Pediocactus nigrispinus

A species separated from the former broad P. simpsonii

POWO accepts Pediocactus nigrispinus as native to Washington, Oregon and Idaho. In older texts, it may appear as P. simpsonii var. nigrispinus. Names with the epithets beastonii and puebloensis are its synonyms in this treatment. P. indranus remains a separate accepted species. [2-WWW] [2-WWW]

Branched stems and dark spines

Stems may be branched, ranging from flattened and ovoid to elongated. Flora of North America gives a height of 5–30 cm and width of 5–15 cm. There are 10–30 radial spines, 8–20 mm long. There are 6–12 central spines, reaching 15–35 mm and darkening from reddish brown to almost black. They may be yellowish or cream-coloured at the base. Spring flowers are 2.5–5 cm in diameter and white, pink, purple or yellowish green. [Heil and Porter 2003a, Flora of North America, P. nigrispinus]

Basalt and tussock vegetation

The floristic account gives an elevation range of 400–2000 m above sea level. The study published in 2025 compared seventeen sites in Washington and Oregon, mainly associated with volcanic substrates. The grasses Poa secunda and Pseudoroegneria spicata grew on all plots. In Washington, Artemisia rigida was a characteristic shrub, while in Oregon Juniperus occidentalis often accompanied the cacti. [Heil and Porter 2003a, Flora of North America, P. nigrispinus. Dillon-Zuppelli et al., pp. 65–68] [Dillon-Zuppelli et al. 2025, Flora of North America, P. nigrispinus. Dillon-Zuppelli et al., pp. 65–68]

Vegetation, surface cover, substrate properties and climatic parameters were compared. All plots represented known cactus sites. Since comparable places without it were not included, the similarities found describe occupied habitats. They do not independently establish causes of species absence elsewhere. [Dillon-Zuppelli et al. 2025, pp. 65–68, 72–74]

Adult plants maintaining the population

Four plots at Wild Horse in Washington State were censused in 2016, 2017, 2018, 2019 and 2022. Results give 415 recorded plants in the first year and 344 in the last. Stem clusters were assigned a shared number because offsets and neighbouring plants were difficult to separate in the field. No new seedlings were detected during these censuses. [Dillon-Zuppelli et al. 2025, pp. 63–65, 68–71]

Matrices for three successive annual transitions, from 2016–2017 to 2018–2019, had λ below one, ranging from 0.935–0.974. This means a modelled decline in abundance if that survival and size-class transition pattern persists. Remaining in the same class had the greatest elasticity, particularly for stems 7–10 cm in diameter. The result concerns the monitored plots, rather than the species' entire range. [Dillon-Zuppelli et al. 2025, pp. 69–75, Table 3–4]

Germination in an experiment and establishment in the field

A greenhouse trial obtained 79% for the combined outcome of germination and survival to six months. Plants grew in a mix of mineral components and horticultural growing medium, with air temperatures of 21–38°C and irrigation changed during the experiment. This is the result of that procedure, rather than a determined optimum temperature range. The value incorporated into the field-recruitment model came from the authors' assumptions because no new seedlings were observed on the plots. [Dillon-Zuppelli et al. 2025, p. 64–65, 70–75]

Bacterial 16S rRNA gene-sequence analysis compared nine samples of biological soil crust near cacti with seven near grasses. Most detected groups were shared, while some occurred more often near cacti. The effects of individual bacteria on growth and nitrogen fixation were not tested experimentally here. The data document microbial community composition, leaving its function to be investigated. [Dillon-Zuppelli et al. 2025, p. 66–67, 71–74, ryc. 5]

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