Sclerocactus tobuschii

Name and plant form

Sclerocactus tobuschii is the species name accepted in POWO, with the combination published in 2024. In older accounts, it appears under names including Ancistrocactus tobuschii. The 2021 study of fire effects uses Sclerocactus brevihamatus subsp. tobuschii, referring to the same studied taxon at subspecies rank. [Reemts and Ferrato 2021] [2-WWW]

The plant forms a small, somewhat spherical stem, usually less than about 7 cm in diameter. A substantial part of its volume may lie at ground level or below it. Its surface is divided into spine-bearing tubercles. The low growth form influences the effects of fire because it limits the extent to which living tissues project into the flame zone above the ground surface. [Reemts and Ferrato 2021]

Limestone sites and winter flowering

The species occurs on the Edwards Plateau in Texas. It occupies open, rocky places on shallow limestone soils, in a landscape with grasslands and oak and juniper vegetation. Rocks and small amounts of fine plant fuel divide the surface into patches across which fire passes at different intensities. [Reemts and Ferrato 2021] [2-WWW]

In the species description cited by Reemts and Ferrato, flowering occurs from mid-January to mid-March, depending on the weather. Fruits ripen from late April to early May. Reproductive size, around 2 cm in diameter, may be reached only after roughly nine years. The long growth period of the youngest plants increases the importance of adult survival. [Reemts and Ferrato 2021]

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At Love Creek Preserve in Bandera County, six 10 × 10 m plots were established and monitored from 2013 to 2019. Three underwent a prescribed burn on 1 December 2016, and a fourth on 10 January 2018. Two remained unburned. The experiment included more than 600 marked cacti and allowed survival, diameter growth and fruiting to be compared. [Reemts and Ferrato 2021]

These were winter burns conducted under specific weather conditions and with the local fuel distribution. Plants were checked in subsequent growing seasons, around 4–5 and 16–17 months after treatment. The authors relate the results to this form of fire exposure, in which parts of the area were only lightly burned or bypassed by the flames. [Reemts and Ferrato 2021]

Survival and signs of damage

One cactus died as a direct result of fire. Visible damage affected 4–14% of plants on individual plots burned in 2016 and 27% on the plot burned in 2018. Scorched spines, damaged tubercles and scarring of the stem surface were observed. No individual with visible fire damage died during the two growing seasons observed. [Reemts and Ferrato 2021]

Diameter growth in damaged cacti was sometimes slower in the first or second season. The proportion of plants producing fruit was maintained. These two results describe different plant functions, since survival and fruiting can accompany reduced stem growth. The study measured the presence of fruits, while subsequent seed quality and offspring survival are further stages of the reproductive cycle. [Reemts and Ferrato 2021]

Some plants developed lateral stems after damage, forming a multiheaded plant. Regeneration therefore changes both growth form and the way an individual is subsequently measured. Such regrowth depends on living parts of the plant surviving, from which new growth can arise. [Reemts and Ferrato 2021]

The importance of size and nearby grasses

Damage occurred more often in larger plants and those projecting farther above ground. Proximity to grasses also mattered. Most damaged cacti grew very close to grass tussocks, often less than 1 cm away. When the distance exceeded 5 cm, the probability of damage fell markedly. This relationship corresponds to local variation in the amount of fuel immediately beside the stem. [Reemts and Ferrato 2021]

The authors suggest that high survival reflects the low growth form, partial burial of plants in the soil and rocky surroundings that interrupt fuel continuity. These mechanisms agree with the spatial distribution of damage, although temperatures inside stems were not measured. The result describes this population of S. tobuschii tolerating the winter burns studied, including temporary slowing of growth in some individuals. [Reemts and Ferrato 2021]

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