Sclerocactus wrightiae

Structure and identification

Sclerocactus wrightiae usually has a solitary, pale green stem, flattened to shortly cylindrical. In the flora of North America description, the most frequent height is 4–8 cm, within an extreme range of 1–11 cm. Tubercles are visible on the 13–16 ribs. An areole usually contains 5–10 radial and 3–4 central spines. The lower central spine is hooked. Pale flowers are 2–4 cm in diameter and have purple-pink stamen filaments. [Heil and Porter 2003b]

In the field, the species may grow near S. parviflorus. During studies in Utah, flower and stamen colour were used in identification. In S. wrightiae, the perianth was white or cream, sometimes with a pink tint, and the stamen filaments were magenta. The S. parviflorus used for comparison more often had pink flowers and greenish or whitish filaments. Identification rests on a set of characters of the whole individual. [Bates et al. 2022]

Habitat and seasonal stem changes

The natural range covers central Utah, including areas in Emery, Sevier and Wayne counties. The monitoring account gave sites at 1280–2320 m elevation. Plants occupy open surfaces with sparse vegetation. The habitats analysed had sandy-loam soils, where the small, spine-sheltered stem readily blends in with stones and dry plant remains. [Bates et al. 2022] [2-WWW]

During drought, the stem contracts and may sink below the ground surface. After hydration, its volume increases again, and it produces flowers in spring. At the site described in the seed bank study, flowering occurred from late April to May. Hydration state and developmental stage therefore change both the individual’s appearance and the likelihood of finding it during a field survey. [Lariviere et al. 2024]

Pollination and seed production

Experiments published in 2023 demonstrated self-incompatibility in the plants studied. Fruit formation required pollinators. Among flower visitors, ground-nesting bees of the family Halictidae, the sweat bees, were identified as the likely main pollen carriers. [Tepedino et al. 2023]

Seed number also depends on plant size and seasonal conditions. In the experiment comparing pressure on the soil, larger individuals produced more seeds, and yield in the wetter 2020 exceeded that in the dry 2018. The range per plant was zero to 506 seeds. Variation among individuals and years determines the amount of material that can enter the soil around adult cacti. [Lariviere et al. 2023]

A persistent seed bank

In June 2021, Lariviere and colleagues collected 500 soil samples at the four-hectare Last Chance Wash site. The study took place before seeds from the current season had been dispersed. The recovered seeds had therefore remained in the environment since at least the previous year. This provides direct evidence of a persistent seed bank, a stock retained in the soil for more than one growing season. [Lariviere et al. 2024]

The samples came from around 50 reproductively mature plants, within 20 cm of the stem, and from two soil layers, 0–3 and 3–6 cm. This sampling also included soil beneath potential nurse plants some distance from the cacti. A total of 43 seeds were recovered. The highest density occurred in the nearest band, within 5 cm of the parent plant. Distance from the stem and position relative to the slope explained the spatial differences in the results well. [Lariviere et al. 2024]

The 0–3 cm layer contained 56% of the recovered seeds, and the deeper layer contained 44%. Fully 98% were found in samples taken downslope. However, this direction was sampled five times as often as the opposite direction, so the percentage itself describes the set of samples obtained. The authors link seed distribution to the slope and transport across the soil surface. They also observed ants removing seeds from fruits and from the ground. [Lariviere et al. 2024]

Viability and conditions for recruitment

The tetrazolium test indicated that 32% of the recovered seeds were viable. This method detects activity in living embryonic tissues through a colour reaction. The result gives the proportion of seeds that retained viability in the collected material. Becoming a seedling still requires germination to begin, emergence through the soil surface and survival through subsequent periods of water shortage. [Lariviere et al. 2024]

No seeds were found in 20 samples beneath potential nurse plants at least 3 m from adult cacti. Together with their concentration near the stems, this result indicates the local nature of the detected stock. The study covered one site and one sampling date. It documents seed persistence into the following year, while their maximum survival time in natural soil remains to be established. [Lariviere et al. 2024]

Pressure on the soil around a plant

Field experiments in 2018 and 2020 tested plant responses to one or four applications of pressure from a model hoof within 15 cm of the stem. The device produced approximately 245 kPa of pressure, matching the assumptions for a walking cow weighing 400 kg. The first year included 64 cacti and the second 146. The treatment affected the surrounding soil without directly crushing the stem. [Lariviere et al. 2023]

Under these conditions, the numbers of flowers, seeds and aborted reproductive structures did not differ significantly between pressure treatments and the control. All plants survived the observation period. The result applies to a specific treatment applied on a single occasion to individuals 2.1–9 cm in diameter. Direct crushing of plants, repeated trampling of the same place and survival of the smallest seedlings remain separate processes. [Lariviere et al. 2023]

How well a population can be seen from the air

During flowering in spring 2019, ground censuses were compared with manual examination of orthophotomaps, photographs combined into an image at a uniform scale. In the 14 plots analysed, 480 plants were found on the ground. Images taken from 10 m correctly identified 183 of them, those from 15 m identified 89, and those from 20 m identified 46. Higher flights reduced image detail and made small stems harder to recognise. [Bates et al. 2022]

At a height of 10 m, correct identifications accounted for 38.1% of the actual census. Some objects marked as cacti also proved to be other features of the ground. Plants up to 2 cm in diameter were particularly difficult to detect. Pale spines, dust and dry surroundings reduce contrast, while open flowers can improve visibility. A reliable census therefore combines images with identification on the ground and checks on the proportion of the smallest individuals. [Bates et al. 2022]

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