Climate change is reducing the extent of cold aquatic habitats and their unique biodiversity in mountain areas. However, a variety of cold rocky landforms (CRLs) are thermally buffered and feed cold springs (<2 °C) that may represent climate refugia for cold-adapted organisms. These landforms, hitherto overlooked by freshwater research, include rock glaciers, debris-covered glaciers, talus slopes, protalus ramparts, and young moraines. Here, we investigated the warm-season water temperature of 228 springs from clean (ice) glaciers, CRLs, and reference slopes (not sourced by any of these features) in 13 mountain ranges of Europe, South America, and North America. Only springs from glaciers (90%) and CRLs (45%) had average stream temperatures below the thermal optimum for coldwater organisms of 2 °C. Springs fed by CRLs were 3 °C–5 °C (up to 9 °C) colder than those from nearby reference slopes. In general, cold springs were rarer in Mediterranean/semi-arid climates than in temperate and sub-polar climates. Landforms comprising barren and coarse rocky surfaces or ice/rock mix, having a simple or absent soil/vegetation structure, and higher likelihood of permafrost more often supported cold springs. When water temperatures were compared to air temperature, most CRL springs were thermally buffered against warm periods, cumulative heat, and daily temperature fluctuations. With cold conditions maintained in a variety of climates and mountain landscapes, CRL springs in mountains likely have high conservation value. We call for integrated ecological and hydrological research for these ecosystems, aimed at understanding their potential as climate refugia.

The Late Holocene Dry Period (LHDP, 3100-1800 cal yr BP) is well established as a period of extreme aridity across lowlands of the Great Basin (GB) yet little has been elaborated about high mountain conditions. Further, temperatures have not been comprehensively addressed at any elevation, although the period is often implied to have been warm. Despite a literature on the early Neoglaciation in the GB, conditions at high elevations have not been incorporated in previous treatments of the LHDP. We compile existing and new data on locations and ages for early Neoglacial advances in mountains of the GB, documenting its existence in 33 mountain ranges that extend from 36.47 degrees N to 44.30 degrees N and 110.69 degrees W to 121.36 degrees W. Dates indicate the early Neoglaciation occurred before, during, and after the LHDP; starting as early as 3800-3700 cal yr BP, with subsequent advances at 3400, 3200, 3100-3000, 2930, 2900, 2800, 2700, 2600, 2550-2640, 2400, 2240, 2200, 2150, 2100, 2000, 1955, 1880, 1700, 1600, 1500, and 1400 cal yr BP. We estimate summer temperature decreases from modern conditions that would enable rock glaciers to have advanced in the LHDP as similar to 1.3 degrees C-similar to 4.5 degrees C, with no or slight increase in winter precipitation for mountain ranges with modern winter snowpacks >54 cm. Cooler summers would translate to lower evaporation and overall maintenance of wetter conditions in the uplands than modern summer conditions, supporting healthy high-elevation ecosystems. Middle Archaic GB communities significantly transformed their lifeways at the start of the LHDP, bringing entire families to summer at alpine plateaus and providing a new proxy for cool, wet uplands during this millennium. To underscore the importance of glacial conditions at high elevations in the GB, we propose that this period be referred to as the "Early Neoglaciation/Late Holocene Dry Period."

The American Pika (Ochotona princeps) has been considered a species at risk due to warming temperatures associated with climate change. Many life-history attributes of pikas contribute to the sensitivity of pikas to warming temperatures. Repeated censuses of a marginal (warm, low-elevation) population of pikas at Bodie State Historic Park, California, conducted from 1972 to 2022, are presented to track the population trajectory of pikas for a time period predating recent awareness of global warming to the present day, thus giving a comprehensive portrayal of how American pikas may be responding to climate change. The northern constellation of anthropogenic habitat patches (mine ore dumps) showed no decline in percent of patches occupied or in total number of pika territories over time, suggesting that pikas in that area have not been at risk of extirpation resulting from climate change. In contrast, the pika population in the southern constellation of patches showed significant declines in percent of patches occupied and number of pika territories occupied. That area was unoccupied for about a decade beginning in 2006, but was recently recolonized from the northern constellation of patches. The most likely cause of the initial decline and transient extirpation in the south appears to result from fragmentation and stochastic population dynamics, independent of climatic factors that we investigated. Assessments of climatic impacts on American pikas should take into account the dynamics documented in the Bodie pika population and its resilience over time. Surveys of American pikas from 1972 to 2022 at a marginal locality (Bodie, California) found contrasting population responses; the northern population was stable, whereas the southern population experienced a metapopulation collapse; climate was not implicated in either situation.

AbstractThe Late Holocene Dry Period (LHDP) was a one-plus millennial megadrought (3100–1800 cal BP) that delivered challenges and windfalls to Indigenous communities of the central Great Basin (United States). New pollen and sedimentation rate studies, combined with existing tree-ring data, submerged stump ages, and lake-level evidence, demonstrate that the LHDP was the driest Great Basin climate within the last 6,000 years—more extreme than the well-known Medieval Climatic Anomaly. New evidence reported here documents that most Great Basin archaeological sites south of 40° N latitude were abandoned during the long dry phase of the LHDP (3100–2200 cal BP), sometimes reoccupied during a wet interval (2200–2000 cal BP), and abandoned again during the most extreme drought (2000–1800 cal BP). Even in the face of epic drought, this is a story of remarkable survivance by some people who adjusted to their drought-stricken landscape where they had lived for millennia. Some moved on, but other resilient foragers refused to abandon their homeland, taking advantage of glacier-fed mountain springs with cooler alpine temperatures and greater moisture retention at high altitude, a result of early Neoglaciation conditions across many Great Basin ranges, despite epic drought conditions in the lowlands.

American pikas (Ochotona princeps), small mammals related to rabbits, occur in mountainous regions of western North America, where they live in shattered-rock habitats (talus). Aspects of their physiology and life history create situations that appear to put pikas at risk from warming climates. Some low-elevation, warm sites that historically harbored pikas have become extirpated, and the assumption is that these will not be re-colonized under current climate trends. Unexpectedly, in 2021, we found that pikas had re-colonized two very warm, low-elevation, dry sites in eastern California, USA, in the Bodie Mountains and Mono Craters. Resident pikas appear to have been absent at both sites for >= 10 years. These findings suggest that pikas, which are normally diurnally active, are able to overcome thermal dispersal barriers and re-colonize long-extirpated sites, perhaps by moving during cool nights. Our data also highlight the often unrecognized suitability of pika habitat in warm regions where the interiors of taluses can remain stably cool even when external air temperatures are hot.

High-elevation five-needle pines are foundational species and iconic components of subalpine forests across western North America. Because they often grow at environmental extremes, high-elevation pines are vulnerable to changing climate conditions. In addition to the direct effects of recent climatic changes, these species are increasingly threatened by biotic disturbances that thrive in the warming and drying conditions now occurring at higher elevations. Among the high-elevation pines, Great Basin bristlecone pine (Pinus longaeva) is revered for its extreme longevity and has been considered an icon of stability during periods of change. Life history strategies of Great Basin bristlecone pine that contribute to its longevity include physiological traits that enhance survival in harsh and dry habitats, and defensive traits that make it less vulnerable than other high-elevation pines to tree-killing bark beetles. Recent increases in growing degree days with no associated increase in precipitation is causing temperature-amplified tree drought stress, while warming temperatures positively influence bark beetle population growth. We report on preliminary investigations into recent and unexpected Great Basin bristlecone pine mortality at two sites, including the potential roles of weather-induced stress and bark beetles. At both sites climatic water deficit (CWD), a cumulative measure of moisture stress, and mean annual temperature increased during the 2010 decade and CWD was the highest in 2020 relative to any time during the past 40 years. Although Great Basin bristlecone pine mortality has not previously been attributed to bark beetles, we observed recent (i.e., 2013 to 2020) bark beetle-attacked trees at both sites, coincident with the timing of increasing temperature and CWD. Few adult beetles were produced, however, and our results support previous research that Great Basin bristlecone pine is a population sink for bark beetles. Because bark beetles are likely not self-sustaining in Great Basin bristlecone pine, bark beetle-caused mortality of this iconic species will most likely occur when it grows mixed with or near other pine species that support bark beetle population growth. We found Ips confusus and Dendroctonus ponderosae attacking Great Basin bristlecone pine in areas where their host trees, P. monophylla and P. flexilis, were also growing. These results suggest that the presence of these infested conifers likely contributed to Great Basin bristlecone pine mortality. We highlight several factors that may be used for prioritizing future research and monitoring to facilitate development of management strategies for protecting this iconic species.

Warming-induced mountain pine beetle (Dendroctonus ponderosae; MPB) outbreaks have caused extensive mortality of whitebark pine (Pinus albicaulis; WBP) throughout the species' range. In the highest mountains where WBP occur, they cross alpine treeline ecotones (ATEs) where growth forms transition from trees to shrub-like krummholz, some of which survived recent MPB outbreaks. This observation motivated the hypothesis that ATEs are refugia for WBP because krummholz growth forms escape MPB attack and have the potential to produce viable seed. To test this hypothesis, we surveyed WBP mortality along transects from the ATE edge (locally highest krummholz WBP) downslope into the forest and, to distinguish if survival mechanisms are unique to ATEs, across other forest ecotones (OFEs) from the edge of WBP occurrence into the forest. We replicated this design at 10 randomly selected sites in the U.S. Northern Rocky Mountains. We also surveyed reproduction in a subset of ATE sites. Mortality was nearly absent in upper ATEs (mean ± SE percent dead across all sites of 0.03% ± 0.03% 0-100 m from the edge and 14.1% ± 1.7% 100-500 m from the edge) but was above 20% along OFEs (21.4 ± 5.2% 0-100 m and 32.4 ± 2.7% 100-500 m from the edge). We observed lower reproduction in upper ATEs (16 ± 9.9 cones/ha and 12.9 ± 5.3 viable seeds/cone 0-100 m from the edge) compared to forests below (317.1 ± 64.4 cones/ha and 32.5 ± 2.5 viable seeds/cone 100-500 m from the edge). Uniquely high WBP survival supports the hypothesis that ATEs serve as refugia because krummholz growth forms escape MPB attack. However, low reproduction suggests ATE refugia function over longer time periods. Beyond our WBP system, we propose that plant populations in marginal environments are candidate refugia if distinct phenotypes result in reduced disturbance impacts.

Abstract Tree-rings representing annual dates from live and deadwood Pinus flexilis at ten sites across the central Great Basin (~38°N) yielded a cumulative record across 4002 years (1983 BC–AD 2019). Individual site chronologies ranged in length from 861–4002 years; all were continuous over their sample depths. Correlations of growth with climate were positive for water relations and mostly negative for summer temperatures. Growth was generally correlated across sites, with the central Nevada stands most distinct. Although growth was low during the Late Holocene Dry Period, variability marked this interval, suggesting that it was not pervasively dry. All sites had low growth during the first half of the Medieval Climate Anomaly, high growth during the mid-interval pluvial, and low growth subsequently. Little synchrony occurred across sites for the early Little Ice Age. After AD 1650, growth was depressed until the early twentieth century. Growth at all sites declined markedly ca. AD 1985, was similar to the lowest growth period of the full records, and indicative of recent severe droughts. A small rebound in growth occurred after ca. AD 2010. A strong signal for Atlantic Multidecadal Oscillation (AMO) occurred in growth response at most sites. The persistence of all stands despite climate variability indicates high resilience of this species.

Mountains are global biodiversity hotspots where cold environments and their associated ecological communities are threatened by climate warming. Considerable research attention has been devoted to understanding the ecological effects of alpine glacier and snowfield recession. However, much less attention has been given to identifying climate refugia in mountain ecosystems where present‐day environmental conditions will be maintained, at least in the near‐term, as other habitats change. Around the world, montane communities of microbes, animals, and plants live on, adjacent to, and downstream of rock glaciers and related cold rocky landforms (CRL). These geomorphological features have been overlooked in the ecological literature despite being extremely common in mountain ranges worldwide with a propensity to support cold and stable habitats for aquatic and terrestrial biodiversity. CRLs are less responsive to atmospheric warming than alpine glaciers and snowfields due to the insulating nature and thermal inertia of their debris cover paired with their internal ventilation patterns. Thus, CRLs are likely to remain on the landscape after adjacent glaciers and snowfields have melted, thereby providing longer‐term cold habitat for biodiversity living on and downstream of them. Here, we show that CRLs will likely act as key climate refugia for terrestrial and aquatic biodiversity in mountain ecosystems, offer guidelines for incorporating CRLs into conservation practices, and identify areas for future research.

A complex landscape lies on an alpine plateau in the high Sierra Nevada, California, and comprises spatially organized physical and ecological features that interact to create a unique ecosystem. At the upslope end a tree-limited ribbon forest of whitebark pines grows on the crest of a short, steep slope, which is the only deformation on the otherwise flat plateau. A long-persistent snowbank forms on the slope in the lee of the ribbon forest; meltwater provides moisture to support a productive wet meadow just below. Below the meadow is a narrow, linear "rampart" comprising large sorted stones, and below this lies a large patterned-ground area of cryogenically sorted circles. The soil domains are densely covered with species-rich vegetation and are abruptly segregated from stone domains. The latter serve as distributed springheads in the otherwise dry patterned-ground terrain. American pikas have colonized the rocky nets of the stone domains and forage plants of the soil circles. The unusual context of this landscape enables pika territories to be more tightly packed than in traditional habitat. We propose a scenario for the Pleistocene origin of this ecosystem and a unique lithologic subsurface barrier that contributes to spring formation during warm intervals.

Custom camera traps were positioned at American pika (Ochotona princeps) haypiles in 12 warm, low-elevation locations of eastern California over 5 years and during warm and cold seasons. These camera traps detected 26 mammal and 10 bird species, including 4331 pika events as well as visits by 16 sympatric herbivores and 7 pika predators. Camera traps documented pika occupancy at some sites that had been evaluated from field surveys as extirpated, and they also confirmed field assessments of extirpation at other sites. Individual pikas could be distinguished by scars, size, and pelage diagnostics, allowing animals to be followed through sequences of photos and enabling behavioral interpretations and documentation of winter and warm-season activities. Temperature measurements at haypiles and talus interiors corroborated prior findings that rocky interiors are much cooler than surfaces, have highly attenuated daily temperature fluctuations, and offer refuge for pikas from high daytime temperatures. Nocturnal activity was recorded for pikas as well as for many other species, but we found little evidence that night activity of pikas at haypiles increased when prior day temperatures were excessively warm. The capacity of pikas to be active at all times of the day adds to their resilience in the face of predators, foraging needs, and changing climates.

Climate-change adaptation focuses on conducting and translating research to minimize the dire impacts of anthropogenic climate change, including threats to biodiversity and human welfare. One adaptation strategy is to focus conservation on climate-change refugia (that is, areas relatively buffered from contemporary climate change over time that enable persistence of valued physical, ecological, and sociocultural resources). In this Special Issue, recent methodological and conceptual advances in refugia science will be highlighted. Advances in this emerging subdiscipline are improving scientific understanding and conservation in the face of climate change by considering scale and ecosystem dynamics, and looking beyond climate exposure to sensitivity and adaptive capacity. We propose considering refugia in the context of a multifaceted, long-term, network-based approach, as temporal and spatial gradients of ecological persistence that can act as "slow lanes" rather than areas of stasis. After years of discussion confined primarily to the scientific literature, researchers and resource managers are now working together to put refugia conservation into practice.

A multiyear study of forest-to-alpine ecotones across extensive krummholz zones in whitebark pine (WBP; Pinus albicaulis), Sierra Nevada, California, resolved mean treeline growing season temperature (GST) of 9.3°C, 2.6°C warmer than global thresholds previously described, and mean growing season length of 143 days. Temperatures declined with increasing elevation; GST at the upper krummholz line (8.9°C), however, was 2.2°C warmer than the mean global treeline threshold, suggesting that by thermal criteria these environments should support tree growth. Possible explanations for the warm conditions and persistence of krummholz rather than treeline advance include a role for moisture limitations, disequilibrium with Little Ice Age temperatures, and the influence of krummholz as a buffer to treeline dynamics. Radial growth in treeline WBP trees was negatively correlated with maximum annual temperature and positively correlated to water year precipitation. Krummholz stems had low correlations to one another, to treeline trees, and to climate, suggesting nonclimatic controls on growth. These findings underscore the variable nature of treeline response to climate change, suggest that krummholz ecotones behave differently from diffuse treelines, and add to examples of mountain conifers that may be exhibiting lag effects and have not shifted with contemporary warming.

Efforts to conserve biodiversity increasingly focus on identifying climate‐change refugia – areas relatively buffered from contemporary climate change over time that enable species persistence. Identification of refugia typically includes modeling the distribution of a species’ current habitat and then extrapolating that distribution given projected changes in temperature and precipitation, or by mapping topographic features that buffer species from regional climate extremes. However, the function of those hypothesized refugia must be validated (or challenged) with independent data not used in the initial identification of the refugia. Although doing so would facilitate the incorporation of climate‐change refugia into conservation and management decision making, a synthesis of validation methods is currently lacking. We reviewed the literature and defined four methods to test refugia predictions. We propose that such bottom‐up approaches can lead to improved protected‐area designations and on‐the‐ground management actions to reduce influences from non‐climate stressors within potential refugia.

In response to contemporary changes in climate, many tree species are shifting upslope to find favorable habitat. In the case of obligate ectomycorrhizal species, seedling growth above upper treeline depends on fungal spore availability. In the mountain ranges of the Great Basin, a recent shift in tree species stratification has been recorded, with limber pine (LP, Pinus flexilis) leapfrogging above the ancient bristlecone pine (BCP, Pinus longaeva) forest and establishing above current treeline. We compared the ability of LP and BCP to interact with soil spore banks collected at different microhabitats (next to dead trees, young live trees or in a treeless control) above current treeline in the White Mountains of California. We found an ectomycorrhizal fungal spore bank community composed of 15 species that was dominated by an undescribed and a hitherto unsequenced species of Geopora and Rhizopogon, respectively. This represents a much richer community than was found previously in this system. While both LP and BCP were able to establish ectomycorrhiza, LP was twice as likely to do so, and when comparing only seedlings that were colonized, its root system was colonized to a three-fold greater extent. BCP seedlings grown on soils collected under young live trees were much more likely to be colonized compared to soils from the other two microhabitats.Synthesis. These differences in ectomycorrhizal receptivity might help to explain why LP is currently establishing at higher rates above the BCP treeline. Furthermore, it is possible that LP saplings above treeline can provide ectomycorrhizal facilitation for BCP seedlings, enabling the subsequent shift of BCP above treeline.

The 1351 ha Walker Fire burned in late August 2015 on the eastern Sierra Nevada, California escarpment from sagebrush steppe and pinyon pine woodlands (2150 m) through mid-elevation forests into subalpine whitebark pine (WBP; 3310 m). Fuels were particularly dry due not only to the season but as a result of a record multi-year drought (2012-2015). The fire burned into stands heavily impacted by a mountain pine beetle outbreak (MPB) during a prior drought (2007-2010). Using plot data, we report 100% mortality due to fire of understory vegetation and trees in pinyon pine, aspen, white fir, and lodgepole pine forest types. Fire-caused tree mortality was lower in Jeffrey pine forests (80%), although 100% of the understory was killed by the fire. For WBP plots in areas that had not been affected by MPB, 100% of understory and tree layers also burned. Where the fire burned through plots within the former MPB-outbreak area, 2 of 15 plots had 100% understory vegetation and trees burned by the fire, while the remaining 13 had varying levels of survivorship. In those plots, pines dead from prior beetle-kill as well as live trees remained unburned, as did understory vegetation. Whereas such "gray-stage" stands have been found to reduce fire severity in other regions and forest types, the observed reduction of surface fire was unexpected. In that fires in the subalpine WBP zone and bark beetle-fire interactions have not been studied in California, these preliminary observations suggest that subalpine forests are vulnerable to fire where continuous fuels exist below them. However, in xeric, Mediterranean landscapes, MPB outbreaks in WBP may reduce subsequent burn severity in unexpected ways. These findings may be important given current concerns for WBP's rangewide vulnerability.

New records of Douglas's squirrel (Tamiasciurus douglasii) document the species in 2 mountain ranges of the Great Basin where tree squirrels have not been previously reported, including 2 sites in the White Mountains, California, and 1 site in the Desert Creek Mountains, Nevada. In the White Mountains, squirrels were photographed by a camera trap on 9 different dates in winter and spring 2016-2017 at a site on the east side of the range crest near the Crooked Creek Field Station of the White Mountain Research Center (3125 m). In early winter 2018, two Douglas's squirrels were observed and photographed 25 km distant and 716 m lower in Leidy Canyon (2409 m) on the lower east side of the White Mountains, 3.5 km west of the Nevada state line. In the Desert Creek Mountains, Douglas's squirrels were observed on 4 days in autumn 2017 and winter 2018 along Desert Creek from 2005 m to 2307 m. We present evidence to suggest that at least the White Mountains records represent recent colonization(s) and, as such, call into consideration the question of how montane mammals are able to migrate into isolated mountain ranges of the Great Basin during warm Holocene climates.

We present the first comprehensive inventory and analysis of rock glaciers in the hydrographic Great Basin (GB), United States, documenting 842 features (mean 9.9 ha; range 0.1–201 ha) across thirty-two mountain ranges. These encompassed 8° latitude (from 36.5°N to 44.3°N) and 11° longitude (from −110.7°W to −121.4°W), and composed 83.1 km2, or 1.1 percent, of the cumulative area above the lowest rock glacier elevation. Forty-five percent of the features were mapped as intact (containing ice) and occurred across sixteen mountain ranges. Rock glaciers conservatively contained cumulative water volume of 0.8924 km3. We also mapped 237 persistent ice fields from thirteen GB ranges. Ice-field water equivalent was 0.0653 km3; rock glaciers contributed 93 percent of the total water volume (rock glacier:icefield ratio, 14:1). Rock glaciers occurred on northerly aspects at high elevations (mean, 3,196 m) and had a mean annual air temperature of 1.7°C (range, 1.3–3.3°C). Contributions of water from rock-glacier springs and groundwater have not been included in GB hydrologic assessments, nor have rock glaciers been evaluated for their roles in supporting cold-adapted aquatic fauna and promoting vegetation communities and habitat for alpine terrestrial species. Rock glaciers provide hydrologic and ecologic refugia previously unrecognized in the GB with respect to warming future climates.

Climate refugia are locations where decoupled climate processes enable species to persist despite unfavorable climate changes in surrounding landscapes. Despite theoretic bases and paleo-ecological evidence, refugia have not been widely characterized under modern conditions in mountain regions. Conifers in the Great Basin, USA, provide an opportunity to evaluate the potential of low-elevation ravine and riparian (LERR) contexts to function as climate refugia. We provide evidence for significantly higher than expected occurrence of limber pine (Pinus flexilis E. James) in LERR contexts (mean 64%) across 43 mountain ranges. We document with observed and modeled data that LERR contexts are cooler and wetter than expected for their elevations, have low solar radiation, and produce larger (more positive) lapse rates relative to upland slopes. Together these findings suggest that LERR contexts generate decoupled microclimates that provide climate refugia for limber pine. In that refugia management has been promoted as a contemporary climate adaptation strategy, our findings suggest that LERR contexts be further evaluated for their conservation potential.