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The distribution of whitebark pine across western North America (adapted from Little 1971; Tomback and Achuff 2010).
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The history of whitebark pine (Pinus albicaulis) forests over the past century exemplifies modern environmental change and the particular challenges faced in reconciling the scales of human observation with long-term ecological changes. A number of factors are implicated in driving observed declines in whitebark pine populations including fire supp...
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... modern range of whitebark pine extends from the northern Canadian Rockies (55° N) to the southern Sierra Nevada (37° N) and from the Coast Range of the Pacific Northwest (128° W) to the eastern Rocky Mountains of Montana and Wyoming (107° W) (Figure 2). Several disjunct populations exist in eastern Montana and northeastern Nevada (Weaver and Dale 1974). ...
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Citations
... For the slowgrowing High-5, mature cone-bearing trees lost to MPB can take a century or more to replace, thereby reducing the reproductive capacity of the population during that time. Trees stressed by drought, disease, or other insects can be particularly susceptible to attack by MPB (see Kolb et al., 2016;Larson and Kipfmueller, 2012; note the exception for severe disease reported by Dooley and Six, 2015;Cardinal et al., 2022). Under epidemic conditions, however, MPB is less discriminating; it attacks and kills healthy trees (Gibson et al., 2008), including those that have genetic resistance to WPBR (e.g., Shepherd et al., 2018). ...
Tree mortality rates have been increasing globally with mountainous regions experiencing higher temperatures and impacts from the expansion and intensification of pests and invasion by non-native agents. Western North American high-elevation forests exemplify these trends, and they often include one or more species of five-needle white pines (High-5 hereafter). These species share many characteristics critical to defining the structure and function of many subalpine forests. The main threats to High-5 populations include the non-native pathogen Cronartium ribicola, which causes the disease white pine blister rust, climate-driven drought stress, episodic and high mortality from mountain pine beetle (Dendroctonus ponderosae), and wildfires of increasing frequency, size, and intensity. The six High-5 species occurring in western North America (whitebark pine, Pinus albicaulis; limber pine, P. flexilis; southwestern white pine, P. strobiformis; Rocky Mountain bristlecone pine, P. aristata; Great Basin bristlecone pine, P. longaeva; and foxtail pine, P. balfouriana) differ in their health status and threat level. The convergence of threats impacting the rapidly declining species could portend future declines in the species and populations currently less impacted by recent disturbances. Differences in the innate adaptive capacities of the species affect their population trajectories under these novel combinations of stressors. We evaluate the status and outlook for each species and address the following questions: (1) Is the environment changing too fast and the intensity of stressors too great for the species to adapt and recover? (2) Do the species have the heritable traits necessary to sustain fitness under C. ribicola and climatic stresses? (3) Are other mortality factors increasing to the degree that they reduce the populations further and delay or preclude adaptation and population recovery? (4) Can the species escape the stressors through migration? Insights related to these questions provide guidance for forest management to facilitate adaptation and increase the resilience of these species into the future.
... Second, because blister rust often occurs in montane zones, it has greater exposure to extreme weather and may respond more directly to changes in temperature and humidity 1,14,21 . Third, SEKI is located at the expansion front or current latitudinal range edge of blister rust invasion in California that began in British Columbia over 100 years ago 50 . Many have suggested that the climatic conditions are too hot and dry south of SEKI for blister rust to successfully disperse and infect hosts 9 ; blister rust may be more strongly controlled by climate here than in more northern latitudes where the disease is more abundant. ...
Range shifts of infectious plant disease are expected under climate change. As plant diseases move, emergent abiotic-biotic interactions are predicted to modify their distributions, leading to unexpected changes in disease risk. Evidence of these complex range shifts due to climate change, however, remains largely speculative. Here, we combine a long-term study of the infectious tree disease, white pine blister rust, with a six-year field assessment of drought-disease interactions in the southern Sierra Nevada. We find that climate change between 1996 and 2016 moved the climate optimum of the disease into higher elevations. The nonlinear climate change-disease relationship contributed to an estimated 5.5 (4.4–6.6) percentage points (p.p.) decline in disease prevalence in arid regions and an estimated 6.8 (5.8–7.9) p.p. increase in colder regions. Though climate change likely expanded the suitable area for blister rust by 777.9 (1.0–1392.9) km ² into previously inhospitable regions, the combination of host-pathogen and drought-disease interactions contributed to a substantial decrease (32.79%) in mean disease prevalence between surveys. Specifically, declining alternate host abundance suppressed infection probabilities at high elevations, even as climatic conditions became more suitable. Further, drought-disease interactions varied in strength and direction across an aridity gradient—likely decreasing infection risk at low elevations while simultaneously increasing infection risk at high elevations. These results highlight the critical role of aridity in modifying host-pathogen-drought interactions. Variation in aridity across topographic gradients can strongly mediate plant disease range shifts in response to climate change.
... major threats, predation, fire and fire suppression, and environmental effects of climate change . . . also occur throughout the entire range and have resulted in significant loss of whitebark pine" [2,22]. ...
... This was in part attributed to the release of basal sprouts and already established seedlings, and, therefore, loss of the main canopy as a seed source would therefore not necessarily affect seedling density immediately post-disturbance. Studies conducted in the Rocky Mountains have similarly found enhanced regeneration in canopy openings created by mountain pine beetle attacks [22]. ...
Effective restoration of whitebark pine populations will require a solid understanding of factors affecting seedling recruitment success, which may vary by site and biogeographic region. We examined the relationship between whitebark pine seedling recruitment, disturbance history, and range position in three independent studies in the southern Sierra Nevada, California (CA), USA. In 66 plots broadly distributed across watersheds, we found that whitebark pine seedling density and proportion were greatest at upper elevations, and where canopy cover of whitebark pine was higher (density ranged 0–383 seedlings/ha; x ¯ = 4, σX = 1). Seedling density and proportion were also greater in plots that had recently experienced loss of canopy cover from insects, avalanche, windthrow, or other disturbance effects. In a second study conducted in popular recreational areas, including campgrounds and trailheads, the response of whitebark pine recruitment to disturbance was strongly dependent on the relative position of stands within the range, or proximity to other forest types. Both studies indicated that low to moderate levels of disturbance enhanced whitebark pine recruitment, especially at its range edge, a finding consistent with the early seral status of whitebark observed in previous studies conducted elsewhere in North America. In our third study, a case study at the June Mt. Ski Area, we demonstrate the potential for a downward shift in the whitebark-lodgepole pine ecotone as a result of insect-caused disturbance.
... However, changes in disturbance regimes and other factors are causing widespread mortality in many tree species [68][69][70]. Despite declines in whitebark pine, we did not find evidence of successional replacement by subalpine fir in sub-mesic communities where whitebark pine occurs in mixed-conifer stands, suggesting that this may not be a ubiquitous threat [13,31,71]. Neither subalpine fir nor Engelmann spruce showed significant changes in basal area, perhaps because stand development at these sites is too slow for detectable shifts in species abundance over a 22-year study period (but see Clason et al.'s [13] results from British Columbia). ...
Whitebark pine (Pinus albicaulis Engelm.), an ecologically important tree species in high-elevation ecosystems of western North America, is threatened by white pine blister rust (Cronartium ribicola Fischer) and increased pressure from mountain pine beetle (Dendroctonus ponderosae Hopkins) due to climate warming. In addition, there is concern that fire suppression may be leading to successional replacement of whitebark by late-seral trees. Despite widespread knowledge that the tree is in decline, there is limited understanding of its successional dynamics, particularly in forests disturbed by white pine blister rust and mountain pine beetle. Our objective was to examine how disturbances have affected forest composition, structure, and seedling regeneration over a 22-year period (1990–2012) at 19 sites in the Cascade Mountains of Washington State (USA). Over that time, 13 sites (68%) were infected by white pine blister rust, 11 (58%) were disturbed by mountain pine beetle, and 5 (26%) experienced wildfire. Tree community composition changed significantly during the study period, primarily due to significant mortality of mature (≥20-cm diameter at breast height) whitebark pine. Despite loss of mature whitebark trees, we found little evidence of successional replacement by other tree species. Whitebark seedling density was unrelated to basal area of mature whitebark pine, but positively correlated with the presence of herb and shrub cover. Our results demonstrate the value of long-term repeated measurements for elucidating successional dynamics.
... However, empirical studies in the last two decades have found that frequent fire has lesser impact on subalpine fir and that species composition changes in whitebark pine forests take place over longer time periods than once thought. Species composition change in subalpine forests proceeds over centuries and may never lead to complete replacement of whitebark pine (Campbell and Antos, 2003;Larson and Kipfmueller, 2012) and fire regimes in many whitebark pine communities are still within their historical intervals (Larson, 2009;Larson et al., 2009). Larson et al. (2009) also found that frequent low-severity fires didn't reduce subalpine fir abundance, and that subalpine fir began establishing before fire exclusion began. ...
... These findings suggest that current abundances of subalpine fir may not represent a fire exclusion-induced change in species composition for these types of whitebark pine communities, in contrast with simulation modelling of developmental changes in whitebark pine stands (Keane et al., 1990;Keane et al., 2017). Furthermore, 'seral' whitebark pine communities are not represented throughout the range of whitebark pine, and subalpine fir is absent from some regions (Larson and Kipfmueller, 2012). The ideas of compositional change in whitebark pine stands from fire exclusion stem from deterministic concepts of forest successional change, a framework that may limit understanding of the complexities of vegetation change (Binkley et al., 2015). ...
Silvicultural thinning treatments to restore whitebark pine (Pinus albicaulis) are widely used in subalpine forests throughout the western United States (US) and Canada. The objectives of these treatments are to (1) improve the condition of whitebark pine at all ages, (2) to improve seedling recruitment processes, and (3) mitigate the damage caused by mountain pine beetle (MPB; Dendroctonus ponderosae) and white pine blister rust (WPBR; caused by the fungus Cronartium ribicola). However, there is some disagreement about the ecological basis of restoration and a paucity of information on the effects these activities – few treatments have been monitored to assess their success. We investigated the ecological effects of silvicultural restoration treatments in whitebark pine forests and evaluated their success by retrospectively sampling five treatment sites in the western US 6–10 years after implementation. We found strong evidence of growth release at a site previously characterized by closed-canopy stands. Growth responses in more open, park-like stands, however, were variable: we found weak growth increases at one site, weak growth decreases at another and no response at two other sites. At the site with strong growth increases, trees with previous damage from WPBR infection had growth increases similar to uninfected trees. We found low rates of whitebark pine seedling recruitment overall, and no increase in whitebark pine recruitment associated with treatments at any site. However, at one site, treated stands had higher regeneration of non-target species than did untreated stands. Post-treatment mortality (mostly from the late 2000s MPB outbreak) was significantly lower in the treated stand at the closed-canopy site; at the other sites, there was no difference in mortality between treated and untreated stands. The treatments had little detectable effect on short-term growth-climate relationships, although our analyses revealed that whitebark pine growth at our sites was more temperature limited than water limited. While some management goals were achieved, many were not, and there were some unintended consequences. Our results call for a closer examination of the ecological basis of silvicultural restoration treatments in whitebark pine and an expanded use of adaptive management.
... Whitebark pine exhibits many of the characteristics of a naïve host, including lower levels of defense chemicals and resin (Raffa et al., 2013. Reduced snow packs may also result in greater drought stress that may increase susceptibility (Larson and Kipfmueller, 2012). Outbreaks in this tree have been devastating in some areas, including the Greater Yellowstone Area, contributing to the recommendation that it be listed as an endangered species (United States Fish, and Wildlife Service [USFWS], 2011). ...
Increased mortality of forest trees, driven directly or indirectly by climate change, is occurring around the world. In western North America, whitebark pine, a high elevation keystone species, and lodgepole pine, a widespread ecologically and economically important tree, have experienced extensive mortality in recent climate-driven outbreaks of the mountain pine beetle. However, even in stands experiencing high levels of mortality, some mature trees have survived. We hypothesized that the outbreak acted as a natural selection event, removing trees most susceptible to the beetle and least adapted to warmer drier conditions. If this was the case, genetic change would be expected at loci underlying beetle resistance. Given we did not know the basis for resistance, we used inter-simple sequence repeats to compare the genetic profiles of two sets of trees, survivors (mature, living trees) and general population (trees just under the diameter preferred by the beetles and expected to approximate the genetic structure of each tree species at the site without beetle selection). This method detects high levels of polymorphism and has often been able to detect patterns associated with phenotypic traits. For both whitebark and lodgepole pine, survivors and general population trees mostly segregated independently indicating a genetic basis for survivorship. Exceptions were a few general population trees that segregated with survivors in proportions roughly reflecting the proportion of survivors versus beetle-killed trees. Our results indicate that during outbreaks, beetle choice may result in strong selection for trees with greater resistance to attack. Our findings suggest that survivorship is genetically based and, thus, heritable. Therefore, retaining survivors after outbreaks to act as primary seed sources could act to promote adaptation. Further research will be needed to characterize the actual mechanism(s) of resistance.
... It has been hypothesized that fire exclusion has facilitated regeneration of Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa), while limiting opportunities for successful regeneration of whitebark pine in many stands (Keane et al. 1990;Morgan and Bunting 1990;Murray et al. 1995;Keane and Arno 1996). However, fire regimes vary throughout the range of whitebark pine and investigations of the relative importance of fire, fire exclusion, blister rust and mountain pine beetle show that different agents of disturbance can have variable impacts on whitebark pine stand dynamics (Campbell and Antos 2003;Wong 2012;Larson and Kipfmueller 2012;Daniels and Wong 2016). ...
In the Coast Area of British Columbia, research is underway to refine information on disturbance regimes and seral stage distribution associated with the Biodiversity Guidebook for British Columbia (British Columbia Ministry of Forests and Ministry of Environment, Lands and Parks 1995a), provide improved silvicultural interpretations with new classification (BEC) materials, support habitat and timber supply assessment, and expand guidance and best management practices in the context of climate change in these transition landscapes. This report to the BC Ministry of Forests, Lands and Natural Resource Operations address two objectives:
(1) Review the literature on disturbance regime parameters for dominant disturbance agents in
the maritime to submaritime transition of the Coast Area.
(2) Critically assess the utility of field and lab-based analytical techniques to quantify the range
of magnitudes and spatio-temporal attributes of dominant disturbances in the study region.
... Due to these threats, the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assessed and designated whitebark pine as Endangered in 2010. As the rapid decline of the tree became evident across its range over the past two decades, studies of whitebark pine increased, though maritime whitebark pine communities remain largely unstudied (Larson and Kipfmueller, 2012). We ask the following question: What climate parameters limit the annual radial growth of whitebark pine in the distinctive maritime band of distribution contained within the southern Coast Mountains? ...
Whitebark pine (Pinus albicaulis), an endangered keystone alpine tree species, faces multiple threats across its western North American range. Little is known of whitebark pine in the southern Coast Mountains of British Columbia relative to well-studied Rocky Mountain populations, especially with regard to the effects of climate on annual radial growth. Our results indicate centenarian whitebark pine annual radial growth is negatively influenced by the onset of snowfall in the prior autumn. This unusual growth limitation likely stems from the truncation of the prior year growing season and reduced physiological preparedness for growth in the following year. Autumn snowfall is moderated by temperature and the Pacific Decadal Oscillation (PDO), which controls largescale weather patterns in the study region. Our results are distinct from studies of mature whitebark pine trees in continental populations where growth is typically limited by summer temperature, or occasionally by winter snowfall due to a reliance on snow meltwater during spring/summer. We suggest that predicted warmer and wetter climate and reduced snowpacks in the southern Coast Mountains may benefit the growth of the young population of maritime whitebark pine over the next few decades.
... Our study identifies biophysical parameters to consider when planting blister rust-resistant seedlings and suggests that open mesohabitats along south-facing slopes may be best for regeneration of whitebark pine, particularly near the northern limits of its range. Hoff, 1989;Larson and Kipfmueller, 2012), research on the regeneration niche would benefit from targeted sampling in a variety of regions. Indeed, most recovery plans for whitebark pine suggest research be conducted on its regeneration niche (Aubry et al., 2008;Keane et al., 2012;Alberta Whitebark and Limber Pine Recovery Team, 2014). ...
Knowledge of regeneration processes of the endangered whitebark pine (Pinus albicaulis Engelm.) is critical for developing approaches for recovery and restoration of the species. We investigated biophysical associates of whitebark pine seedling occurrence and density in different mesohabitats (defined by community type and elevation) within the northern Rocky Mountains of Alberta. We developed candidate linear models to examine factors influencing occurrence and abundance. Occurrence was positively related to bare mineral soil and species richness in forest mesohabitats, while in both open and alpine-treeline environments it was positively related to prostrate shrub cover. Negative associates included tree cover, rocky substrates, and seedling cover of other conifers. Model validation showed a strong correlation between observed and predicted occurrence (correlations of 0.60, 0.56, and 0.56 for forest, open, and alpine-treeline mesohabitats, respectively). Climate models best predicted seedling density; abundance was highest on south-facing slopes in all mesohabitats. Correlations between observed and predicted density were 0.83, 0.92, and 0.72 for forest, open, and alpine-treeline mesohabitats, respectively. Our study identifies biophysical parameters to consider when planting blister rust-resistant seedlings and suggests that open mesohabitats along south-facing slopes may be best for regeneration of whitebark pine, particularly near the northern limits of its range.
... Some 53% of the aerial extent of WBP in the U.S. is located in the GYE [20,21]. The GYE WPB population has been particularly hard hit in recent years with over 95% mortality of cone bearing trees [22,23] in some areas due to factors related to warming climate [24]. Beyond the current forest die-off, scientists are projecting that the area of suitable habitat for WBP in GYE will decline dramatically in the coming century under changing climate [25][26][27][28]. ...
... In addition to its physiological tolerances to climate, the performance of WBP in the GYE is structured by competition with lodgepole pine (Pinus contorta), Douglas-fir (Pseudotsuga menziesii), subalpine fir (Abies lasiocarpa), and Engelmann spruce (Picea engelmannii). At the highest elevations, climates are mostly unsuitable for competitors, allowing WBP to persist in pure stands [24,37,38]. At lower elevations in the montane zone, the other conifer species typically grow faster than WBP, and by mid to late seral stages, there is reduced growth, reproduction, and survival of WBP through competition [38]. ...
... WBP regenerates well in burned areas and is among the tree species that initially colonize these open sites and persist in them for decades until outcompeted by other conifer species [40]. WBP's higher resistance to fire [41] delays succession under mixed-severity fire regimes through the preferential removal of competitors such as subalpine fir and Engelmann spruce by fire [24]. ...
Climate suitability is projected to decline for many subalpine species, raising questions about managing species under a deteriorating climate. Whitebark pine (WBP) (Pinus albicaulis) in the Greater Yellowstone Ecosystem (GYE) crystallizes the challenges that natural resource managers of many high mountain ecosystems will likely face in the coming decades. We review the system of interactions among climate, competitors, fire, bark beetles, white pine blister rust (Cronartium ribicola), and seed dispersers that make WBP especially vulnerable to climate change. A well-formulated interagency management strategy has been developed for WBP, but it has only been implemented across <1% of the species GYE range. The challenges of complex climate effects and land allocation constraints on WBP management raises questions regarding the efficacy of restoration efforts for WBP in GYE. We evaluate six ecological mechanisms by which WBP may remain viable under climate change: climate microrefugia, climate tolerances, release from competition, favorable fire regimes, seed production prior to beetle-induced mortality, and blister-rust resistant trees. These mechanisms suggest that WBP viability may be higher than previously expected under climate change. Additional research is warranted on these mechanisms, which may provide a basis for increased management effectiveness. This review is used as a basis for deriving recommendations for other subalpine species threatened by climate change.
Complex Challenges of Maintaining Whitebark Pine in Greater Yellowstone under Climate Change: A Call for Innovative Research, Management, and Policy Approaches. Available from: https://www.researchgate.net/publication/296195710_Complex_Challenges_of_Maintaining_Whitebark_Pine_in_Greater_Yellowstone_under_Climate_Change_A_Call_for_Innovative_Research_Management_and_Policy_Approaches [accessed Apr 9, 2016].
