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Federal Register / Vol. 86, No. 166 / Tuesday, August 31, 2021 / Rules and Regulations populations extirpated due to drying of habitat to rebound over time as suitable habitat conditions would not be present.
Specifically, drying of habitat has been linked to decreased abundance and extirpation of populations in the Chiricahua, Dragoon, Empire, Santa Rita, and Rincon mountains, including a group of plants from the largest population at Juniper Flat. In three of these instances, extirpation was associated with the drying of habitat, which rendered it no longer suitable for the species.
6 Comment: Two commenters suggested that the moist canyons where Bartrams stonecrop have been found are associated with the attraction of the public and botanists to these locations, and that survey bias and poor detectability can result in the mischaracterization of Bartrams stonecrop habitat. Several commenters questioned the characterization of Bartrams stonecrops habitat needs with respect to moisture and proximity to water.
Response: The Service completed a robust SSA based on the best available scientific and commercial information.
Bartrams stonecrop is a species found in Madrean woodlands, and does not appear to be a riparian species dependent on shallow ground water.
The best available information on Bartrams stonecrop indicates the species occurs near water sources springs, seeps, or intermittent streams, which may provide humidity and create suitable microclimate conditions. The deep, narrow canyons and associated overstory species provide shade during a portion of the day and create a cooler temperature, and the vegetation litter promotes retention of soil moisture and contributes to the humid microenvironment. Of 56 extant Bartrams stonecrop subpopulations with microhabitat condition documented, 78.6 percent were found within 10 meters of an intermittent or perennial streambed, an additional 14.3
percent were found between 11 and 20
meters from an intermittent or perennial streambed, and 7.1 percent were located more than 20 meters from an intermittent or perennial streambed.
Researchers searched for plants at varying distances from streambeds, but note most plants were found nearer streambeds. In general, botanists visit many different habitat types in southern Arizona, and few Bartrams stonecrop individuals have been located outside of habitats with relatively humid microhabitat conditions, as described in the SSA report Service 2020a, pp. 18, 24.

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7 Comment: A commenter notes that other Bartrams stonecrop populations are being discovered, and at least one of the presumed extirpated populations Rincon North cannot really be determined to be gone.
Response: The discovery of 872
additional individuals and new groups of plants in Arizona and Mexico, as described in our response to Comment 4, above, represent substantial survey effort by multiple groups. The 2015
survey of the population referenced referred to as Happy Valley North in the SSA report did not locate any Bartrams stonecrop individuals.
However, in 2019, a single plant was located within this population. We have incorporated the updated information into the revised SSA report and describe the Happy Valley North population as extant. However, we note that one individual does not indicate a robust population and consider this population to be in very poor condition.
8 Comment: A commenter indicated we did not solicit information from Cecile Shohet, who conducted research on Bartrams stonecrop for a Masters of Science thesis.
Response: As required by the Act 16
U.S.C. 1533b1, we based the listing decision on the best available scientific and commercial information. We worked in partnership with numerous agencies and organizations to visit most of the known U.S. locations of Bartrams stonecrop occurrences at least once with some long-term monitoring initiated, as well as a portion of the Mexico populations. Although information from 19832010 is limited, we used the best available information to assess the species current and future conditions. The U.S. Forest Service, National Park Service, Service, industry surveyors, and other researchers gathering information on Bartrams stonecrop have increased survey efforts since 2013 in suitable habitat in Arizona and Mexico. At a minimum, recent surveys and research on Bartrams stonecrop have occurred each year from 2013 to 2020.
A solicitation for peer review of the SSA report was sent to Ms. Shohet on October 16, 2017, and no response was received. We solicited Ms. Shohets expert opinion on specific aspects of the SSA and have incorporated all information received following the publication of the December 6, 2019, proposed rule in the revised SSA.
9 Comment: Three commenters stated that there is little to no evidence that drying has contributed to the extirpation of Bartrams stonecrop populations.

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Response: Bartrams stonecrop occurs only in habitat near water sources with a relatively moist and humid microenvironment and occasionally occurs in lower abundance in habitat farther away from water. As such, we determined that the humid microhabitat conditions are a need for species viability ability to sustain populations in the wild over time. Changes to required habitat conditions, including drying, are expected to negatively affect Bartrams stonecrop populations and contribute to reductions in abundance and population extirpation. Bartrams stonecrop seedlings are particularly susceptible to desiccation, and resurveys have shown large losses in this size class.
Drying of habitat associated with population or group extirpations has been observed in the Carlink Canyon, Empire Mountains, and Mule Mountains. Extirpations occurring in drying habitat are unlikely to be recolonized since suitable conditions for Bartrams stonecrop are no longer present. When suitable habitat is lost and not restored, Bartrams stonecrop experiences an increased risk of extirpation and extinction.
10 Comment: The commenters stated that several morphological and metabolic characteristics of Bartrams stonecrop are not discussed in the proposed rule despite their important role in determining the habitat requirements of Bartrams stonecrop.
These characteristics include a thick waxy covering on the epidermis, Crassulacean acid metabolism that results in stomata only opening at night, a shallow root system, and succulent leaves massed together as a rosette.
Response: Crassulacean acid metabolism CAM plants minimize photorespiration and save water by separating the steps of carbon dioxide fixation and the Calvin cycle used to turn carbon dioxide into sugar in time, between day and night. Reducing photorespiration decreases wasted energy and decreases sugar synthesis.
Approximately 6 percent of flowering plants are known to use CAM. CAM
species vary widely in the efficacy and use of CAM, and many maintain the ability to conduct photosynthesis without reducing photorespiration during part of the day, part of the season, and/or part of their lifecycle. All or nearly all members of the nearly worldwide plant family Crassulaceae have the ability to perform CAM, and they occupy a range of microhabitats.
Most taxa grow in arid habitats such as rocks and rock fissures under otherwise more humid climatic conditions, or in mountain regions in moderately arid
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