Toward climate change refugia conservation at an ecoregion scale

Abstract

Climate change uncertainty poses serious challenges to conservation efforts. One emerging conservation strategy is to identify and conserve climate change refugia: areas relatively buffered from contemporary climate change that enable persistence of valued resources. This management paradigm may be pursued at broad scales by leveraging existing resources and placing them into a tangible framework to stimulate further collaboration that fosters management decision-making. Here, we describe a framework for moving toward operationalizing climate change refugia conservation at an ecoregion scale with an analysis for the Sierra Nevada ecoregion (CA, USA). Structured within the Climate Change Refugia Conservation Cycle, we identify a preliminary suite of conservation priorities for the ecoregion, and demonstrate how existing mapping, data, and applications could be used for identifying, prioritizing, managing, and monitoring refugia. We focus on six stakeholder-identified conservation priorities, including two process-based refugial priorities (snow and fire), and four ecosystem-based refugial priorities (meadows, giant sequoia, old growth forests, and alpine communities). This pilot overview of concepts and resources provides a foundation for both near-term implementation and further discussion in moving from science to conservation practice. Such an approach may provide new practical insights for ecosystem management at ecoregion scales in the face of climate change.

1 INTRODUCTION

Anthropogenic climate change poses severe threats to biodiversity and ecosystem function. Climate change adaptation aims to reduce vulnerabilities to these threats. One adaptation strategy is to conserve climate change refugia, defined as “areas relatively buffered from contemporary climate change over time that enable persistence of valued physical, ecological, and socio-cultural resources” (Morelli et al., 2016). More resistant to climate change than surrounding areas, climate change refugia can be further supported and conserved in the short term by implementing best management practices to avoid degradation by immediate, nonclimate-specific threats such as land use practices or invasive species. Knowledge of refugial status could motivate managers to direct extensive management or restoration efforts toward climate change refugia, where the return on investment is expected to last longer. Climate change refugia can be considered “slow lanes”—areas where more gradual change shields species and other valued resources from negative consequences of climate change (Morelli et al., 2020). Realistically, climate change refugia conservation represents a short- to medium-term management strategy on the scale of decades to a century, and many areas identified as refugia now may not remain so in perpetuity (Brown, Wigley, Otto-Bliesner, Rahbek, & Fordham, 2020, Hylander, Ehrlén, Luoto, & Meineri, 2015, Morelli et al., 2016). In the long term, managed refugial areas at various scales could buffer range shifting species with higher but still limited climate tolerance, or might act as “evolutionary cradles” that not only facilitate species persistence, but potentially generate future biodiversity hotspots (Murphy, Guzik, Cooper, & Austin, 2015), serving as havens from negative climate impacts to biodiversity and ecosystem function (Morelli et al., 2020).

Recent focus on operationalizing climate change refugia conservation has called for synthesis of regionally-relevant science and on-the-ground applications; the Sierra Nevada ecoregion (CA, USA; Jepson Flora Project, 2020; Figure 1) provides a compelling case study for such a synthesis. This area covers a vast elevation range that includes the highest peak in the contiguous United States (Mt. Whitney) and the largest alpine lake in North America (Lake Tahoe). It boasts an abundance of endemic flora and fauna, ecological processes, and habitats, including foothill woodlands, conifer forests, alpine tundra, wetlands, meadows, lakes, and rivers (Millar, 1996, NPS, 2019). It also has significant economic and health implications for the millions of Californians who receive their drinking water from its reservoirs. With over 60% of the ecoregion in public holdings (Millar, 1996), implementing effective on-the-ground climate change refugia conservation approaches here is likely to entail substantial coordination among municipal, state, and federal government entities, tribal groups, private landowners, and nongovernmental organizations (NGOs).

FIGURE 1

Extent of Sierra Nevada Ecoregion, comprising the High Sierra and Sierra Nevada foothills subregions as defined by the Jepson Flora Project (Jepson Flora Project, 2020)

The Sierra Nevada ecoregion is already experiencing profound impacts from the intersection of climate change and past management. In the last decade it has experienced its most extreme regional drought event in over a thousand years (which triggered massive tree mortality; Goulden & Bales, 2019), in addition to the largest wildfires ever recorded in the Sierra Nevada. Climate change threats interacting with current land management practices, invasive species, and pest dynamics portend continued loss of iconic species and ecosystems (Bentz et al., 2010; Spencer et al., 2015; USFWS, 2014). Predicted changes to the Sierra Nevada climate—which include end-of-century temperature increases that will continue to shift hydrologic and fire processes in the region, with decreased winter snowpack, earlier snowmelt, reduced summer flows, upslope biome shifts, and more frequent and severe wildfires (Berg & Hall, 2017; Diffenbaugh, Swain, & Touma, 2015, Gonzalez, 2016; Reich et al., 2018; Westerling, 2016)—illustrate the urgency of climate adaptation planning in this ecoregion.

Nevertheless, the Sierra Nevada ecoregion also offers hallmarks of climate change refugia, such as steep elevational gradients and rugged topography that foster cold-air pooling (Wilkin, Ackerly, & Stephens, 2016) and microclimate diversity (Ackerly et al., 2010). Furthermore, some of the most advanced climate change, snowpack, and vegetative modeling, as well as extensive ecological studies, have occurred in California, including in the Sierra Nevada. Finally, consistent with the focus of climate change refugia conservation on “valued resources” (Morelli et al., 2016; Wallace, 2012), the Sierra Nevada holds an exceptional degree of value to Indigenous peoples, scientists, resource managers, local economies, and the general public.

Although refugia conservation approaches may have utility for practitioners both in the Sierra Nevada and elsewhere, major challenges remain. Taking refugia conservation from theory to implementation is demanding due to the challenge of precisely defining conservation goals amid uncertainty in a changing climate, assembling relevant science, tools, and applications, and placing those resources within a clear framework for practical discussion and subsequent decision-making. These obstacles compound at regional scales, where diversity in land tenure and organizational missions affects values and land management goals, as well as project implementation capacity. Because the lens through which conservation is considered and conducted varies, resulting land management objectives may be complementary or in conflict, but the capacity to develop and consider refugia conservation actions offers resource managers the opportunity to pursue tangible decisions around climate change adaptation. Many agencies and organizations have missions to preserve valued natural resources for present and future generations. Climate change refugia conservation may help achieve objectives by promoting the persistence of endangered and rare species, ecologically important vegetation habitats (Thorne et al., 2020), or forest structure and function amid disturbance (Meigs, Dunn, Parks, & Krawchuk, 2020), for example. However, without a concrete starting point for climate change refugia conservation, scientists, and practitioners may lack the foundation to move toward climate change adaptation planning.

2 BACKGROUND AND CONCEPTUAL APPROACH

To address these challenges, our primary goal was to assemble existing science for pursuing a climate change refugia conservation paradigm in the Sierra Nevada. The work detailed herein was catalyzed by a Climate Change Refugia Conservation Workshop held at Yosemite National Park in 2019 through a collaboration among the National Park Service (NPS), other regional stakeholders and researchers, and the Northeast and Southwest Climate Adaptation Science Centers (CASCs; see climaterefugia.org/sierra-nevada). Workshop organizer TL Morelli sought to bring together regional natural resource managers and scientists as part of a broader ongoing effort to integrate climate change refugia into existing management and climate change adaptation planning. Similar workshop efforts have been conducted regionally in the northeastern, northwestern, and southwestern United States (see climaterefugia.org).

In pursuit of translating conservation science into practice, the climate change refugia conservation paradigm builds on the approaches of translational ecology (Enquist et al., 2017) and knowledge co-production (Wall, Meadow, & Horganic, 2016) to connect refugia research with management priorities. A total of 29 Sierra Nevada scientists or practitioners attended the Yosemite workshop (19 from federal agencies, 4 from academia, 4 from NGOs, and 2 from California state agencies), which began with presentations from scientists and resource managers relevant to climate change adaptation in the Sierra Nevada, including topics such as water balance modeling, meadow mapping, vegetation refugia planning, and climate change impacts on Sierran amphibians and mammals. Participants identified a preliminary short list of priority focal resources, and then split into small groups to further define the priority resources, discuss how to apply climate change refugia results to ongoing or future management actions, and brainstorm relevant data and partnerships.

Subsequently, coauthors on this work produced an analysis of existing ecological data resources and management frameworks that might be leveraged for climate change refugia conservation in this ecoregion. To forge a pathway toward Sierra Nevada refugia conservation, and illustrate an example of tangible refugia conservation planning more broadly, we sought to assess the climate change vulnerabilities of priority resources, and gather refugia-relevant data, mapping, tools, and applications that can be leveraged for identifying, prioritizing, managing, and monitoring refugia, framed within an abbreviated version of the Climate Change Refugia Conservation Cycle (CCRCC) (Morelli et al., 2016). The set of refugia priorities and resources outlined herein has been informed and constrained by the backgrounds of coauthors and workshop participants, and should not be considered comprehensive of the various conservation values and management objectives held by different land managers across the ecoregion.

The priority resources identified at the workshop fit broadly into two categories. First, Process-based Refugial Priorities, such as snow and fire, focus on ecosystem processes that support or promote the persistence of valued ecosystems, habitats, or species. Second, Ecosystem-based Refugial Priorities focus directly on the valued ecosystems, habitats, or species themselves, and include meadows, giant sequoia, old growth forests (Appendix Refugia S1), and alpine communities (Appendix Refugia S2; Figure 2). Although each priority is reviewed and discussed separately below or in the Supporting Information (see Appendix), there is often clear overlap between process- and ecosystem-based priorities (e.g., meadows overlap with snow and hydrologic processes). Some priority sections illustrate sub-priorities in greater detail, demonstrating that sub-priorities—like Pacific fisher (Pekania pennanti; Appendix Inset S1) in old growth forests, or whitebark pine (Pinus albicaulis; Appendix Inset S2) in alpine communities—may have their own refugia conservation cycles.

FIGURE 2

Overview of preliminary Sierra Nevada Climate Change Refugia priorities identified at the 2019 refugia conservation workshop. Process-based priorities are denoted with white text and include Snow and Fire. Ecosystem-based priorities are denoted with black text and include Meadows, Giant Sequoia, Old Growth Forests, and Alpine Communities

We organized this analysis into the following five sections for each priority resource:

1. Define Planning Scope defines and describes the priority resource.
2. Assess Climate Change Vulnerability describes the exposure, sensitivity, and adaptive capacity of the priority resource to climate change.
3. Identify and Validate Refugia describes mapping and modeling efforts relevant to delineating refugia for the priority resource.
4. Prioritize Refugial Areas and Implement Management Actions identifies resources for evaluating which identified refugial areas should be prioritized for management, as well as management actions to support the priority resource.
5. Monitor Effectiveness of Refugia describes existing monitoring efforts that may be leveraged to assess whether refugia are functioning as desired.

Figure 3 uses one priority resource, Sierra Nevada meadows, to illustrate this organization style within the context of the Climate Change Refugia Conservation Cycle. Although the Sierra Nevada Climate Change Refugia Workshop touched upon these steps, this article represents the first attempt to formally arrange existing resources within the lens of the Climate Change Refugia Conservation Cycle. Inspired by the Climate-Smart Conservation Cycle of Stein, Glick, Edelson, and Staudt (2014), framing climate change refugia conservation as a cycle rather than as a linear set of steps allows the planning scope, climate change vulnerabilities, refugia identification, and refugia effectiveness to be iterated upon and defined accurately through time as the climate changes, increasing the capacity for adaptive and agile management amid uncertainty. For example, the process of identifying and validating refugia, and/or a season of refugial monitoring, may clarify which specific monitoring metrics should be used to measure refugia effectiveness, and in turn inform a more precise definition of a priority resource and the associated planning scope.

FIGURE 3

The Climate Change Refugia Conservation Cycle (Morelli et al., 2016) provides a framework for approaching climate change adaptation. This work uses a condensed formulation of the cycle to provide a preliminary roadmap toward refugia conservation in the Sierra Nevada ecoregion. Here, the Meadows Priority resource illustrates the organization of the article. For each priority resource, sections are organized into 1. Define Planning Scope (e.g., protect refugial meadows), 2. Assess Climate Change Vulnerability (e.g., meadows are vulnerable to climate-driven hydrologic changes, exacerbated by livestock grazing and trampling), 3. Identify and Validate Refugia (e.g., mapping and modeling efforts relevant to the priority resource, such as Sierra Nevada meadow refugia mapping by Maher et al. (2017)), 4. Prioritize Refugial Areas and Implement Management Actions (e.g., restoration efforts at Halstead Meadow at Sequoia and Kings Canyon National Parks provide an example of management action that can be taken for refugial meadows), and 5. Monitor Effectiveness of Refugia (e.g., a vegetation monitoring plot at Soda Springs Meadow in Devils Postpile National Monument provides an example of existing monitoring practices that might help assess whether meadow refugia are being conserved, based on defined metrics such as grass height and plant community composition). Photo credits: 1. National Park Service/B.Blackburn; 2, 4, 5: National Park Service; 3. Maher et al. (2017)

Table 1 summarizes the resources identified as priorities for Sierra Nevada climate change refugia conservation. It maps to steps in Figure 3 to provide a summary of priority resources expected to persist within refugia (Figure 3.1: Define Planning Scope), their projected climate change vulnerabilities (Figure 3.2: Assess Climate Change Vulnerability), examples of physical or ecological features expected to buffer refugia from climate change (Figure 3.3: Identify and Validate Refugia), and examples of monitoring metrics that might be used as indicators of refugial effectiveness (Figure 3.5: Monitor Refugia Effectiveness). Meanwhile, Table 2 is a compilation of region-specific resources to support identification and validation of refugia in the Sierra Nevada.

TABLE 1. Summary of priority resources expected to persist within refugia, their projected climate change vulnerabilities, features expected to buffer refugia from climate change, and monitoring indicators of refugial effectiveness

Priority resource	Refugial resource(s) expected to persist within the refugium	Projected climate change vulnerabilities	Example features that may buffer refugia from climate change	Example monitoring indicators of refugial effectiveness
Snow	Seasonal snow cover and persistence patterns resembling historical snowpack regime prior to modern-day anthropogenic climate change	Precipitation falling as rain instead of snow Decreased snow water equivalent Decreased snow depth at lower elevations Earlier snowmelt	Topographic shading Cold-air pooling Higher elevations North-facing slopes Alliance with persistent glaciers	Selected snow cover metrics, such as snow water equivalent, April 1st snowpack, and normalized difference snow index (NDSI)
Fire	Fire frequency and severity patterns resembling historical fire regime prior to modern-day anthropogenic climate change	More frequent and severe fire Changing vegetation dynamics Drought	Riparian zones Cold-air pooling Low productivity soils (e.g., serpentine)	Metrics assessing fire severity and frequency; persistence or timely return of target species or habitats postfire
Meadows	Sierran meadows with finely textured surficial soils, shallow groundwater, and dominance of herbaceous vegetation, supporting wildlife such as sensitive amphibians, willow flycatcher, great gray owl, and California golden trout	Drought Decreased snowpack Earlier snowmelt and shift in runoff timing to earlier in year Slow recovery from disturbance	Higher subsurface water storage capacity Low perimeter to area ratio Higher soil available water capacity Higher watershed forest cover Precipitation historically occurring as rain rather than snow	Meadow quality metrics, such as end-of-growing-season grass height, percent willow cover, plant species composition, occupancy/abundance models for focal meadow wildlife
Giant Sequoia	Giant Sequoia	Drought Decreased snowpack More frequent and severe fire Bark beetle attacks Low genetic diversity	Existing sequoia grove locations Elevation at projected rain-snow transition Positive site water balance metrics	Metrics for giant sequoia grove health, such as densities and condition across age classes, and moisture stress indicators such as foliage dieback
Old growth forests	Forest characteristics: many large and old trees (150–200 years), complex canopy and overstory structure Tree species: ponderosa pine, Jeffrey pine, sugar pine, red fir, white fire, California black oak, Douglas fir Wildlife species: California spotted owl, southern Sierra Nevada Pacific fisher	Drought Pests and pathogens More frequent and severe fire	Areas predicted to have less stressful climate conditions (e.g., Thorne et al., 2018, 2020) Presence of fire-adapted traits High number of seed dispersal modes Seed longevity	Metrics for old growth forest quality, such as species composition, health, mortality, status, and trend
Alpine communities	Bighorn sheep American Pika Sierra Nevada Red Fox Rosy-finches Sensitive amphibians Endemic plants Whitebark pine	Increased temperatures Decreased snowpack Earlier snowmelt Upslope shifts of lower elevation communities	Landscape resiliency features based on land facets and topoclimate diversity (e.g., Buttrick et al., 2015) Rock glaciers and related cold rocky landforms (e.g., Brighenti et al., 2021)	Demographic indicators (occupancy, abundance, productivity, survivorship) for focal alpine wildlife species; forest health indicators for whitebark pine

TABLE 2. Compilation of resources to support identification and validation of refugia in the Sierra Nevada ecoregion

Data resource type	Description	References
Snow
For identifying refugia	Snowpack dynamics models, such as future snow residence time and snow water equivalent (SWE)[1–7], some of which are available through Cal-Adapt (https://cal-adapt.org/tools/snowpack/) [8,9] Models for cold-air pooling potential [10]	(1. Rice & Bales, 2013, 2. Luce, Lopez-Burgos, & Holden, 2014, 3. Thorne, Boynton, Flint, & Flint, 2015, 4. Gergel, Nijssen, Abatzoglou, Lettenmaier, & Stumbaugh, 2017, 5. Lute & Luce, 2017, 6. Roche, Bales, Rice, & Marks, 2018, 7. Sun et al., 2019, 8. Livneh et al., 2015, 9. Pierce, Kalansky, & Cayan, 2018, 10. Curtis et al., 2014)
For validating refugia	Snow telemetry network (SNOTEL) data [1] California Cooperative Snow Surveys [2] Moderate Resolution Imaging Spectroradiometer (MODIS) data for snow cover mapping (with model accuracy assessments in the Sierra Nevada, e.g., [3,4]) Remote sensing-based spatial distributions of SWE [5] Hydrological resources to assess/validate downstream function of snow refugia (e.g., California Basin Characterization Model (BCM, [6]) Climatic water balance data [7,8] National Park Service river monitoring data [9] Historical and projected streamflow metrics [10] Sierra Nevada Sentinel Streams Project [11] Data from downstream water users (municipal, agriculture) Normalized difference water/vegetation indices (NDWI, NDVI) [12]	(1. NRCS: https://www.wcc.nrcs.usda.gov/snow/, 2. CDWR, 2020, 3. Raleigh et al., 2013, 4. Rittger, Painter, & Dozier, 2013, 5. Rice, Bales, Painter, & Dozier, 2011, 6. Flint, Flint, Thorne, & Boynton, 2014, 7. Dobrowski et al., 2013, 8. Abatzoglou, Dobrowski, Parks, & Hegewisch, 2018, 9. Andrews, 2012, 10. Wenger, Luce, Hamlet, Isaak, & Neville, 2010, 11. Thorne et al., 2015, 12. Huntington et al., 2017: ClimateEngine)
Fire
For identifying refugia	Fire refugia identification models and modeling approaches [1–6] Future fire regime predictions under climate change [7,8] Fire return interval departure (FRID) [9]	(1. Safford & Harrison, 2008, 2. Krawchuk et al., 2016, 3. Wilkin et al., 2016, 4. Meddens et al., 2018, 5. Jeronimo et al., 2019, 6. Koontz, North, Werner, Fick, & Latimer, 2020, 7. Parks, Holsinger, Miller, & Parisien, 2018, 8. Westerling, 2018, 9. Safford & Van de Water 2014)
For validating refugia	Field study of identified refugia (e.g., [1–3]) Satellite imagery [4–6] Combination of field study and satellite imagery [7] Federal Wildland Fire Occurrence Data [8] Persistence of target species postfire (e.g., [9]) Postfire severity mapping (e.g., Rapid Assessment of Vegetation Condition after Wildfire, RAVG) [10]	(1. Van de Water & North 2011, 2. Berry, Lindenmayer, & Driscoll, 2015, 3. Blomdahl, Kolden, Meddens, & Lutz, 2019, 4. Kolden, Lutz, Key, Kane, & van Wagtendonk, 2012, 5. Kane et al., 2015, 6. Meigs & Krawchuk, 2018, 7. Meddens, Kolden, & Lutz, 2016, 8. USGS, 2019, 9. Schofield, Eyes, Siegel, & Stock, 2020, 10. Miller & Quayle, 2015: https://fsapps.nwcg.gov/ravg/)
Meadows
For identifying refugia	Meadow refugia maps [1] Meadow prioritization tools [2–4]	(1. Maher et al., 2017, 2. Vernon, 2019, 3. Albano et al., 2019, 4. Gross, McClure, Albano, & Estes, 2019)
For validating refugia	Cross-reference refugia against survey or genetic data for defined meadow species (e.g., [1–3]) Maps, assessments, projections, and reports available through the UC Davis Sierra Nevada Meadows Clearinghouse [4] and Sierra Meadows Partnership [5], such as meadow boundary mapping on public lands (e.g., [6,7].)	(1. Morelli et al., 2017, 2. Keene, Ernest, & Hull, 2011, 3. Wu, Loffland, Siegel, & Stermer, 2016, 4. https://meadows.ucdavis.edu/, 5. https://www.sierrameadows.org/, 6. Fryjoff-Hung & Viers, 2013, 7. Pyrooz et al., 2015)
Giant Sequoia
For identifying refugia	Current locations of sequoia groves [1,2] Remote sensing-based vulnerability maps [3] Satellite imagery-based maps [4] Projections of vegetation exposure under climate change [5]	(1. Rundel, 1972, 2. Willard, 1994, 3. Brodrick, Anderegg, & Asner, 2019, 4. Su et al., 2017; Thorne et al., 2017)
For validating refugia	Monitoring efforts such as Leaf to Landscape [1] Combinations of field and remote sensing data to measure drought stress [2] Conservation efforts such as Save the Redwoods League [3]	(1. Ambrose et al., 2018, 2. Martin et al., 2018, 3. Burns, Campbell, & Cowan, 2018)
Old growth Forest
For identifying refugia	Forest refugia identification maps [1,2] Mapping and syntheses relevant to major forest stressors such as drought [3–6], fire [7], and pests [8]	(1. Thorne et al., 2018, 2. Thorne et al., 2020, 3. Flint et al., 2014, 4. Asner et al., 2016, 5. Byer & Jin, 2017, 6. Brodrick et al., 2019, 7. Safford & Van de Water, 2014, 8. Larvie, Moody, Axelson, Fettig, & Cafferata, 2019)
For validating refugia	Mapping efforts, range maps, and historic maps on individual tree species (e.g., whitebark pine [1], California black oak [2]). Forest Inventory and Analysis (FIA) data [3] Yosemite Forest Dynamics Plot [4] Sierra Nevada Forest Dynamics Plot Network [5] Cross-reference refugia against survey or genetic data for defined old growth forest species (e.g., Pacific fisher)	(1. Nesmith, Wright, Jules, & McKinney, 2019, 2. Gaman & Casey, 2002, 3. FIA: https://www.fia.fs.fed.us/, 4. Lutz et al., 2015, 5. Das, Stephenson, & Davis, 2016)
Alpine communities
For identifying refugia	Landscape resiliency data [1] Species-specific data (Bighorn sheep, [2], pika [3–5], rosy-finch [6], amphibians [7])	(1. Buttrick et al., 2015, 2. CDFW, 2020, 3. Millar & Westfall, 2010, 4. Stewart & Wright, 2012, 5. Stewart, Wright, & Heckman, 2017, 6. Siegel et al., 2014, 7. MYLF ITT, 2018)
For validating refugia	Existing population monitoring efforts (Bighorn sheep [1], rosy-finch [2], amphibians [3–6])	(1. CDFW, 2020, 2. Steel, Bond, Siegel, & Pyle, 2012, 3. Brown, Kiehl, & Wilkinson, 2012, 4. Brown & Olsen, 2013, 5. Fellers, Kleeman, Miller, Halstead, & Link, 2013, 6. Fellers, Kleeman, & Miller, 2015)

3 PROCESS-BASED REFUGIAL PRIORITIES

Although snow and fire are abiotic factors that drive species and habitat distributions across the Sierran landscape, snow refugia and fire refugia can also be spatially delineated on a map for management. Briefly, these can be considered as areas where snow cover dynamics or fire frequency/severity are less departed from recent snow or fire regimes prior to anthropogenic climate change.

3.1 Snow

3.1.1 Define planning scope

Snow refugia (Table 1) are areas predicted to sustain snow seasonally on the landscape in the face of climate change. Snow is a primary driver of hydrologic processes in the Sierra Nevada, and snowpack functions as California's largest natural water storage system. Snowmelt is slowly released into streams and stored as groundwater in meadows, offsetting the typically dry summer season. Drought in the ecoregion is triggered largely by deficient winter snowfall or inadequate snowpack (Edwards & Redmond, 2011). Runoff from snowmelt contributes up to 80% of annual streamflow (Rice et al., 2011), with alpine regions of the Sierra Nevada serving as water sources for downstream agriculture and municipal uses. Due to tight linkages with hydrological processes, snow refugia can overlap with and support other refugial priorities, such as old growth forest, the high water needs of giant sequoia (Sequoiadendron giganteum; Ambrose et al., 2016), and other critical ecosystems (e.g., montane meadows and alpine areas).

3.1.2 Assess climate change vulnerability

Snow droughts—periods with abnormally low snowpack— are increasing in the Sierra Nevada. In recent decades, climate change has produced a shift to more rain and less snow (Hatchett et al., 2017; Knowles, Dettinger, & Cayan, 2006; Safeeq et al., 2016), with decreases in snow water equivalent (e.g., Biondi & Meko, 2019; Moser, Franco, Pittiglio, Chou, & Cayan, 2009), April first snowpack (Moser et al., 2009), and snow depth at low elevations (e.g., Grundstein & Mote, 2010). Snow drought has been linked to extreme early season precipitation, frequent rain-on-snow events, and low precipitation (Hatchett & McEvoy, 2017). In the latter half of the 21st century, snowpack decreased at low-to-mid elevations, but increased at higher elevations (Howat & Tulaczyk, 2005). Additionally, whitebark pine (P. albicaulis) is invading formerly persistent snowfields (Millar, Westfall, Delany, King, & Graumlich, 2004); concomitant forest canopy snow interception diminishes snow accumulation (Helbig et al., 2020).

Warming temperatures are expected to shift snowmelt timing earlier by approximately 1 week per °C in the Sierra Nevada (Rice & Bales, 2013), with snowmelt occurring up to 50 days earlier by the end of the century (Reich et al., 2018). An increase of 2°C could decrease average spring snowpack by 30–50% (Mote, Hamlet, Clark, & Lettenmaier, 2005; Reich et al., 2018), with the loss of snow albedo feedback causing increased warming at mid-elevations (~1,500–2,400 m) (Reich et al., 2018). Climate change is predicted to increase the proportion of winter precipitation falling as rain, and lead to a shift in peak snow mass to earlier in the year, reduced water tables, reduced summer base flows, and drying of perennial streams (NPS, 2019; Reich et al., 2018; Theobald, Merritt, & Norman, 2010; Thorne et al., 2015).

Snow refugia may be less vulnerable due to features such as topographic shading and cold-air pooling, wherein cold, dense air collects in topographic depressions to reduce snowmelt (Curtis et al., 2014). Higher elevations (Howat & Tulaczyk, 2005) and snow drifts that are especially deep, on north-facing slopes, or are related to persistent glaciers may have a higher capacity to persist (Morelli et al., 2016).

3.1.3 Identify and validate Refugia

Quantifiable measures of features that reduce climate change exposure (e.g., high elevation, topographic shading, and north-facing slopes) can help in identifying snow refugia. Table 2 summarizes a sampling of relevant tools and research products for snow refugia identification, such as future snow residence time and snow water equivalent (Luce et al., 2014; Lute & Luce, 2017; Rice et al., 2011). In the validation stage, independent data are used to test predictions for desired ecological characteristics to ground-truth whether an identified refugium is functioning to conserve the valued resources (Barrows et al., 2020). Depending on the locations of identified snow refugia, validation efforts might use the automated, sensor-based snow telemetry (SNOTEL) network, which collects sub-daily data such as snow depth, precipitation accumulation, and snow water equivalent, with approximately 30 sensors located within the Sierra Nevada (https://www.wcc.nrcs.usda.gov/snow/). SNOTEL data are supplemented by the California Cooperative Snow Surveys (CCSS) program, which comprises an automated network of 130 snow sensors and manually-collected data from 265 snow courses (CDWR, 2020).

3.1.4 Prioritize refugial areas and implement management actions

If many areas are identified as snow refugia, it may not be possible to manage or protect all of them. In the broad ecoregion-scale perspective we outline here, process-based refugia might be prioritized for management action based on overlap with allied ecosystem-based refugia priorities. For example, snow refugia might be prioritized for management particularly if they overlap with areas identified as meadow refugia (Figure 4). After prioritization, conservation actions to protect snow refugia include strategic forest management to retain snow on the landscape (Lundquist, Dickerson-Lange, Lutz, & Cristea, 2013), such as creating canopy gaps that improve snow retention and fire resilience (Schneider, Affleck, & Larson, 2019). Reduction or modification of recreation or development activities that compromise snowpack may also help (Rixen & Rolando, 2013).

FIGURE 4

Illustration of possible intersecting climate change refugia priorities at Yosemite National Park. This example shows where potential snow refugia based on projected end-of-century April 1st snow water equivalent (SWE; Luce et al., 2014; Lute & Luce, 2017) do or do not overlap with meadows that rank highest by hydrological importance using the Sierra Meadows Prioritization Tool (Vernon, 2019). Snow data are only shown within the park boundary and do not indicate SWE outside the park

3.1.5 Monitor effectiveness of refugia

Monitoring efforts for snow refugia may be driven by an adaptive management approach that uses the normalized difference snow index (NDSI), SNOTEL, and CCSS sensor data, paired with sub-basin-specific scenario planning for temperature, precipitation, snowpack, snowmelt, and streamflow (Rice & Bales, 2013). National Park Service hydrology inventory and monitoring efforts and existing data (e.g., Andrews, 2012) can track the status of snow refugia by assessing downstream hydrologic function. Additionally, effectiveness may be assessed using monitoring methods directly relevant to the ecosystems with which snow refugia are closely linked—for example, by looking at normalized difference water and/or vegetation indices (NDWI and NDVI) available via ClimateEngine (Huntington et al., 2017), wherein a lack of decline in these metrics may provide further evidence of effective snow refugia.

3.2 Fire

3.2.1 Define planning scope

Fire is a natural ecosystem process in the Sierra Nevada that shapes the fates of several other priority resources outlined here (meadows, giant sequoia, and old growth forests). Fire protects meadows from forest encroachment, supports the establishment and regeneration of fire-dependent species like giant sequoia, and affects and maintains forest structure and composition (Agee, 1996; Sugihara, Van Wagtendonk, Fites-Kaufman, Shaffer, & Thode, 2006). Prescribed burns and natural and managed wildfires have long been used by Indigenous peoples to promote the persistence of valued species and ecosystems (Anderson & Rosenthal, 2015; Hankins, 2015; Kimmerer & Lake, 2001), and cultural burning remains in practice to manage ecological communities today (Long et al., 2016; Long, Goode, Gutteriez, Lackey, & Anderson, 2017). However, the region is recovering from over a century of Euro-American fire suppression concomitant with the exclusion of Indigenous cultural burning practices, which enabled excess vegetation growth, fuel accumulation (Stephens, Martin, & Clinton, 2007), and ecosystem homogenization (Koontz et al., 2020), all of which contributed to a departure from the Sierra Nevada fire regime that existed prior to modern-day anthropogenic climate change.

For a fire-adapted landscape like much of the Sierra Nevada, it is critical to note that fire refugia (Table 1) are not necessarily areas that never burn. Rather, fire refugia are areas not highly departed from the fire regime that existed prior to anthropogenic climate change in this ecoregion, which allow the persistence (or timely return postfire) of existing vegetative communities (like Sierra Nevada mixed conifer forests, meadows, giant sequoia, and old growth forests). Broadly, fire refugia are patches disturbed less severely or frequently by fire relative to the surrounding vegetation matrix (Krawchuk et al., 2016; Meddens et al., 2018), thereby preserving habitat persistence and connectivity for plants and wildlife (Hylander & Johnson, 2010; Robinson et al., 2013). Areas less frequently or severely disturbed by fire compared to surrounding areas under climate change can act as legacies, conserving variation in structure, function, and genetics to promote postfire recovery (Krawchuk et al., 2020).

3.2.2 Assess climate change vulnerability

The Sierra Nevada is already experiencing more stand-replacing fires and higher proportions of area burned at high severity due to interactions between past land management and climate change (Miller & Safford, 2012; Miller, Safford, Crimmins, & Thode, 2009; Steel, Koontz, & Safford, 2018; Westerling, 2018; Westerling, Hidalgo, Cayan, & Swetnam, 2006). Most recently, the 2020 Creek Fire burned over 1,537 km²—the largest known single source wildfire ever recorded in the Sierra Nevada (CalFire, 2020). Wildfires have been occurring at higher elevations in the Sierra Nevada over the past century (Schwartz et al., 2015). As disturbance regimes shift due to climate change and land use practices, fire refugia are expected to decline in their capacity to support priority resources (Krawchuk et al., 2020). Changing vegetation dynamics—such as invasion by fire-prone species—may fundamentally alter fire regimes by shortening fire return intervals, and by carrying fire into sparse-vegetation environments that normally do not burn (Barrows et al., 2020). Additionally, under extreme heat and drought conditions, certain areas identified as fire refugia could ultimately burn more severely if they contain accumulated fuel loads (Kolden, Bleeker, Smith, Poulos, & Camp, 2017; Krawchuk et al., 2016; Safford & Harrison, 2008).

3.2.3 Identify and validate refugia

Fire refugia are typically defined in post hoc fire analyses as areas in which the predominant vegetation regime has persisted through multiple wildfires, forming as a consequence of topography, fuels, and weather, which control fire spread and intensity (Meddens et al., 2018). Riparian zones, wet meadows, topography, and soil characteristics can help predict fire refugia occurrence (Krawchuk et al., 2016), with refugia more likely to occur in valley bottoms, gullies, and local concavities, potentially due to cold-air pooling and soil moisture (Meddens et al., 2018; Wilkin et al., 2016). These standard definitions may not capture the diversity of fire refugia types in the fire-adapted Sierra Nevada, however, where refugia may also be identified on low productivity soils like serpentine (Safford & Harrison, 2008) or areas where exposed bedrock or boulders protect vegetation rooted therein (Hylander & Johnson, 2010; Koontz et al., 2020). In the fire-suppressed Sierra Nevada, areas with restored (or less departed) fire regimes may also act as a form of fire refugia through perpetuation of fine-scale heterogeneity (Koontz et al., 2020). Such areas are correlated with higher lightning strike densities in this ecoregion (Jeronimo et al., 2019).

One way to validate areas identified as fire refugia is to assess the persistence or (timely) return of target species postfire. For example, after the 2013 Rim Fire, spotted owls (Strix occidentalis) were recorded within certain areas of the fire perimeter at rates similar to those observed prefire, suggesting that forest characteristics remained consistent with owl habitat requirements (Schofield et al., 2020). See Table 2 for additional region-specific tools and research products to identify and validate fire refugia.

3.2.4 Prioritize refugial areas and implement management actions

Fire refugia might be prioritized for conservation particularly if these refugia overlap with ecosystem or species-based priorities such as meadows, giant sequoia, or old growth forests. Fire management decision-making and response planning frameworks (e.g., Thompson et al., 2016) may further help prioritize refugial areas for management. Management approaches may include collaborating with local Indigenous communities to restore cultural burning practices (Aldern & Goode, 2014). Conservation of fire refugia can involve managing other stressors, such as addressing competing vegetation during postfire reforestation (North et al., 2019) and preventing heavy visitor use. Climate-informed restoration (Ng et al., 2020; North et al., 2019) and the Climate-wise Reforestation Toolkit (Steel et al., 2020) likely offer management insights postfire. Fire refugia management decisions could fit into existing fire management frameworks, such as National Park Service resource stewardship strategies and U.S. Forest Service fire management plans in the region, wherein landscape-scale management approaches are combined with scenario planning, vulnerability assessments, and structured decision making (e.g., Nydick & Sydoriak, 2011).

3.2.5 Monitor effectiveness of refugia

Under the definition that fire refugia are areas less departed from the historical fire regime that allow existing vegetation communities to persist, postfire severity mapping tools, such as the Rapid Assessment of Vegetation Condition after Wildfire (RAVG), provide a direct means for evaluating severity and frequency of occurrence of fires in identified fire refugia (RAVG: https://fsapps.nwcg.gov/ravg/, Miller & Quayle, 2015). Beyond this, fire refugia effectiveness might be defined based on metrics relevant to the vegetation type(s) encompassed by the fire refugium area, whether it is a meadow, giant sequoia grove, old growth forest, some combination therein, or otherwise. For example, long-term data collection efforts tracking tree mortality factors, like the Yosemite Forest Dynamics Plot and the Sierra Nevada Forest Dynamics Plot Network (e.g., Das et al., 2016; Lutz, 2015), may be invoked to monitor the effectiveness of fire refugia for protecting forests. Evaluating persistence of target species within old growth forest postfire provides another means for monitoring refugia effectiveness (e.g., spotted owls: Schofield et al., 2020, Pacific fisher: Blomdahl et al., 2019). Monitoring that considers interactions between different process-based refugial priorities could also be fruitful: for example, drought increases wildfire activity (Abatzoglou & Williams, 2016), and severity (i.e., kills more trees; van Mantgem et al., 2013), so hydrology-focused monitoring, modeling, and mapping (e.g., Flint et al., 2014) may provide indirect indicators for fire refugia effectiveness when combined with fire data and/or stand structure and species composition data.

4 ECOSYSTEM-BASED REFUGIAL PRIORITIES

4.1 Meadows

4.1.1 Define planning scope

We define Sierran meadow ecosystems (Table 1) according to Viers et al. (2013), where meadows are characterized by finely textured surficial soils, the presence of shallow groundwater, and the dominance of herbaceous vegetation (trees or shrubs may occur densely in some meadow areas, but are not dominant). Meadows are a rare ecosystem in the Sierra Nevada, making up 2% of the ecoregion (Viers et al., 2013). Meadow ecosystems intersect strongly with hydrologic and snow processes, potentially acting as hydrologic refugia where water availability is greater than that of the surrounding landscape (McLaughlin et al., 2017). Meadows provide ecosystem services including groundwater recharge, surface water retention and runoff, carbon sequestration, water quality improvements, and late season base flow (Ankenbauer & Loheide, 2017; Drew et al., 2016; Loheide et al., 2009; Micheli & Kirchner, 2002; Norton et al., 2011). Identifying and protecting refugial meadows can improve the chances of conserving a wide diversity of flora and fauna, including the federally-threatened Yosemite toad (Anaxyrus canorus), Cascades frog (Rana cascadae), California state-endangered willow flycatcher (Empidonax traillii), California state-endangered great gray owl (Strix nebulosa), California golden trout (Oncorhynchus aguabonita), and a host of other birds, fish, and amphibians.

4.1.2 Assess climate change vulnerability

Current climate change-based threats to meadow persistence have been magnified by past land use practices. California has lost over 90% of its wetland area since 1780 (National Research Council, 1992), and a period of high intensity grazing and burning in the late 1800s produced long-lasting impacts on herbaceous vegetation in Sierra Nevada meadows (McKelvey & Johnston, 1992). Meadows have also been historically degraded or lost due to intensified livestock grazing during the gold rush, drainage for railway and road placement, home construction, introduced non-native species, surface and groundwater diversions, fire suppression, agricultural conversion, flooding under reservoirs, and recreation (Kattelmann & Embury, 1996).

Additionally, sensitive species composition dynamics and hydrologic structure make meadows highly vulnerable to climate change (Viers et al., 2013; Hauptfeld, Kershner, & Feifel, 2014). Meadows are formed and maintained by feedbacks between hydrologic processes, vegetation, and soils (Wolf, 2017). If these feedbacks are lost or degraded under climate change, new meadows are unlikely to form on timescales meaningful for the persistence of valued resources they support, and recovery from disturbance may be slow (Maher et al., 2017). Montane meadow ecosystems are sensitive to projected decreases in spring snowpack, particularly in areas with limited subsurface storage (Albano et al., 2019), where reduced water availability in summer and fall may enable transition toward upland vegetation communities (Drexler, Knifong, Tuil, Flint, & Flint, 2013). Meadows are at risk from encroachment by upland shrubs and trees due to past fire suppression, overgrazing, shifts to competitive interactions, and climate change, which interact to degrade hydrologic function (Darrouzet-Nardi, D'Antonio, & Dawson, 2006; Millar et al., 2004). Climate change impacts on hydrology are further exacerbated by livestock grazing and trampling, which affects channel morphology and soil and nutrient dynamics (Ostoja et al., 2014; Vernon, Campos, & Burnett, 2019). Meadow-dependent wildlife species are directly vulnerable due to loss of suitable habitat from hydrologic degradation, livestock grazing and trampling, and conifer encroachment (Brown, Hayes, Green, MacFarlane, & Lind, 2015; Green, Bombay, & Morrison, 2003; Kalinowski, Johnson, & Rich, 2014; USFWS, 2014).

Meadows with higher watershed subsurface water storage capacity are likely to be more resistant to climate change; likewise, meadows for which more precipitation historically occurs as rain rather than snow (i.e., not dependent on snowpack) may also be less vulnerable (Albano et al., 2019). Such features may serve as hallmarks of meadow refugia.

4.1.3 Identify and validate refugia

Sierra Nevada meadow refugia have been mapped using a species-agnostic approach (Maher et al., 2017). This work can be integrated with species-specific data to identify refugial meadows for prioritized wildlife—for example, using great gray owl or Yosemite toad landscape habitat suitability modeling in Yosemite (e.g., Keene et al., 2011; Liang, Grasso, Nelson-Paul, Vincent, & Lind, 2017; Liang & Stohlgren, 2011). To validate meadow refugia, independent data can be used to test whether an identified refugial meadow is functioning as anticipated. To assess whether a refugial meadow is supporting prioritized species, the refugium can be cross-referenced against species survey or genetic data (e.g., Belding's ground squirrel, Morelli et al., 2017). Validation of refugial meadow hydrologic function might be accomplished by using the Climate Engine meadow monitoring tool (Albano et al., 2019; Gross et al., 2019). See Table 2 for additional resources.

4.1.4 Prioritize refugial areas and implement management actions

Meadow management, restoration, and monitoring in the Sierra Nevada is a multi-agency collaboration (Drew et al., 2016). The Sierra Meadows Partnership (www.sierrameadows.org) aims to restore and protect 30,000 acres of meadow habitat by 2030 and to provide climate-informed guidance for restoration projects (Vernon et al., 2019). As such, this partnership may be leveraged to prioritize and manage meadow refugia to maximum effect.

Meadow refugia can be ranked for management based on existing prioritization and decision-making tools for Sierra Nevada meadows. The Sierra Meadow Prioritization Tool (Vernon, 2019) has been used to identify areas where refugial meadows overlap with priority species, ecosystem services, or other management goals. The Meadow Decision Support Framework (Albano et al., 2019; Gross et al., 2019) can evaluate climate change vulnerability and inform where conservation and restoration actions should be focused. For some management entities, the top priority management action in the face of climate change is to restore, support, and protect functional meadow hydrology. Managers can protect meadow refugia by removing livestock (Vernon et al., 2019), rerouting recreational trails that undermine meadow hydrological and ecological function (sensu Yosemite's Lyell Canyon, Yosemite Conservancy, 2020), restoring incised streams (Hammersmark, Rains, & Mount, 2008; Long, Lake, Goode, & Burnette, 2020), and raising the water table with beaver dam analogs or maintenance of beaver populations (Fair et al., 2018; Greenwood et al., 2018; Pollock et al., 2014) or via controlled burns that stave off encroaching vegetation (Aldern & Goode, 2014; Meddens et al., 2018). Additional actions may include restoration of willow stands (Green et al., 2003), and maintaining meadow grass at sufficient heights to support vole habitat for great gray owl (Kalinowski et al., 2014).

4.1.5 Monitor effectiveness of refugia

Meadow refugia managers can adopt adaptive management-driven monitoring plans, assessing metrics such as grass height at the end of the growing season, percent willow cover, plant species composition, bank stability, groundwater levels, floodplain inundation, focal bird species richness and abundance, and percent cover of bare soil (Vernon et al., 2019). The Climate Engine meadow monitoring tool can be used to assess meadow responses to climate change and refugia management actions in near-real time (Albano et al., 2019, Gross et al., 2019). Monitoring data about meadow ecosystem characteristics can then be combined with monitoring efforts for prioritized species. For example, great gray owls have previously been monitored in Yosemite under an occupancy monitoring design that may inform future monitoring (Keene et al., 2011; Wu et al., 2016).

4.2 Giant sequoia

4.2.1 Define planning scope

Giant sequoias (S. giganteum; Table 1) provide an example of how climate change refugia conservation may be focused on a single species. Giant sequoias can grow past 90 m in height, can live for over 3,000 years (Sillett et al., 2015), and largely only occur naturally on public land in the Sierra Nevada (Stephenson, 1999). Drought-tolerant compared with other tree species in the region (Nydick et al., 2018), giant sequoias depend on fire for reproduction (Harvey, Shellhammer, & Stecker, 1980; Swetnam, 1993). Decades of prescribed burning in Sequoia and Kings Canyon National Park have helped create sunny, open forest conditions that allow giant sequoia seedlings to establish in mineral-rich soils exposed by fire (Meddens et al., 2018), reducing fuel loads and creating conditions necessary to encourage giant sequoia reproduction (Harvey et al., 1980; Stephenson, 1999). Given their longevity and that the species has managed to persist through millennia of different climates, giant sequoia groves could themselves serve as an indicator of hydrologic or fire refugia more broadly (Nydick et al., 2018; Su et al., 2017).

4.2.2 Assess climate change vulnerability

Giant sequoias have low genetic diversity compared with other trees, potentially limiting their adaptive capacity in a changing climate (Dodd & DeSilva, 2016). Additionally, they require substantial amounts of water, taking in >2,000 kg day⁻¹ (Ambrose et al., 2016). Thus, drought and snowpack decline in the Sierra Nevada create management uncertainty for this species in coming decades. California's severe drought from 2012 to 2015—wherein sequoias fared better than other species—nevertheless revealed new insights about their drought vulnerability (Nydick et al., 2018; Su et al., 2017). Sequoias regulate water loss through leaf-level shedding, but under severe drought they make crown-level adjustments to maintain favorable water status (Ambrose et al., 2018). Though very few giant sequoias died in the 2012–2015 drought, they exhibited varying signs of drought stress: drought-induced foliage dieback was higher at lower elevations, in areas with low densities of adult sequoia, and on steep slopes, suggesting that variation in sequoia drought vulnerability is driven by site water balance metrics (Stephenson et al., 2018). Future severe fire and drought may interact to make giant sequoias more vulnerable to attacks by cedar bark beetle (Phloeosinus spp; Nydick et al., 2018; Stephenson et al., 2018).

4.2.3 Identify and validate refugia

Current locations of ~70 sequoia groves are well-established (Rundel, 1972; Willard, 1994). Climate change refugia mapping for giant sequoia could synthesize existing grove locations, vulnerability maps (Brodrick et al., 2019), satellite imagery-based maps (Su et al., 2017), modeled projections of vegetation exposure under climate change (Thorne et al., 2017), combined field and remote sensing approaches (Martin et al., 2018), and hydrologic refugia mapping or hydrology data (Flint et al., 2014) to identify areas where giant sequoias are likely to persist and be less sensitive to future severe drought. A refugia mapping approach may help assess whether current sequoia refugia areas will remain refugia under severe drought. For example, the existing distribution of giant sequoia groves occurs along a narrow elevation band that tracks the rain-snow transition (Nydick et al., 2018). Future climate projections might identify hydrologically appropriate areas that will track this rain-snow transition.

4.2.4 Prioritize refugial areas and implement management actions

Drought vulnerability maps can help guide management action (Nydick et al., 2018). Where climate projections and mapping show sequoia groves that are outside the future climate envelope they are predicted to need, managers are faced with tangible decisions about whether and where to prioritize conservation action. In addition to protecting existing stands where changes in climate are predicted to be less severe, sequoia might be experimentally planted in areas that have been predicted to be climatically suitable. In experimental planting areas, managers can use prescribed fire to facilitate natural regeneration and pest control (Anderson, 2006; Meddens et al., 2018), mechanical treatment to remove competition from smaller neighboring trees (York, Louen, & Thomson, 2015), and planting at low density or thinning early on in dense stands (York, O'Hara, & Battles, 2013). In areas more vulnerable to drought and beetle attacks, tree species diversity can be increased to enhance resistance to pests.

4.2.5 Monitoring effectiveness of refugia

The Leaf to Landscape project measured the physiological consequences of drought stress in individual giant sequoia trees (Ambrose et al., 2018), combining field and remote sensing data to measure drought stress (Martin et al., 2018), and using remotely-sensed data to generate maps of forest vulnerability to hot droughts (Brodrick et al., 2019). In addition, the conservation goals of the Save the Redwoods League may align with sequoia refugia planning and assessment to evaluate the status of giant sequoia forest ecosystem health (Burns et al., 2018). To assess refugia effectiveness, these efforts may be combined with other long-term forest monitoring data (e.g., FIA data, Lutz, 2015, Das et al., 2016).

5 DISCUSSION

This work has illuminated a path toward climate change refugia conservation at an ecoregion scale in the Sierra Nevada, demonstrating six priority resources arranged within the framework of the Climate Change Refugia Conservation Cycle (CCRCC). Because these refugia priorities have been both guided and limited by the perspectives and experiences of coauthors and workshop participants, this work should be considered an early roadmap toward climate change refugia conservation in the Sierra Nevada. Like any roadmap, to remain current and maintain utility, this one can be updated through time to refine or expand priorities, precisely define effective refugia characteristics and monitoring metrics for priorities, include additional perspectives, incorporate more diverse knowledge and new climate research, and adapt to shifting goals in a changing climate.

Refugial status is merely one indicator that may be incorporated into decision-making around choosing sites for management action, and it is important to contextualize this approach with respect to emerging climate change adaptation dialogue. Within the Resist, Accept, or Direct (RAD) framework (Thompson et al., 2021), rather than accepting ecological transformation or directing change toward a future ecological condition, a simplistic argument might be made that climate change refugia conservation offers a resistance approach that strives merely to preserve existing or historical ecological configurations. However, climate change terminology regarding resistance, resilience, and transformation can be vague (St-Laurent, Oakes, Cross, & Hagerman, 2021). If implementing a refugia conservation approach iteratively within the CCRCC, the planning scope definitions, expectations, and adaptation actions for priority resources may in fact vary along a continuum from resistance (e.g., protection of refugial areas) to resilience (e.g., meadow restoration via beaver re-introduction) to transformation (e.g., assisted range expansion; St-Laurent et al., 2021). Indeed, as sites change too much (e.g., warm, dry) to serve as refugia for current resources in centuries to come, refugial “slow lanes” can still buffer climate change for other resources or emerging ecological communities, including northward shifting species (Morelli et al., 2020).

Climate change refugia conservation is not immune to the semantic ambiguities that shape emerging climate adaptation discourse, but these apparent conceptual conundrums may invite the flexibility to remain agile in managing resources amid constraints and uncertainty. For example, the distinction between resources used to identify refugia and resources used to validate refugia is not always clear; a resource used to validate refugia might alternatively be used to identify refugia, but if the resource was used to identify refugia, it should not also be used as a validation source. Thus, the categorizations of identify/validate in Table 2 are often suggestions. Refugia identification and validation also links with the final step in the CCRCC, “Monitor Effectiveness of Refugia.” Metrics used to identify and validate refugia today may be the metrics by which to quantify refugial effectiveness in conserving the resource in the future—provided that the first step in the cycle, “Define Planning Scope,” adequately captures the gestalt of a management objective to begin with. Additionally, rather than binary styling as refugia or non-refugia, it may be useful—where possible—to conceptualize refugia as a gradient, varying along a continuum as climate and disturbance regimes change through time (Krawchuk et al., 2020). A gradient approach to refugia identification aligns well with the cyclical framework of the CCRCC, wherein refugia identification, management, and monitoring may be adapted amid uncertainty as knowledge is gained through time.

Indeed, placing Sierra Nevada refugia conservation efforts into adaptive management frameworks can help account for current limitations by reducing system uncertainty under climate change and providing clear triggers for management action (Williams, 2011). However, challenges remain. Though adaptive management is discussed in several priority management contexts (e.g., meadows: Vernon et al., 2019; Pacific fisher: Spencer et al., 2016, pika, whitebark pine: Kellermann, Rodhouse, Nesmith, & Chung-MacCoubrey, 2019), an integrated, dynamic, ecoregion-based approach to refugia conservation faces both hurdles and opportunities in the form of diverse land tenure and differences in land management mandates, objectives, and capacity. The USDA Forest Service holds the majority of land in the Sierra Nevada, with the Bureau of Land Management, National Park Service, and other federal, state, tribal, and municipal entities also managing substantial areas. Agency mandates and funding cycles often incentivize managers to confront immediate threats (e.g., fire, invasive species), making it harder to prioritize projects that support the decadal-scale management required for climate change adaptation planning, although there are efforts to shift this trend (e.g., 2012 Forest Service Planning Rule; 77 FR 21162).

Nevertheless, a large-scale, ecoregion approach to refugia conservation may make it easier to identify where priority resources overlap. A climate change refugia conservation perspective that iteratively considers climate vulnerabilities as well as interactions between priorities presents the opportunity for dynamic management of climate change refugia that act as “slow lanes” within the Sierra Nevada (Morelli et al., 2020). At broad scales across the ecoregion, process-based refugia (e.g., snow and fire) may overlap with and support all of the ecosystem-based refugia discussed here, and some refugia types may be co-located (e.g., Figure 4). For example, snow refugia may overlap directly with meadow or alpine community refugia, or fire refugia may be prioritized for management based on their capacity to conserve old growth forest refugia. In turn, management actions to protect certain refugia types may overlap to support other refugia types; for example, functional wet meadows can act like a sponge, absorbing snowmelt in groundwater and surface water sources, and releasing it slowly throughout the dry season (Hunt, Fair, & Odland, 2018), which may support snow refugia that drive Sierra Nevada hydrologic processes.

Conversely, some priority resources may directly conflict. In an integrated prioritization process, managers may facilitate transitions of one type of vegetative refugia community to another—such as allowing a meadow refugium to transition to forest if the meadow is not expected to persist without prohibitively intensive management. Not all refugial priorities will present straightforward alliances or conflicts, and temporal management scales are important to consider, as areas predicted to be refugial in mid-century may no longer function as refugia by the end of the century. For example, managers may grapple with fire refugia management decisions that either benefit or conflict with management decisions for old growth forest refugia, given the potential temporal conflicts and co-benefits of fire and old growth forest density.

Varying management objectives mean that management capacity and approach may be different within each management unit. For instance, barriers to using prescribed fire to meet management objectives, along with the motivations for using fire as a management tool, differ in national forests vs. national parks (Doane, O'Laughlin, Morgan, & Miller, 2006; Quinn-Davidson & Varner, 2012), and social, economic, and regulatory hurdles constrain forest management options (Schwartz, Thorne, Collins, & Stine, 2020). Meanwhile, cattle grazing is prohibited in meadows within the national parks, but allowed on national forest land in the Sierra Nevada (Multiple Use Sustained Yield Act, P.L. 104–333). Units designated as Wilderness Areas may have additional management prohibitions (Wilderness Act, P.L 88-577). This diversity in management motivation and capacity likely compounds on private lands. Approximately ⅓ of land in the Sierra Nevada is privately owned, particularly at lower elevations (Millar, 1996), and engagement with landowners through land trusts or conservation easements will be critical where identified refugia overlap with private lands. However, coordinated, regional planning can meet multiple objectives at large scales and promote greater resilience under climate change; existing cross-jurisdictional partnerships aim to address such challenges in the Sierra Nevada ecoregion (e.g., Tahoe-Central Sierra Initiative [TCSI]: Manley, Wilson, & Povak, 2020).

The climate change refugia conservation framework presented here provides one perspective on navigating climate change adaptation within the Sierra Nevada. This work does not currently include Indigenous perspectives. Herein, we used language like “priorities” and “resources,” but acknowledge that Indigenous-led discourse might favor terms such as “relatives” and “beings” (Long et al., 2020; Tribal Adaptation Menu Team, 2019), in recognition of animals, plants, and water as full citizens within the community (Aldern & Goode, 2014). The Sierra Nevada is considered sacred and within the ancestral homelands of many tribal communities, and for Indigenous people who have stewarded these lands since time immemorial, protecting these areas in the face of climate change may be considered a critical responsibility. Cultural practices, such as cultural burning to foster regeneration of food-providing plants like sourberry and oaks (Aldern & Goode, 2014), are not currently represented in the priorities detailed here, but could be a focus of Sierra Nevada climate change adaptation. For example, cultural burning practices in the Sierra Nevada serve to balance ecosystem health and may be synergistic with climate change refugia conservation (Aldern & Goode, 2014; Long et al., 2020; Tribal Adaptation Menu Team, 2019).

This work reviews a pathway toward climate change adaptation planning in a region currently facing tangible threats from climate change. Future climate change refugia conservation in the Sierra Nevada might incorporate regional landscape connectivity work (Buttrick et al., 2015; McRae et al., 2016), considerations for increased human-wildlife conflict and zoonotic disease (e.g., Hammond, Liebman, Payne, Pigage, & Padgett, 2020; MacDonald, McComb, O'Neill, Padgett, & Larsen, 2020), additional existing climate change refugia conservation approaches in the region (Buhler et al., 2019), and much more. Placing priorities and resources into an actionable framework provides ideas for near-term application, and can stimulate additional collaboration to meet the challenges of climate change adaptation in the Sierra Nevada ecoregion. To help address pressing challenges posed by climate change, the management paradigm of climate change refugia conservation can be similarly pursued in other ecoregions, wherein marshaling existing resources within a climate adaptation framework can help translate science into conservation practice. Although refugial status is just one of many priorities for choosing an area for conservation action, it can provide an extra layer of information by which to select a finite number of areas for management, given limited opportunities and resources for action. By creating a clear foundation within the CCRCC, efforts to assemble existing resources may help leverage refugia conservation in a landscape conservation framework to confront the pressing challenges posed by climate change.

ACKNOWLEDGMENTS

This research was supported by funding from the U.S. Geological Survey Southwest, Northeast, and National Climate Adaptation Science Centers (CASCs). Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Thank you to Serra Hoagland, Cathy Brown, our lab group at UMass, and especially to three reviewers for thoughtful comments and feedback that have improved this manuscript. Thank you to everyone who participated in the 2019 Southwest Climate Change Refugia Research Coalition meeting that laid the foundation and motivation for this work, and to Mitzi Thornley for helping make it happen.

CONFLICT OF INTEREST

The authors declare no conflicts of interest.

AUTHOR CONTRIBUTIONS

Toni Lyn Morelli, Cathleen Balantic: Conceived the project; Cathleen Balantic: Created the article figures; Cathleen Balantic, Toni Lyn Morelli: Wrote first draft of the article; Cathleen Balantic, Andrea Adams, Shana Gross, Rachel Mazur, Sarah Sawyer, Jody Tucker, Marian Vernon, Claudia Mengelt, Jennifer Morales, James H. Thorne, Timothy M. Brown, Nicole Athearn, Toni Lyn Morelli: Wrote subsequent article drafts and critically revised them for important intellectual content.

Open Research

DATA AVAILABILITY STATEMENT

No original data were collected for this article; all information is contained within the article or supporting information.