Saving California’s Great Basin Bristlecone Pine in the Face of Climate Change December 10, 2021 Produced by : Joe Lewis, William Hirsh, Kelli Allen, Fiona Osborn 1 Table of Contents Table of Contents 1 Executive Summary 3 Acknowledgements 4 Introduction 4 Description of Species 4 Biological Classification of Species and General Geographic Range 4 Current Climate 5 Associated Vegetation 5 Natural Disturbance Regimes 6 Past and Current Non-Climatic Anthropogenic Disturbances and Threats 6 Climate Change Vulnerability 7 Predicted Climatic Changes in the White Mountains 7 Maxent Analysis 8 Data 8 Habitat Correlation with Climate Variables 8 Habitat Vulnerability to Climate Change 9 Specific Threats Posed by these Climatic Changes to the Species 10 Changing Volatile Organic Compound Production and the Mountain Pine Beetle 10 Increasing Risk of Fire 11 Leapfrogging by Limber Pines 12 Adaptive Capacity/Vulnerability of the Species to Climate Change in Current Location 13 Adaptive Capacity/Vulnerability of the Species to Climate Change Via Migration 14 Suggested Climate Change Adaptation Strategies 15 Fuel Clearance 15 2 Transplanting vs. Preservation 16 Limber Pine Maintenance 16 Conclusions 17 References 18 Appendices 21 3 Executive Summary The oldest trees in the world, the Great Basin bristlecone pine, are an iconic California species and one that will face several existential threats due to climate change in the coming century. This report assesses these threats and proposes effective solutions to protect this species. The Great Basin bristlecone pine, or Pinus longaeva, is a species native to California, occurring at high elevations in the Panamint, Inyo, and White Mountain ranges of eastern California (Appendix: Figure 4). They are able to grow in relatively unfavorable conditions on steep slopes and rocky soils, and in areas with little precipitation due to their adaptations for drought, such as wide, shallow roots and thick, waxy needles (Taylor, 2018). Historically, they have faced relatively few natural and anthropogenic disturbances. Their long lifespan—averaging about 2000 years—means that they have lived through several past climate fluctuations and been able to survive. Yet despite its extraordinary ability to adapt with these changes, the biggest threat currently facing this species is climate change. The climate is changing at a larger scale than ever before, both in terms of its speed and its severity. The White Mountain region, where the bristlecone pines are located, is expected to see at least 4º C of warming, with subsequent increases in drought and wildfire risk (“Local climate change snapshot”, 2021). Although these predicted changes will alter the bristlecone pine’s range and wipe out its southern stands, these changes themselves will not decimate the species, and the area currently containing the main stand of bristlecones will remain habitable. Instead, the bristlecone pine will be threatened by three climate-related changes. Increasing temperatures could alter the tree’s volatile compounds which would result in fewer defenses against encourching pine beetle populations (Gray, Runyon, & Jenkins, 2019). These insects could prove devastating. Increased temperature and less precipitation could also result in high fire risk (Gray, Runyon, & Jenkins, 2019). These non-fire-adapted trees would suffer under such conditions. Finally, a competitive species, the limber pine, is projected to encroach on the bristlecone’s small range through a slow “leapfrogging process” (Smithers et. al., 2017). To address these threats we recommend three core adaptation strategies. Intensive fuel clearance methods are strongly encouraged. Through the use of both manual and mechanical clearance fire risk can be mitigated. With a small range within the White Mountains, this strategy will be focused and viable with clearance costs averaging around $134 per acre (Loomis, et. al. 2019). In conjunction, we highly recommend the preservation of the species within its naturally established range rather than focusing on transplantation. This ensures resources and costs will be focused on existing trees. Finally, maintenance of encroaching limber pines is needed to reduce competition with existing bristlecone pines. A balanced approach which preserves the ecological niche of the limber pine while preventing it from invading sensitive areas like the Methuselah Grove is critical for the native preservation of the bristlecone pine. 4 Acknowledgements We would like to thank Dr. Glen Macdonald and Ben Nauman for providing us with the space and resources to learn about California’s iconic vegetation like the bristlecone pine and witness first-hand both the beauty and ongoing risks facing these species today. We would also like to acknowledge the Western Shoshone and Southern Paiute people, who are the traditional stewards of the land encompassing our study area along the eastern mountains now referred to as the White Mountains of California. Introduction This report is concerned with the Great Basin bristlecone pine, the longest living tree in the world, and its potential reactions to climate change within California. More than just an iconic California species, these ancient trees have been invaluable to climate research, and their rings have been used to gain a better understanding of past climate change. But with the emergence of climate change, these species are now at risk. Exactly how will this species endure through a warming California climate? In addition, what can be done to ensure the preservation of the Great Basin bristlecone pine under these new conditions? Using climate data and Maxent GIS analysis, we will demonstrate the potential threats which will face this species by the year 2100 and what strategies can be used for its adaptation and preservation. First, an ecological overview of the species will be presented. This will be followed by a Maxent analysis to determine the species reaction to changing climate variables, three primary areas of threats to the species will then be identified, and finally, three adaptive and preservation strategies will be discussed as possible avenues for the species’ survival. Description of Species Biological Classification of Species and General Geographic Range While it goes by the common names western bristlecone pine and intermountain bristlecone pine, Pinus longaeva is most commonly referred to as the Great Basin bristlecone pine; Pinus longaeva falls in the Pinaceae or pine family (Calflora, 2021). Pinus longaeva is closely related to foxtail pine ( P. balfouriana ) and Rocky Mountain bristlecone ( P. aristata ), though their ranges do not overlap in California (FEIS, 2021). In terms of basic physiology, Pinus longaeva has needles which occur in fascicles of five and can live for three to four decades (FEIS, 2021) They can grow from 6 to 12 m depending on elevation and they have a more twisted structure at higher elevations. They can have one or multiple trunks which contain a high percentage of dead wood (FEIS, 2021). In California, Pinus longaeva occurs in Inyo and Mono counties at high elevations in the Panamint, Inyo, and White Mountain ranges (Appendix: Figure 5 4). While Pinus longaeva has a smaller range in California, it has an extensive range outside the state; it is found across the Great Basin, particularly in the Nevada White Mountains, southern and eastern Nevada, and across the western half of Utah (FEIS, 2021). Pinus longaeva occurs at roughly 2,000-3,7000m in elevation, though it becomes the dominant species at the higher end of its elevation range (NPS, 2021). It can occur on moderate to steep slopes of about 10 to 50 degrees (Taylor, 2018). Pinus longaeva favors a northeast-facing slope at lower elevations and aspect greatly impacts average stand age; while stands on north-facing slopes have an average stand age of about 2,000 years, south-facing slopes have a stand age of only about 1,000 years (FEIS, 2021). This disparity can be attributed to the rate at which evaporation occurs; evaporation and evapotranspiration occur faster on south-facing slopes due to increased sunlight exposure throughout the growing season, which hinders water conservation – a limiting factor when it comes to Pinus longaeva age. In terms of pedologic setting, Pinus longaeva frequently grows on rocky dolomite and limestone soils, which generally have a high erosion potential and a low water capacity (Taylor, 2018). Current Climate In the White Mountains, the temperature range of Pinus longaeva stands extends from a July average of 10° C to a Winter average of <0° C; at higher elevations, extreme cold temperature is a main limiting factor on distribution (NPS, 2021). Generally, Pinus longaeva occurs on drier mountain ranges, where it can outcompete other species due to its drought tolerance. Since California’s White Mountains fall on the rain shadow of the Sierra Nevadas, they receive significantly less rain (Calflora, 2021). At their White Mountains sites they get about 300 mm of precipitation per year, however only 60 mm of this occurs as rainfall during their growing season (Taylor, 2018). Growing seasons can last as long as three months or as short as six weeks (FEIS, 2021). Precipitation is a major limiting factor at lower elevations of Pinus longaeva ’s range. In order to cope with such little precipitation, Pinus longaeva has adapted many drought-tolerant features including wide, shallow roots and waxy, thick, long-living needles (Taylor, 2018). Associated Vegetation Pinus longaeva occurs in denser heterogeneous forests at lower elevations, while it occurs in sparse homogenous stands at higher elevations (Taylor, 2018). The limber pine ( Pinus flexilis) is a codominant species with Pinus longaeva in the White Mountains up to a certain elevation extent (FEIS, 2021). Numerous shrubs and herbs occur within Pinus longaeva stands, including sagebrushes, currants, and various grasses. At lower elevations of Pinus longaeva ’s extent, it merges with Great Basin sagebrush, limber pine forest, or pinyon pine-juniper woodland communities (Gymnosperm Database, 2021). 6 Natural Disturbance Regimes Fire constitutes the main natural disturbance regime for Pinus longaeva , though it is not particularly common. Stands are only ever subjected to infrequent, low intensity fires and these are more common at lower elevations where vegetation density is greater (FEIS, 2021). Due to such limited exposure, Pinus longaeva individuals are only adapted to survive low-intensity fires. Their thin cambial layer is protected by a thin layer of bark, which makes individuals extremely vulnerable to fire damage. Due to these reasons, prescribed burning cannot be recommended as an option for vegetation thinning at any point or in any capacity. However, since fire return intervals are so long, fire regimes in the Great Basin bristlecone pine forest are largely under researched; more investigation is in order on topics like post-fire succession and post-fire seed behavior (Gymnosperm Database, 2021). Mountain pine beetle epidemics potentially pose another disturbance to Pinus longaeva, but in practice, they pose no immediate concern. Pinus longaeva naturally resists mountain pine beetle boring due to its built-in chemical defences, even in recent epidemics (Gray, 2017). Climate change could affect this stability and this consideration is explored in later sections. Pinus longaeva are periodically subject to droughts, yet due to their drought-tolerant adaptations, regular drought regimes pose no significant threat to stand health and structure (FEIS, 2021). Past and Current Non-Climatic Anthropogenic Disturbances and Threats One anthropogenic disturbance, discussed yet still not fully understood, is the impact of CO2 fertilization – how more carbon in the atmosphere could alter photosynthesis rates for vegetation. One study involving Pinus longaeva and CO2 fertilization concluded that while increased CO2 would increase needle photosynthesis rates and productivity, the concurrent negative effects of climate change such as warming and particularly drying would confound any increase in the overall growth of Pinus longaeva individuals (de Boer et. al., 2019). Great Basin bristlecone pines also face threats from direct mechanical human disturbances. Recently popular “last chance” tourism could potentially drive up Pinus longaeva visitation to the point where damage (intentional or not) could occur due to over-exposure to humans. Additionally, the threat of vandalism or “arboreal violence” could put charismatic long-lived individuals at risk (Moore, 2012). While their intentions are often good, researchers could also put individual trees at risk through poor practices such as careless tree coring. UNC graduate student Donald Rusk Currey gained infamy for destroying the oldest known Pinus longaeva individual, when he chopped down the 5,000 year old tree “Prometheus” in 1964 to remove his stuck tree coring device (Eveleth, 2012). Many are hopeful that older individuals exist, though they have not been recorded. 7 Finally, the white pine blister rust could pose a potential threat to Pinus longaeva. It is an invasive fungus infecting high-elevation pines though its significance to the Great Basin bristlecone pine remains uncertain. Originally transported on ships from Asia a few centuries ago, the blister rust has had damaging impacts on many tree populations, though even if any Pinus longaeva individuals have contracted it, they do not seem to be noticeably affected (US Forest Service, 2021). Climate Change Vulnerability Predicted Climatic Changes in the White Mountains Although the Great Basin bristlecone pines have experienced serious climate fluctuations in the past—and been able to successfully adapt—the current anthropogenic-driven changes are happening at a speed and intensity unlike anything seen before. Natural fluctuations of carbon dioxide in the atmosphere fall between 180 and 320 parts per million (ppm) by volume, but current predictions show future carbon dioxide concentrations exceeding 550 ppm (Besworth et. al, 2018). This high concentration of carbon dioxide is expected to cause record-breaking temperatures, droughts, and wildfires. Temperatures in the White Mountains are expected to warm between 4º C and 6º C, with average annual temperatures increasing from 13.8º C up to 19.2º C under a high emissions scenario (see Figure 3) (Gray & Jenkins, 2017; “Local climate change snapshot”, 2021). Rising temperatures will impact evapotranspiration rates in the area as well as the snowpack in the mountains, leading to more severe droughts (Besworth et. al, 2018). These changes will be dramatic: the number of days considered extreme on the Keetch-Byram drought index, which represents the amount of precipitation necessary for soil to return to its field capacity and can be used as a measure of drought and wildfire risk, will increase from 7 days to 22-50 days depending on the emissions pathway used (see Figure 1) (“Local climate change snapshot”, 2021). Consequently, fires in California are expected to become more extensive and severe, burning upwards of 100 hectares each year in the Eastern mountain ranges of California where the bristlecone pines are located (see Figure 2) (Besworth et. al, 2018). Precipitation, on the other hand, is not expected to change much, with projections showing a slight increase (“Local climate change snapshot”, 2021).With all of these changes happening so fast, the bristlecone pines will not be able to adapt like they have in the past, putting them at greater risk. 8 Maxent Analysis Data In order to better understand how these predicted changes in climate will impact the suitable habitat and distribution of the bristlecone pine, we used the environmental modeling software Maxent. The input data included the current locations of bristlecone pine individuals and WorldClim’s current and future (2081-2100) bioclimatic variables at a 2.5 minute resolution, which incorporate 19 different climate variables representing annual trends, seasonality, and limiting environmental factors (CalFlora, 2021; Fick and Hijmans, 2017). The future climate variables were based on the global climate model CANESM5, which is an updated version of the climate model recommended to the state of California as the best “average” climate model that should be used if an analysis, such as ours, could not compare all of the global climate models (Pierce, Kalansky, and Cayan, 2018). However, we did choose to compare several pathways to understand if there would be differing risks to the bristlecone pine under different emissions scenarios. These pathways included ssp126, a low carbon emissions scenario, ssp245, which corresponds to the state standard RCP 4.5 and is considered to be the “business as usual” scenario, and ssp585, which falls at the high end of the predicted emissions range and corresponds to RCP 8.5. Below we focus our discussion on ssp245, as there were no significant differences between the three emissions scenarios and their impact on the bristlecone pine outputs. Habitat Correlation with Climate Variables As a part of the Maxent analysis, we ran a Jackknife analysis to examine the influence of each individual bioclimatic variable on the overall model output. These results showed that the bioclimatic variables are all highly correlated, and no single variable had significant impacts on the overall distribution of bristlecone pines. However, there were a few variables that stood out, including mean temperature of the driest quarter and mean temperature of the coldest month, suggesting that temperature is one of the more important climatic factors in determining where bristlecone pines can survive. 9 Habitat Vulnerability to Climate Change Output of the Maxent analysis depicting the current and future probability of occurrence for the bristlecone pine throughout California (CalFlora, WorldClim, UCLA Geoportal). The Maxent output depicts the probability of a bristlecone pine occuring at any given location. As seen above, the current range of suitable habits is already limited and concentrated in the White Mountain region in eastern California, with a few smaller strands appearing further south. The changes between the current and predicted distributions are subtle—and not as dire as expected. The future distribution, based on ssp245, is visibly smaller than the current distribution, with the southern strands completely disappearing, unable to adapt to changing climate features. Although the main concentration of suitable habitat also shrinks slightly, seen in the decrease of colored pixels in the inset map, it remains mostly intact, with the probability of occurrence even rising in some areas. This is promising; it shows that the main concentration of bristlecone pines as it stands today is not completely threatened by future climate change, providing hope of successfully preserving them. There is also an expansion of suitable habitat along the eastern edge of the Sierra Nevada. Though the probability here is still lower than the areas where there are currently bristlecones, this expansion demonstrates that the climate itself may not be the most significant factor putting the bristlecone pines at risk. 10 Specific Threats Posed by these Climatic Changes to the Species This report will focus on three main areas in which climate change will induce threats to the bristlecone pine. Rising temperatures will change the pine’s volatile organic compound production and make it more attractive to mountain pine beetles, as well as increase fire risk and encourage leapfrogging of the bristlecones by limber pines. Each of these threats will be considered below. Changing Volatile Organic Compound Production and the Mountain Pine Beetle Rising temperatures cause plants to alter their emissions of volatile organic compounds, causing some chemicals to be produced at higher rates and others at lower rates. Historically, bristlecone pines have emitted relatively high ratios of the compounds α-pinene/D-limonene and α-pinene/3-carene. The high levels of these compounds in bristlecone pines have acted as a deterrent to mountain pine beetles, which choose their host pines based on the trees’ chemical composition. As temperatures in the White Mountains increase due to climate change, the chemical makeup of bristlecone pines will change as their production of these two compounds decreases. Bristlecones that emit reduced levels of these compounds will have a chemical composition that more closely resembles that of other pines that the mountain pine beetle proliferates in (Gray, Runyon, & Jenkins, 2019). Bristlecone pines will no longer be distinguishable from other pines, and their defense mechanism against these beetles will be lost. Once mountain pine beetles become more attracted to bristlecone pines, the survival of the species could be at tremendous risk. These beetles have successfully destroyed entire forests in other regions, and they would be able to kill entire bristlecone pine stands very easily. Mountain pine beetles burrow through the bark of their host trees and lay eggs in the trees’ cambium layer. The larvae then mature inside the cambium, getting their nutrients from the phloem of the trees. In doing so, they effectively steal the trees’ nutrients, causing the tree to die (Gibson et. al., 2008). Given the already low numbers of bristlecone pines in California, the presence and predation of the mountain pine beetle in bristlecone stands could potentially decimate the remaining groves. Additionally, mountain pine beetles thrive in warmer environments, because their larvae freeze over if temperatures are too cold. Climate change in the White Mountain region will make conditions more hospitable for the beetles (Gibson et. al., 2008). This doubly disadvantages bristlecones, because they will face an increasing abundance of mountain pine beetles while simultaneously losing their defense mechanism against such pests. Mountain pine beetles are also a tremendous threat to limber pines, which is an advantage for bristlecone pines in the context of leapfrogging, but if there are limber pines to provide habitat for the beetles in the same environment as bristlecone pines, the probability of the beetles spreading increases. Climate change thus endangers the bristlecone pine because it affects their ability to produce certain volatile organic compounds, making them more susceptible to the mountain pine beetle. 11 Increasing Risk of Fire Rising temperatures also increase fire risk, in several ways. Volatile organic compounds are important in this context too, because higher temperatures cause bristlecone pines to produce more terpenes, which increase the flammability and burn rates of the trees (Gray, Runyon, & Jenkins, 2019). This will be particularly pronounced for lower elevation bristlecone stands, because they will face hotter temperatures than bristlecones at treeline. Lower elevation stands are also more dense, because they share the habitat with other conifers and spruces. Since hotter temperatures make fire conditions more extreme, especially because of more dramatic wind and fuel moisture, the severity of the fires that ignite within these lower elevation stands will be especially pronounced (Gray & Jenkins, 2017). If conditions are harsh enough, particularly with longer drought periods and hotter days, these fires will have little difficulty spreading upslope. Bristlecone pines have not had to deal with fires much throughout their long lifespans (see Figure 5). They inhabit very open stands with little vegetation to provide fuel for fire. Consequently, they are only adapted to withstand low-severity fires. Bristlecones have thin bark, low branches, and adult trees retain a lot of needles. Because the trees are very short and their needles remain close to the ground, they could very easily catch on fire. They also produce a significant amount of litter and duff at their bases, which can ignite in extreme conditions (Gray & Jenkins, 2017). Fires spreading upslope from higher density stands may not need to travel far as long as there is enough litter on the ground to spread the flames. Warmer temperatures in the White Mountains will decrease ignition times for all trees in the area, and there will be more days per year with temperatures at which ignition is likely to take place (Gray & Jenkins, 2017). Climate change is threatening the bristlecone by providing weather conditions that increase the likelihood of intense fires for which it is not adapted. However, increasing temperatures also put bristlecone pines at greater fire risk because they make higher elevations more hospitable to a greater variety of species. As conifers and highly flammable invasive weeds from lower elevations move upslope because of the expansion of suitable habitat for them, they will bring greater amounts of vegetative fuel for fires (Gray & Jenkins, 2017). The increasing abundance of other species in historically open stands around bristlecones introduces the possibility of more fuel by which fire can spread and build intensity. This is especially consequential in the context of the limber pine, because limber pine juveniles have demonstrated a profound ability to rejuvenate above the treeline where bristlecones live. Limber pine juveniles are even less adapted to fires, as they have extremely thin bark and they regenerate very quickly, providing more fuel within the next season or two (Gray & Jenkins, 2017). The prevalence of their saplings on the surface upslope of bristlecones could create a situation in which bristlecones are surrounded on all sides by vegetation that is ripe to burn. Since bristlecone pines do not stand a chance by themselves against large-scale fires, this climate change-induced problem must be recognized and handled appropriately. 12 Leapfrogging by Limber Pines Increasing temperatures at higher elevation sites due to climate change make upslope conditions more habitable for limber pines. Limber pines commonly share habitat with bristlecones in lower elevation sites, and they are abundant just below treeline in many stands (see Figures 11 and 12). There are important differences between the two species which have prevented much competition thus far, besides the advantage bristlecones have to grow at higher elevations with colder temperatures. Limber pine adults favor granitic soils, whereas bristlecone pines favor carbonate, calcareous, or dolomite soils (Smithers et. al., 2017). Limber pines also have large seeds that are spread and planted by the Clark’s Nutcracker at distances up to twenty-two meters away from the parent tree. Bristlecone pines have small seeds, and they rely instead on wind and gravity to disperse. They are heavily preyed upon by rodents, so limber pines have greater dispersal advantage because their seeds have higher chances of being distributed. Collectively, these species have achieved a mean vertical shift of 19.1 meters above treeline since 1950, because of rising temperatures at higher elevations (Smithers et. al., 2017). But most of that advance has been on behalf of limber pines, because they have a significant migration advantage. Limber pine juveniles do not share the same environmental associations, or ‘qualifications’, for habitat as limber pine adults do. Consequently, they are able to migrate in abundance on a variety of soils and in relatively unfavorable conditions. They will grow on any soil type, including dolomite soils that are core habitat for bristlecone pine. In contrast, bristlecone pines have very slow regeneration times and their juveniles share the same environmental preconditions as the adults, making them much more ‘picky’ as to where they can migrate (Smithers et. al., 2017). Bristlecone pine juveniles are also completely shade intolerant, so where limber pines can become the initial colonizer, they create shade and competition for space in which bristlecones are comparatively weak for regeneration. Bristlecone seedlings have no chance to establish where limber pines have gotten a head start, and this seems to be occurring across their territory in the White Mountains (Smithers et. al., 2017). 13 These panels compare the bristlecone pine distributions to the limber pine distributions. The third image in each series overlays the two and demonstrates the seriousness of leapfrogging (CalFlora, WorldClim). To understand the severity of this issue, we ran a secondary Maxent analysis with the limber pines to see how their range would change under future climate conditions. Two things stand out. First, the limber pine occurrences almost completely overlap those of the bristlecone pine, with limber pines completely overtaking the bristlecone in future predictions. Second, unlike the bristlecone pine, the probability of occurrence for the limber pine actually increases across its range under future climate conditions, particularly in the area that is currently home to the main strand of bristlecones. Because of the dispersal advantage limber pines have, they are able to leapfrog over the bristlecone pines at treeline. Rising temperatures are allowing them to proliferate below and above bristlecone pines in elevation, and once they are established they will outcompete bristlecone pines for the minimal remaining upslope habitat. The lag in the range shift of bristlecone pines is allowing limber pines to progressively dominate above treeline, making it increasingly likely that bristlecones could be outcompeted in the struggle to survive climate change’s consequences. Adaptive Capacity/Vulnerability of the Species to Climate Change in Current Location Great Basin bristlecone pines are already restricted to a relatively small elevational range of about 2,700 to 3,700 meters, and they live in sky island environments. This means that even though there is some territory upslope for the trees to inhabit, there is not a significant amount of new habitat (Gray, Runyon, & Jenkins, 2019). And if climate change progresses at the extreme speed with which it is already developing, bristlecone pines might run out of this territory 14 quickly, while becoming ill-adapted for the warming climate of their current range. However, bristlecones are not impacted by the changing climate equally. Those living at lower elevation stands are more sensitive to changes in precipitation, whereas those living in higher elevation stands near or at treeline are more sensitive to changing temperatures (Salzer et. al., 2014). Because they are not uniformly affected by climate change, they have varying vulnerabilities that make helping them adapt more complex. Bristlecones living at treeline appear to have a positive growth response to rising temperatures, as there has been some expansion of bristlecone juveniles roughly twenty-five to seventy-five meters above treeline in recent years (Salzer et. al., 2014). Even though this seems promising, it is important to keep in mind that limber pines still have the ability to outcompete bristlecone pine juveniles above treeline, and if they do not expand quickly enough they will struggle to find suitable habitat. Additionally, lower elevation stands are most vulnerable because they will face the warmest temperatures, which will cause excess heat stress, drought stress, competition from other species moving upslope, and a higher likelihood of frequent and severe fires (Gray, Runyon, & Jenkins, 2019). Adaptive Capacity/Vulnerability of the Species to Climate Change Via Migration The bristlecone pine’s ability to migrate in the face of climate change is hindered because of the threat of leapfrogging by limber pines. The ease with which limber pines spread above treeline makes it much more difficult for bristlecones to regenerate at elevations higher than their present range. One of the most important aspects of this competition is the shade that results from the establishment of limber pine saplings, which prohibits bristlecone juveniles from growing (Smithers et. al., 2017). Bristlecone pines have one advantage over limber pines, in that their extreme upper range limit is thirty meters higher than that of limber pines, but they still migrate too slowly to utilize this potential. The soil conditions above treeline in the White Mountains are also not particularly hospitable to bristlecone pine juveniles, which gives limber pines an even greater advantage because their juveniles have much broader environmental tolerances. Moreover, upper treeline has less hospitable upslope geomorphology. Even though bristlecones are well adapted to grow on steep slopes, the conditions for regeneration are still difficult (Smithers et. al., 2017). All of these factors, combined with the slow regeneration time of bristlecones, suggest that there is a very real possibility for local extirpation at some sites. 15 Suggested Climate Change Adaptation Strategies When approaching the best ways to maintain the bristlecone pines in the face of the climatic threats mentioned above, we highlight three primary areas of consideration. Fuel Clearance In order to mitigate the impact of fires in the coming century, fuel clearance will play an essential role. Due to climate projections which imply an upslope movement of vegetation in addition to an increase in fire severity, it is very likely that fire will be one of the most pressing threats facing the bristlecone pines. In order to address this, the areas where the bristlecones are located will need to be diligently maintained in regards to fuel cutback and maintenance. To some extent, this strategy could also limit potential exposure to bark beetle populations by preventing interactions with other species. Clearance will largely involve manual and mechanical clearance with other methods, such as prescribed burning, used with the utmost caution and only in specified areas far away from the bristlecone pine itself, since fire poses a huge risk to them. One fortunate aspect of these procedures is that the area where bristlecone pines are likely to habitat in the coming century will be particularly small and concentrated. Specifically, the total area projected through Maxent analysis suggests that around 97,781 hectares in the White Mountain region will be habitable for them (Figure 14). Focusing on fuel clearance in and around this range would likely secure a much more fire protected area for the species and ensure their preservation. Proposed focus of future preservation efforts 16 In regards to cost, combined efforts of fuel clearance can vary. With manual labor, mechanical clearing, fire break clearance, and other methods put in place, it is likely that the average cost of maintenance per acre would come out to be around $134 (Loomis et. al., 2019). If this same procedure were applied to the entire habitable zone for the bristlecone pine, this would result in a total estimated cost of approximately $32.3 million. However, applying these procedures to the entire 378 square mile range within the White Mountains is an inaccurate representation of how clearance would take place, and the total would likely be highly reduced with an assessment of which areas should be prioritized for clearance. With proper attention, costs in fuel maintenance can be optimized to a prudent degree. It does remain consistent however, that in order to avoid fire having a devastating effect on the bristlecones, fuel clearance will be an imperative procedure. Transplanting vs. Preservation In regards to whether to transplant the bristlecones or to preserve them in their current range, we highly encourage the latter. As demonstrated through Maxent analysis, the projected range of the bristlecone will likely only decrease within the next century. As a result, potential locations for new bristlecone habitation are extremely limited. While there are a few locations where the trees could expand, they would likely also run into competition, as it was also demonstrated that much of the area opened up to bristlecone pines will likely be dominated by limber pines. In order to concentrate preservation and conservation efforts, resources should be directed to the 378 square mile range mentioned above. This will allow for a focused conservation effort and once that emphasizes the survival of the most charismatic members of the species found within the methesula grove. When looking at the preservation of the species as a whole, options in California are limited. It is highly encouraged that areas of expansion found in nearby states such as Nevada and Utah be looked into when considering species wide conservation. Collaboration with these respective state agencies would greatly benefit the overall survival of the Great Basin bristlecone as more land could be utilized for the expansion of the species. Limber Pine Maintenance One final area of consideration crucial to the survival of the bristcones will be addressing the issue of limber pine imposition. As demonstrated previously, the leapfrogging and upslope movement of limber pines pose a significant threat in the preservation of bristlecones in their current range. In order to mitigate the impact of this movement, the careful removal of limber pine juveniles will be an important part in adaptation strategy. In addition to general clearance, special care will need to be put in place in order to limit the movement of limber pines into sensitive bristlecone habitat. This will come in the form of general clearance, both manual and 17 mechanical, along with the fuel clearance strategies mentioned above. Specifically pertaining to the limber pine, however, there are few considerations necessary to make. Though less threatened than the bristlecones, the limber pine still fills an ecological niche which should be considered when removing them from bristlecone habitat. In particular, there is no need to eliminate the species from below treeline where the area is not critically dominated by bristlecones. Instead, more consideration should be paid to the limber pines which are observed to be “leapfrogging” bristlecones into higher elevation. Furthermore, a select amount of limber pines should be allowed to remain within the general area in order to support the species, such as birds, which rely on limber pines seeds for food. By focusing on the removal of encroaching juvenile limber pines from particularly sensitive areas such as the Methuselah Grove, we hope to allow the Great Basin bristlecone pine to persist without the competitive pressures of limber pines potentially harming the species in select areas. Conclusions Throughout this report we have discussed the Great Basin bristlecone pine, its ecology, its projected reactions to climate change, the greatest threats facing the species, and concentrated adaptation strategies to be used in its preservation. In particular we have found: ● Conditions in the White Mountains are projected to grow warmer and drier by the year 2100. ● In reaction, habitable zones for the bristlecone pine will likely decrease beyond their already small extents. ● The species will likely face a series of threats under these changing conditions. ● The first threat is a possible change in soil composition which would make bristlecones susceptible to bark beetles. ● The second threat is an increase in fire frequency and intensity. ● The final threat is the leapfrogging and imposition of limber pines in bristlecone habitat. With these facts in mind we recommend three core strategies for adaptation and preservation: ● The first involves general fuel clearance through manual and mechanical means in order to mitigate fire risk. ● The second encourages the preservation of the species within its already established range in the White Mountains to reduce costs and feasi