates are lower). However, the observed steeply declining trends in white pine populations in the northern Cascade and Rocky Mountain ranges, coupled with the identification of blister rust and mountain pine beetle in our parks is a significant cause for concern about the future status of these valuable communities. Monitoring of white pine community dynamics will enable parks the opportunity for early detection of downward trends and perhaps more effective management intervention. Also, monitoring information from these parks will contribute meaningfully to the broader regional assessment of the status and trend of white pine species across western North America. The objectives of the protocol are to determine the status and trends in: 1. Tree species composition and structure 2. Tree species birth, death, and growth rates 3. Incidence of white pine blister rust and level of crown kill 4. Incidence of pine beetle and severity of tree damage 5. Incidence of dwarf mistletoe and severity of tree damage 6. Cone production of white pine species Each network will establish a set of randomly selected permanent macroplots 50 m x 50 m in dimension. Plots are divided in five 10 x 50 m subplots, facilitating comparison of results from xiv this protocol with other programs, including the NPS Greater Yellowstone Network, which uses the smaller plot size. Within each plot, trees ≥1.37 m (DBH) in height will be tagged and their status tracked over time. Seedlings and saplings will be monitored in a set of nine 3 m x 3 m regeneration plots systematically arranged within each macroplot. The use of a common database structure by each Network will facilitate combined analyses and reporting. This protocol details the why, where, how, and when of the PWR‘s white pine community dynamics monitoring program. As recommended by Oakley et al. (2003), it consists of a protocol narrative and a set of standard operating procedures (SOPs) which detail the steps required to collect, manage, and disseminate the data representing the status and trend of white pine community dynamics and the condition of associated communities in each of the three networks. The protocol is a ―living‖ document in the sense that it is continually updated as new information acquired through monitoring and evaluation leads to the refinement of program objectives and methodologies. Changes to the protocol are carefully documented in a revision history log. The intent of the protocol is to ensure that a scientifically credible story about the ecological condition of white pine communities and their responses to invasive pathogens, park management actions, changing precipitation patterns, and other stressors can be told to park visitors and managers alike. These long-term data can contribute to the development of informative models of relationships between white pine community dynamics and key environmental factors and management actions specific to each park.
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Context Milder winters have contributed to recent outbreaks of Dendroctonus ponderosae in Canada, but have not been evaluated as a factor permitting concurrent outbreaks across its large range (ca.1500 9 1500 km) in the western United States (US). Objectives We examined the trend in minimum air temperatures in D. ponderosae habitats across the western US and assessed whether warming winters explained the occurrence of outbreaks using physiological and population models. Methods We used climate data to analyze the history of minimum air temperatures and reconstruct physiological effects of cold on D. ponderosae. We evaluated relations between winter temperatures and beetle abundance using aerial detection survey data. Results Extreme winter temperatures have warmed by about 4 °C since 1960 across the western US. At the broadest scale, D. ponderosae population dynamics between 1997 and 2010 were unrelated to variation in minimum temperatures, but relations between cold and D. ponderosae dynamics varied among regions. In the 11 coldest ecoregions, lethal winter temperatures have become less frequent since the 1980s and beetle-caused tree mortality increased—consistent with the climatic release hypothesis. However, in the 12 warmer regions, recent epidemics cannot be attributed to warming winters because earlier winters were not cold enough to kill D. ponderosae. Conclusions There has been pronounced warming of winter temperatures throughout the western US, and this has reduced previous constraints on D. ponderosae abundance in some regions. However, other considerations are necessary to understand the broad extent of recent D. ponderosae epidemics in the western US. Keywords Climate change Á Demography Á Mountain pine beetle Á Process-based model Á Bark beetles Á Pinus Á Cold tolerance
Whitebark pine (Pinus albicaulis Engelm.) is vulnerable to mountain pine beetle (Dendroctonus ponderosae Hopkins) attack throughout western North America, but beetle outbreaks in the southwestern portion of the range (i.e., Sierra Nevada) have been spatially limited until recently. We examined patterns of mortality, structure, and regeneration in whitebark pine stands impacted by mountain pine beetle in the southern Sierra Nevada. Mortality was greatest in medium to large diameter (>10-20 cm dbh) trees, resulting in declines in mean and maximum tree diameter and tree size class diversity following an outbreak. Severity of beetle attack was positively related to mean tree diameter and density. Density of young (<3 years old) whitebark pine seedling clusters was positively related to severity of beetle attack on mature stands. All sites showed a stable production of whitebark pine regeneration within at least the past 30-40 years, with a pulse of new seedlings in the past 3 years in beetle-impacted stands. Our results show that mountain pine beetle outbreaks in the southern Sierra Nevada result in substantial changes in whitebark pine stand structure and suggest low resistance but high resilience to initial attack, especially in the absence of white pine blister rust. © 2016, National Research Council of Canada. All rights reserved.