**2. Methods**

We accessed the United States Fish and Wildlife Service's (USFWS) website to obtain the most up to date list of plant species identified as threatened or endangered under the ESA [31]. This list was reduced to only those species occurring in the Mojave Desert, as defined by the EPA Level III Ecoregions [32]. We overlaid spatial data of plant species distribution with a boundary of the selected ecoregion? (Mojave Basin and Range) to verify species occurrence, which resulted in 24 plant species in the Mojave Desert listed as threatened or endangered (**Figure 2**).

We used NatureServe's Climate Change Vulnerability Index (https://www. natureserve.org/conservation-tools/climate-change-vulnerability-index, last accessed 19 Nov 2020) to investigate the susceptibility of Mojave Desert listed plants to climate change (CCVI) [33]. This index incorporated future climate projections, along with data for each species related to natural history, current distribution, and ecological associations, to predict range contraction and/or population extirpations. The CCVI used a total of 24 factors in several categories

#### **Figure 2.**

*Plant species listed as threatened or endangered under the United States Endangered Species Act in the Mojave Desert.*

Extremely Vulnerable (EV): Abundance and/or range extent within geographical area assessed extremely likely to substantially decrease or disappear by 2050.

Highly Vulnerable (HV): Abundance and/or range extent within geographical area assessed likely to decrease significantly by 2050.

Moderately Vulnerable (MV): Abundance and/or range extent within geographical area assessed likely to decrease by 2050.

Less Vulnerable (LV): Available evidence does not suggest that abundance and/or range extent within the geographical area assessed will change (increase/decrease) substantially by 2050. Actual range boundaries may change.

Insufficient Evidence (IE): Information about a species' vulnerability is inadequate to calculate an index score.

#### **Table 2.**

*Climate Change Vulnerability Index Score Descriptions [33].*

to assess the three major elements of vulnerability; exposure to climate change (direct and indirect), sensitivity, and species-specific adaptive capacity. We reviewed information about the species and entered a score for each factor according to guidance and criteria set by the CCVI [33]. Direct climate exposure was calculated by estimating projected temperature and moisture change within the assessment area, while indirect exposure was assessed by evaluating future distribution relative to barriers that may restrict a species ability to shift its' range. Mid-century (2040–2069) climate projections were used based on an ensembleaverage of general circulation models and a medium emissions scenario [33]. For sensitivity and adaptive capacity, we evaluated a variety of factors including dispersal capability, reliance on specific thermal or hydrological conditions, dependence on disturbance regime or snow/ice cover, restriction to uncommon habitat types or landscape features, reliance on interspecific interactions, and genetic variation. For each sensitivity factor, we assigned a score of decrease, somewhat decrease, neutral, somewhat increase, increase, or greatly increase vulnerability to climate change. A score of unknown was given when information was lacking for a particular factor. Although some sensitivity factors were optional, the CCVI required a minimum number of factors (10) in order to avoid a determination of insufficient evidence [33].

Each species was assigned one of the following five categories based on CCVI scores: extremely vulnerable, highly vulnerable, moderately vulnerable, less vulnerable, and insufficient evidence (**Table 2**). The CCVI also calculated a categorical confidence estimate (very low, low, high, very high) for each species ranking, which was based on certainty in the factor values as represented by the frequency of multiple categories of vulnerability being selected for a given factor.

#### **3. Results**

The majority of the species assessed (N = 21, 88%) were found to be moderately, highly, or extremely vulnerable to climate change (**Figure 3**). Five species (21%) were extremely vulnerable to climate change, six species (25%) were highly vulnerable, and ten species (42%) were moderately vulnerable to climate change (**Figure 3**). Only three species were determined to be less vulnerable to climate change and no species received a score of insufficient evidence. The confidence

#### **Figure 3.**

*Proportion of Mojave Desert listed plants categorized by the following climate change vulnerability indices: EV = extremely vulnerable, HV = highly vulnerable, MV = moderately vulnerable, LS = less vulnerable. N is the number of species within each category, out of 24 species total.*

estimate for each species score was very high, indicating a large degree of certainty in the vulnerability ranking.

Among key factors, limited dispersal capability increased climate change vulnerability for almost every species assessed (N = 23), with the one exception being the only grass species, Eureka Dune Grass (*Swallenia alexandrae*), which is wind pollinated (**Table 3**). Likewise, natural or anthropogenic barriers that may impede range shifts, increased or somewhat increased climate change vulnerability for the majority of plant species assessed (N = 22, **Table 3**). Physical habitat, or restriction to uncommon geological formations or substrates, also emerged as one of the most important factors in our assessment, as it somewhat increased or increased vulnerability for most species (N = 22). The potential for climate change mitigation projects (e.g., alternative energy facilities) also somewhat increased vulnerability for many species (N = 15, **Table 3**), which is not surprising given that these desert plants inhabit areas often identified as prime locations for solar energy projects. Additionally, physiological hydrological niche was a key factor affecting vulnerability ranking in many species (N = 9), with species reliant on a particular hydrologic regime (e.g., desert springs) being assessed as more vulnerable than species not dependent on these habitats. Competition from other native or non-native species favored by climate change somewhat increased vulnerability for roughly half of the species assessed (N = 11). Genetic variation appeared to be a less important factor (N = 1), similar to reproductive system which somewhat increased vulnerability in only a few species (N = 6, **Table 3**).

The five species classified as extremely vulnerable occurred in the same geographic area (southwest Utah and northwest Arizona, **Figure 2**). Among species classified as highly vulnerable (N = 6), five of these were located primarily in a protected area at Ash Meadows National Wildlife Refuge in Nevada (**Table 3**). Species classified as less vulnerable (N = 3) also included one that inhabited a protected area (**Table 3**). Roughly 38% of the species assessed (N = 9) were located exclusively or primarily on lands under protection status, with seven species endemic to Ash Meadows National Wildlife Refuge in Nevada, one species occurring in Death Valley National Park in California, and one species in Zion National Park in Utah. In addition to climate change, many plant species were also found to be threatened by other anthropogenic factors such as agriculture, grazing, groundwater pumping, invasive species, mining, recreational off-highway vehicle (OHV) use and activities, and urban development (**Table 3**).





*Key factors contributing to vulnerability for each species. Factors related to indirect climate exposure include natural and anthropogenic barriers that restrict species movement in response to changing climatic conditions, and the degree to which the species may be affected by climate change mitigation actions (ie, placement of solar arrays in desert plant habitat). All other listed key factors relate to species sensitivity and adaptive capacity. Dispersal refers to the ability to move through unsuitable habitat, while physiological hydrological niche pertains to plant species dependent upon a narrowly defined water source (e.g., desert springs). Physical habitat refers to a species dependence upon a particular uncommon landscape or geological feature (e.g., gypsiferous soils). Pollinators relates to pollination strategy (e.g., dependence on only one pollinator may increase vulnerability), while competition identifies species that may be outcompeted by another species (native or non-native) favored by climate change. Reproductive system serves as a measure of genetic diversity in plants where genetic information is lacking, and genetic variation serves as a proxy for a species capacity to adapt to novel conditions. Protection area status is indicated by whether or not a species occurs exclusively or primarily in a protected area (e.g., wildlife refuge, park, area of environmental concern). Non-climate related anthropogenic stressors that are negatively affecting plant populations are also displayed.*

#### **Table 3.**

*Climate change vulnerability index score for threatened and endangered plants in the Mojave Desert (EV = extremely vulnerable, HV=highly vulnerable, MV = moderately vulnerable, LV = less vulnerable).*
