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Federal Register / Vol. 86, No. 166 / Tuesday, August 31, 2021 / Rules and Regulations
2013, pp. 6, 119. Reduced rain and snow, earlier snowmelt, and drying tendencies cause a reduction in latespring and summer runoff. Together these effects, along with increases in evaporation, result in lower soil moisture by early summer Garfin 2013, p. 117.
Precipitation timing and amount impact the germination, growth, and flowering of Bartrams stonecrop, resulting in the loss of individuals and recruitment, and overall reducing the population size. Climatic events such as reduced snowpack, earlier snowmelt, and increased drought are regional and will impact all populations of Bartrams stonecrop.
In the Southwest, the period since 1950 has been warmer than any period of comparable length in at least 600
years, and average daily temperatures for the 20012010 decade were the highest in the time period including 19012010 Garfin et al. 2013, p. 3.
Fewer cold waves and more heat waves occurred over the Southwest during 20012010 compared to average decadal occurrences in the 20th century. More frequent hot and fewer cold temperature extremes over most land areas are predicted on daily and seasonal timescales, as global mean surface temperature increases IPCC 2014, p.
58. Heat waves are predicted to occur with a higher frequency and longer duration IPCC 2014, p. 58. Occasional cold winter extremes will continue to occur IPCC 2014, p. 60. Surface temperatures in the Southwest are predicted to increase substantially over the 21st century, with more warming in summer and fall than in winter and spring. Summer heat waves will become longer and hotter, while winter cold snaps will become less frequent but not necessarily less severe Garfin et al.
2013, p. 6; Garfin et al. 2014, p. 464.
When temperatures rise, evapotranspiration rates also increase and soil moisture decreases. An increase in evapotranspiration results in water loss from the plant and increases stress on the plant. This increase in stress impacts photosynthesis, respiration, transpiration, water use efficiency, leaf conductance, growth rate, vigor, and gas exchange. These impacts result in reduced growth, flowering, and seed production, and, therefore, reduce overall recruitment and population numbers.
Along with projected warming and increased evapotranspiration, droughts in parts of the Southwest will become hotter, more severe, and more frequent Garfin et al. 2013, pp. 6, 137138.
Future droughts are projected to be substantially hotter, and for major river
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basins such as the Colorado River Basin, drought is projected to become more frequent, intense, and longer lasting than in the historical record. This projection of intensified drought conditions on the Colorado River is not due to changes in precipitation, but rather due directly to warming and its effect on reducing soil moisture Garfin 2013, p. 138.
Although rare species in the southwestern United States evolved with drought, recent changes in temperature and rainfall patterns present stressful conditions of increased magnitude compared to what the species faced. Some species may shift their distributions in response to warming of the climate McLaughlin et al. 2002, p. 6070. However, it is highly unlikely that Bartrams stonecrop would be able to shift its range naturally to keep up with current and high projected rates of climate change due to its overall population decline and inability to maintain current populations. Because plants are not mobile, expanding the distribution of this species is dependent on seed dispersal. Bartrams stonecrop seeds are small and limited in dispersal ability Ferguson 2020. Given their geographic location in the landscape i.e., in canyons with springs and streams, it is possible that seeds are transported by water and that populations may have been founded by a single individual plant or seed Shohet 1999, p. 58. Seeds may also be dispersed via gravity and wind.
Seedling distribution studies indicate gravity is the most likely dispersal mechanism as seeds are fusiform shaped elliptical like a football Ferguson 2020, pers. comm.. Further, extant populations are small, which limits the amount of seed production for dispersal.
It is highly unlikely that under elevated environmental stress associated with climate change, the species would be able to both maintain populations and colonize new areas with more suitable climate conditions. Thus, localized extirpations over portions of Bartrams stonecrops range could result.
Erosion, Sedimentation, and Burial Bartrams stonecrop typically occurs on steep slopes with erodible soils and in areas susceptible to rock fall, making the plant particularly vulnerable to physical damage to its environment Phillips et al. 1982, p. 10; Shohet 1999, p. 50; Ferguson 2014, p. 42; Ferguson 2016a, pp. 15, 26. Soil erosion can result in the burial of individual plants, loss of soil where the plant is rooted, or dislodgment of plants. While displaced plants may re-root Shohet 1999, pp.
5051, 60, it is more likely that these
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plants will not survive Ferguson 2015, p. 2. Soil disturbance and erosion within or above Bartrams stonecrop habitat may occur from a variety of activities, including livestock and wildlife movement; the placement and maintenance of infrastructure, trails, and roads; and recreationists or other individuals traveling along established trails or cross country Phillips et al.
1982, p. 10; Shohet 1999, p. 60;
Ferguson 2014, p. 42; NPS 2015, p. 4;
Ferguson 2016a, p. 26.
Direct removal of Bartrams stonecrop individuals and substrate due to erosion or burial of individuals may also occur due to the placement of mineral extraction sites and debris piles. Erosion from test pits an excavation made to examine the subsurface conditions of a potential mine site has been documented to remove portions of habitat occupied by Bartrams stonecrop in Flux Canyon Phillips et al. 1982, pp.
910.
Trampling The trampling of individual Bartrams stonecrop plants may occur from a variety of activities, including livestock and wildlife movement; the placement and maintenance of infrastructure, trails, and roads; and recreationists or other individuals traveling along established trails or cross country Phillips et al. 1982, p. 10; Shohet 1999, p. 60; Ferguson 2014, p. 42; NPS 2015, p. 4; Ferguson 2016a, p. 26.
Populations may be particularly impacted during periods of unusual recreational use. We considered trampling as a stressor in our analysis of future viability only when it may impact a population with fewer than 50
individuals, as more minor stressors are exacerbated in small populations.
Illegal Collection The illegal collection of succulents is known to occur, and is often difficult to detect. Illegal collection of Bartrams stonecrop individuals has been reported, and the effect of collection is more pronounced in small populations.
More than half 58 percent of Bartrams stonecrop populations contain fewer than 50 individuals. The lifespan of Bartrams stonecrop plants has been estimated at 510 years, allowing sufficient time for discovery and collection.
Bartrams stonecrop is an attractive and small plant not available from nurseries that can be easily collected by gardeners and succulent enthusiasts.
This stressor was first noted in 1982, when exact localities were excluded from a summary report due to the possibility of illegal collection. Tagged
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