Oenothera acutissima

Wagner (1981) described large-flowered, self-compatible but presumably outcrossing plants endemic to the mountains of nw CO and ne UT as Oenothera acutissima, citing several vegetative and reproductive autapomorphies that distinguish it from O. flava. Subsequently, O. acutissima was treated as a variety of O. flava by Welsh (2015). A recent study of this and related taxa in Oenothera sect. Lavauxia found that this taxon was clearly distinct in terms of floral morphology and odor chemistry; combined with its edaphic specialization, autapomorphic vegetative characters, and the poor viability (and sterility) of artificial hybrids between O. acutissima and O. flava, these findings supported maintaining O. acutissima as a distinct species (Raguso et a. 2007).

Oenothera acutissima is a perennial herb of the western United States that is endemic to the mountains of northeastern Utah and adjacent Moffat County, Colorado where it is restricted to seasonally moist habitats with sandy to gravelly soil. It is known from approximately 23 occurrences including one large occurrence in Utah containing approximately 160,000 plants. Most of the other occurrences have more moderate numbers, <100 to several thousand individuals. The principal threats to this species are loss of wetland habitat due to stock pond creation and the associated hydrologic changes, trampling from livestock grazing, and off- road vehicle use. Oenothera acutissima is predicted to be highly vulnerable to the effects of climate change. Few or no occurrences are considered appropriately protected and managed at this time. Four occurrences in Colorado are considered extirpated or possibly extirpated. There are numerous historical occurrences that should be prioritized for site visits.

Oenothera acutissima occurs in the western United States where it is restricted to the Uinta Mountains of Daggett, Uintah, and Duchesne Counties, Utah and adjacent areas of Moffat County, Colorado (Welsh et al. 2015). The Center for Native Ecosystems and Colorado Native Plant Society (2006) describe the range as "in the vicinity of the Flaming Gorge National Recreation Area and around Diamond Mountain, Cold Spring Mountain, and Douglas Mountain at the eastern end of the Uinta Mountains. It has been found as far west as Burnt Mill Spring, northwest of Roosevelt, and as far east as Boone Draw, below Sand Wash Basin in Moffat County, Colorado... The Uinta Mountains are relatively isolated from other mountain environments with similar elevational characteristics, and this isolation likely sets a natural limit on the geographic distribution." Range extent is approximately 5,000 - 5,500 square km when calculated using GIS tools.

Rangewide there are 23 occurrences with 11 in Colorado and 12 in Utah. As of 2022, there are 11 occurrences documented in the Colorado Natural Heritage Program database, however, 2 of the 11 occurrences have not been observed in over 20 years. There are an additional four occurrences that are considered extirpated or possibly extirpated in Colorado. Surveys for this species are relatively complete and thorough with little chance of additional discoveries (Center for Native Ecosystems and Colorado Native Plant Society 2006).

Threats to this species' limited habitat include livestock trampling, expanding off- road vehicle (ORV) use, noxious weeds, road construction and maintenance, and drought.
A few occurrences are seriously threatened by impacts such as heavy trampling or ORV damage, extensive water diversions and meadow channeling, or housing and infrastructure development; however, in other parts of the range, the rocky terrain and remote location may help to insulate populations. Nevertheless, most, perhaps all, occurrences are exposed to some degree of disturbance. Livestock trampling is the most significant threat; most to all sites are affected. Many of the springs that this species inhabits are used to supply water for livestock, exposing plants to trampling. Hydrologic alterations related to livestock grazing, including the drying of wetlands from upstream earthen dams, stream downcutting and channelization, and the loss of habitat due to stock pond creation are a concern. In addition, a few sites are found adjacent to infrastructure such as stock ponds, corrals, and fence lines that concentrate cattle and exacerbated impacts from trampling. Cattle herbivory does not appear to be a serious threat at present, but this species is palatable to livestock and wildlife, and increases in grazing intensity may increase herbivory in the future. In addition, at least at two sites, significant herbivory by wild ungulates (mule deer and elk) has been noted (Raguso et al. 2007, Denver Botanic Gardens 2008). ORV use, increasing in this species' range, is allowed almost everywhere O. acutissima occurs, and ORV impacts have been noted at several sites. The wet soils inhabited by this species are easily rutted and displaced by tires. Non-native invasive plant species were observed in several populations, including the aggressive rhizomatous species, Canada thistle (Cirsium arvense) and smooth brome (Bromus tectorum). Livestock and ORVs often serve as vectors for these weeds. This species seems to prefer bare patches of soil or gravel within the stream channels it occupies, and there is concern that weed invasions could eliminate or alter this microhabitat, increasing substrate stability and percent ground cover. These alterations might allow other species to gain a competitive advantage over O. acutissima. Most occurrences are near roads, and some are directly bisected. Roads are a vector for ORVs, noxious weeds, and livestock, and road maintenance and road repair activities can directly damage plants. Roads are also a concern because of their impacts on site hydrology. Oenothera acutissima is predicted to be highly vulnerable to climate change (Handwerk et al. 2015) but at least two observations indicated the species was stable or reproducing during drought. Climate change is of particular concern for this species because it has a narrowly defined suitable habitat with a fairly specific hydrology, within a region that is experiencing increasing drought. Nevertheless, despite these factors, in 1988 Franklin commented that the species seemed relatively secure, at least within the Ashley National Forest (Franklin 1988 cited in Center for Native Ecosystems and Colorado Native Plant Society 2006).