Sclerocactus from flower to a new generation

The flower opens the way to the next generation

In spring, the pale flowers of Sclerocactus wrightiae make the plants more visible on the stony surfaces of Utah. After the reproductive season, new seeds reach the area around the stem, and some remain in the soil until the following year. The adult cactus, its seeds and the smallest seedlings also form three distinct parts of the population. Each responds to its own set of conditions and requires a different observation method. [Lariviere et al. 2024] [Bates et al. 2022]

Pollination studies published in 2023 distinguished self-incompatible S. wrightiae from partially self-compatible S. mesae-verdae. Both species required pollinators for fruit formation. Ground-nesting halictid bees were among the probable main pollen vectors. The capacity to use their own pollen and autonomous flower pollination therefore describe two separate elements of reproductive biology. [Tepedino et al. 2023]

Seeds remaining near the plant

At Last Chance Wash, 500 soil samples were collected around S. wrightiae and beneath potential nurse plants. Sampling took place in June 2021, before seeds were released from fruits formed in the current season. The recovered material therefore documented seed survival since at least the preceding year. Such a reserve is called a persistent soil seed bank. [Lariviere et al. 2024, pp. 3–7]

Samples around 50 adult cacti covered distances up to 20 cm from the stem and two layers, 0–3 and 3–6 cm. A total of 43 seeds were found. The highest density occurred in the nearest five-centimetre band of soil. More than half of the recovered seeds came from the upper layer, but 44% were at a depth of 3–6 cm. The reserve therefore also occurred below the immediate surface along which newly dispersed seeds move. [Lariviere et al. 2024, pp. 5–7]

Most were found on the downslope side. The authors relate this distribution to the direction of transport across the soil surface. Ants provide an additional means of movement and were observed carrying seeds from fruits and the ground. A seed's eventual position therefore results from several overlapping processes, while the neighbouring stem marks the location where the reserve was most often detected. [Lariviere et al. 2024]

A living embryo and a future seedling

The viability of recovered seeds was assessed using a tetrazolium test. The colour reaction occurs in living tissues and allows assessment of the embryo's condition after recovery from the soil. 32% of the material proved viable. Between this assessment and the appearance of a new cactus lies a sequence of further events, including germination, growth through the substrate layer and survival of the first drought periods. [Lariviere et al. 2024]

This study documented survival until at least the next season. The exact age of individual seeds and their maximum persistence in soil remain unknown. The few seeds found near a plant form a potential source of recruitment, but the number of future seedlings also depends on conditions at germination. Distinguishing these stages explains why adult plant numbers can change more slowly than their capacity to produce a new generation. [Lariviere et al. 2024] [Coles et al. 2012]

The first years on clay soil

The longest observation series discussed concerns S. mesae-verdae. Plants were monitored on three one-hectare plots in southwestern Colorado from 1986 to 2005, with 19 annual censuses. During this period, 569 seedlings were recorded. At the end of the study, 209 were alive, including 63 individuals known for 10–18 years. Among seedlings that died during observation, mean survival was less than three years. [Coles et al. 2012, pp. 312–315]

The number of seedlings detected was associated with April rainfall. The statistical relationship explained about 49% of variation between years. However, the spring census included both newly emerged seedlings and plants that might have germinated in autumn. The association with rainfall therefore describes the outcome of germination and survival in the field. Separating these processes would require more frequent observations of the youngest plants. [Coles et al. 2012, pp. 316–320]

During the 2001–2002 drought, flowering and fruiting were severely reduced, with little mortality attributed directly to water shortage. Adult cacti persisted at the site while the supply of new seeds declined. For assessing population recruitment, counts of flowering stems and seedlings therefore provide information that complements a census of living plants alone. [Coles et al. 2012]

A replacement shoot benefits from the older plant's history

S. mesae-verdae seedlings required 7–11 years to reach a size allowing flowering. Shoots regrowing after damage could flower in their first year and usually within three years. A replacement shoot is part of an existing individual and begins developing under conditions different from those of a plant growing from seed. This large difference in maturation time changes the rate at which reproductive function returns after stem loss. [Coles et al. 2012]

In the population studied, serious damage was caused by larvae of the beetle Moneilema semipunctatum, which feed inside the tissues. About one fifth of damaged plants survived and produced new shoots. Some individuals previously regarded as apparently dead revealed a replacement shoot only the following year. The long-term census therefore distinguished stem death from death of the whole plant, and determining an individual's fate sometimes required another inspection. [Coles et al. 2012]

Damage depends on the type of event

An experiment with S. wrightiae in 2018 and 2020 investigated pressure on the soil beside the stem. A hoof model was applied once or four times within 15 cm, avoiding the plant itself. Under this defined treatment, no significant differences in flower and seed numbers were found between treatments and the control. Differences associated with plant size and seasonal moisture were more pronounced. [Lariviere et al. 2023]

A different type of disturbance was assessed in S. tobuschii (named S. brevihamatus subsp. tobuschii in the 2021 paper). Two winter burns in Texas directly killed one plant and left some others with damaged spines and stem surfaces. The damaged cacti survived the two seasons observed, although their diameter growth was sometimes slower. Damage frequency increased near grasses that supplied fuel. [Reemts and Ferrato 2021]

Both studies establish the response to a specific process. Pressure on surrounding soil, direct crushing of the stem, partial scorching of the surface and larval feeding within tissues involve different parts of the plant. Their effects can be compared using common measures such as survival, growth and reproduction, while retaining a precise description of the damage itself. [Coles et al. 2012] [Reemts and Ferrato 2021] [Lariviere et al. 2023]

How many plants the camera detects

In 2019, drone photographs were taken from three heights during flowering of S. wrightiae, and manual identifications on orthophotomaps were compared with a ground census. On the 14 plots analysed, 480 cacti were found on the ground. The comparison shows how rapidly the number of correctly identified plants falls as flight height increases. [Bates et al. 2022, Table 2, p. 7]

The smallest plants, up to 2 cm in diameter, were difficult to detect at all heights studied. Stems covered with pale spines contrast poorly with the dry substrate. Other objects were also mistakenly identified as cacti. Flower visibility improves the chance of finding an adult plant but directs attention towards individuals already reproducing. [Bates et al. 2022]

Assessing population recruitment requires identifying its least visible parts. Soil samples reveal seeds, detailed surface surveys reveal seedlings, and finding a marked plant again reveals regeneration. Aerial photographs help describe distribution across the landscape, while ground inspections retain information about small individuals. Combining these observations reveals continuity between current flowering and the future generation. [Lariviere et al. 2024] [Bates et al. 2022] [Coles et al. 2012]