Sclerocactus mesae-verdae

A plant of clay badlands

Sclerocactus mesae-verdae has also been described under the names Coloradoa mesae-verdae and Pediocactus mesae-verdae. Its stem is small, somewhat spherical and usually solitary. In the long-term study, it rarely exceeded 10 cm in diameter, and the largest measured 11.1 cm. Damaged individuals can produce lateral stems, so that one plant forms several separate heads visible above ground. [Coles et al. 2012] [2-WWW]

The sites described in southwestern Colorado lie on substrates derived from Mancos Shale and deposits of the Fruitland Formation. The soils contain a high clay fraction, swell when wet and shrink as they dry. They have elevated salt contents, including calcium, sodium and magnesium compounds. Water infiltrates slowly, and sparse vegetation, often including saltbushes, covered around 5–15% of the area of the habitats studied. [Coles et al. 2012]

The regional climate description reported around 150–230 mm of annual precipitation, with roughly half falling between July and October. The plants therefore experience both long periods of water deficit and brief episodes when the heavy soil becomes moist. These field conditions describe the species' natural environment and its seasonal variability. [Coles et al. 2012]

Twenty years of observations

Coles, Decker and Naumann followed populations on three one-hectare plots in Ute Mountain Ute Tribal Park. The work covered 1986–2005, with no census in 1993. In total, 659 plants and 1629 stems were recorded. Distinguishing these two units allowed loss of above-ground parts and death of whole individuals to be described separately. [Coles et al. 2012]

Long-term tracking of marked plants revealed events invisible in a single census. The stem of an apparently dead cactus could regrow after a year, with the new growth already supported by surviving parts of the older plant. For adjacent stems, the authors also considered the difficulty of distinguishing regrowth from a nearby seedling without disturbing the underground system. [Coles et al. 2012]

Seedlings and offsets mature at different rates

Individuals grown from seed reached the size associated with the onset of flowering after 7–11 years. Offsets could flower in their first year, and usually did so within three years of appearing. The smallest flowering stem observed was 1.4 cm in diameter, although the more common reproductive size threshold was around 2.1 cm. Stem size and origin therefore jointly describe its maturity. [Coles et al. 2012]

During the full monitoring period, 569 seedlings were found. At the end of the study, 209 were alive, including 63 individuals already observed for 10–18 years. Seedlings that died during the observations survived for an average of less than three years. The youngest developmental stage therefore imposed strong selection, while some individuals that survived it persisted for many further seasons. [Coles et al. 2012]

The number of seedlings recorded was positively related to April rainfall. The relationship explained around 49% of the variation between years, using rainfall data from a regional weather station. Censuses took place in spring, so the number of newly detected plants combined the effects of germination and earlier survival. A wet April could also benefit seedlings that had begun developing in the previous autumn. [Coles et al. 2012]

Flowering, pollination and drought

At the sites observed, flowering was concentrated in the last two weeks of April and the first two weeks of May. Larger stems produced more flowers. The mean was 4.6 flowers per flowering stem per season, with a recorded maximum of 30. Late frosts could interrupt fruit development, linking reproductive success to the weather during a brief spring period. [Coles et al. 2012]

Experiments published in 2023 demonstrated partial self-compatibility in the plants studied. Fruit set nevertheless required pollinators. Ground-nesting halictid bees were identified as the likely main visitors transferring pollen. [Tepedino et al. 2023]

During the extreme drought of 2001–2002, flowering and fruiting were strongly inhibited. For almost two seasons, plants produced very few reproductive structures, while direct mortality attributed to drought was low. A reduced supply of new seeds can therefore precede a clear decline in adult plants in the census. [Coles et al. 2012]

Beetles, stem loss and regeneration

The main identified cause of death was the beetle Moneilema semipunctatum. Its larvae feed inside stems and destroy tissues. Over twenty years, it was assigned responsibility for the deaths of 170 of the 659 plants recorded and the loss of 320 of the 1629 stems. These are proportions of the entire tracked group, which also included individuals alive at the end of the study. [Coles et al. 2012]

Damage particularly affected large individuals, which also had a greater capacity to produce flowers. Losing such a plant therefore removes an important seed source from the population. Around one fifth of cacti damaged by beetles survived and produced new stems. Regeneration allowed an individual to persist despite the destruction of a substantial part of its visible surface. [Coles et al. 2012]

Changes in abundance depended on overlapping episodes of feeding damage, seedling emergence and regeneration. Two decades of observations gave an overall picture of relative stability on the study plots, with substantial changes over shorter periods. The result applies to these three plots in 1986–2005. Its most important biological detail is the distinction between the fate of the whole plant, an individual stem and the new generation. [Coles et al. 2012]

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