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Pediocactus knowltonii

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  © 2005 Robert Sivinski
Gucker, Corey L. 2007. Pediocactus knowltonii. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: [].



Knowlton's miniature cactus
Knowlton's minute cactus

The scientific name of Knowlton's miniature cactus is Pediocactus knowltonii L. D. Benson (Cactaceae) [3,7]. Knowlton's miniature cactus belongs to the Echinocactanae subtribe [1].

Within this review, genus level information will be distinguished using scientific nomenclature.

Pediocactus bradyi L. D. Benson var. knowltonii (L. D. Benson) Backeberg [3,5]
Pediocactus simpsonii (Engelmann) Britton and Rose var. knowltonii (L. D. Benson) Halda [3]


Endangered [16]

Information on state-level protected status of plants in the United States is available from Plants Database.

In January of 2007, an extensive search was done to locate information on Knowlton's miniature cactus with little success (see FEIS's list of source literature). The following paragraphs summarize the available information.


SPECIES: Pediocactus knowltonii
Knowlton's miniature cactus has a very limited distribution and occurs in a small area on the New Mexico-Colorado border. Knowlton's miniature cactus is restricted to an area that includes the borders of New Mexico's San Juan and Rio Arriba counties and Colorado's Archuleta County [10,17]. There may be only 2 Knowlton's miniature cactus populations, the largest of which occurs in a 15-acre (6 ha) area south of LaBoca, Colorado and the other occurs in nearby Reese Canyon [6,11].

In 1960 there were an estimated 100,000 Knowlton's miniature cactus plants in the La Boca area. This population decreased to less than 1,000 in 1979 and recovered to approximately 9,000 by 1987 [6,11]. Extensive plant collection by "cactus poachers" reduced populations to 1% of their original levels in nearly 20 years following their discovery [14]. The small size of Knowlton's miniature cactus made it a prized botanical collection and rendered it easily accumulated and concealed [6]. Today's threats to Knowlton's miniature cactus include continued illegal collection but also include habitat destruction related to grazing and oil and gas development [11]. Flora of North America provides a distributional map of Knowlton's miniature cactus.

Knowlton's miniature cactus occurs in Colorado pinyon-Rocky Mountain juniper (Pinus edulis-Juniperus scopulorum) woodlands and sagebrush (Artemisia spp.) shrublands [3,6].


SPECIES: Pediocactus knowltonii


  © 2005 Robert Sivinski

This description provides characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available (e.g. [3,10,17]).

As the name implies, Knowlton's miniature cactus is a tiny plant. Solitary or clustered stems measure up to 2.2 inches (5.5 cm) tall and 1.2 inches (3 cm) in diameter. Clusters of up to 30 cylindrical stems are reported. Knowlton's miniature cactus lacks a central spine, and radial spines are hard, pubescent, and measure 1 to 1.5 mm long. There are typically 18 to 26 spines per areola. Areolas are circular and elongate. Young areolas are woolly but become smooth with age. Flowers are produced when plants reach 3 or 4 years of age and last only 2 to 3 days. Flowers measure 0.4 to 1.4 inches (1-3.5 cm) long by 0.4 to 1 inch (1-2.5 cm) in diameter. Most plants produce 2 or 3 fruits, which are flat on top and approximately 4 mm long. Fruits are dehiscent and on average contain 10 to 12 seeds. Seeds are approximately 1.5 mm long and 1 to 1.2 mm wide [2,3,5,6,10,11,17].

Much of the available information on the botanical and ecological characteristics of Knowlton's miniature cactus comes from a recovery plan written by Heil [6] and a reintroduction study [11,12]. Many personal communications and personal observations are included within these sources. Methods used and early success in the reintroduction effort are described in Reintroduction.

Stem succulent

Knowlton's miniature cactus reproduces both sexually and asexually; however, predominance of either reproduction type is unknown. Seedlings have been observed in large numbers [6], but seeds planted in the fall as part of a reintroduction effort had not germinated by the following spring [12]. Asexually generated stems are typically produced in the fall [5].

Pollination: The principal Knowlton's miniature cactus pollinator is unknown. Ants and native bees have been observed in the short-lived (2-3 days) flowers [6].

Breeding system: Discussions of the importance of Knowlton's miniature cactus pollinators in recovery plans [6] and reintroduction efforts [11,12] suggest that male and female flowers are separate.

Seed production: In the following studies, 11% to 23% of Knowlton's miniature cactus plants produced fruits. Additional studies are necessary to determine year-to-year variability. A study by Knight described in [6] revealed that just 22 of 100 flowering Knowlton's miniature cactus plants developed fruits. Sixty-two percent of seedlings transplanted as part of a reintroduction effort in northwestern New Mexico produced flowers but just 11% produced fruits in the first transplant year [11]. In the second transplant year, 23% of plants produced flowers or fruits. Over a 3-year period, 21% to 34% of Knowlton's miniature cactus produced flowers or fruits in the native source population in northwestern New Mexico [12].

Seed dispersal: The limited distribution of and the dry fruits produced by Knowlton's miniature cactus suggest that wildlife species are unlikely dispersal agents. Dispersal is probably a result of water and wind movement that results in scattered high density colonies [5,6]. Researchers observed Pediocactus seeds at the base of parent plants as late as early fall although fruits were likely released by the end of June [5].

Seed banking: Recovery of decimated Knowlton's miniature cactus populations from less than 1,000 in the mid-1970s to nearly 7,000 in the early 1980s was considered a result of soil-stored seed germination [6]. However, duration of seed persistence in the seed bank is unknown.

Germination: Precise Knowlton's miniature cactus germination requirements and timing are unknown. Alternating freezing and thawing temperatures are considered important [6], and Brack (personal communication, in [12]) reports that germination often exceeds 75%. Stratification of Pediocactus seed by freezing at 30 °F (-1 °C) for 30 days was often sufficient to encourage germination. Stratified seeds kept at 90 °F (32 °C) germinated in 7 to 14 days [5].

Seedling establishment/growth: Requirements conducive to Knowlton's miniature cactus seedling establishment and growth are largely unknown. Many Knowlton's miniature cactus seedlings were observed in northwestern New Mexico in the early 1980s. Old plants were lacking, suggesting that illegal collections may still occur. A fructose lichen, Hypogymnia physodes, is often abundant in Knowlton's miniature cactus habitats and may play a role in successful seedling establishment [6]. Heil and others [5] report that Pediocactus establish slowly and that root development is poor when grown from seed.

Knowlton's miniature cactus seedlings were not observed in the spring following planting of seed at a reintroduction site in the fall in northwestern New Mexico. Researchers planted seeds near the surface and at 0.5 cm and 1 cm depths [12].

Reintroduction: Much of the above information regarding Knowlton's miniature cactus fecundity has come from the following reintroduction study. Researchers obtained cuttings of plants from the largest Knowlton's miniature cactus population in northwestern New Mexico. Nearly all cuttings (96%) developed roots in the greenhouse. Transplanting occurred in the fall (1985) and spring (1986) approximately 5 miles (8 km) from the source population. In the first transplant year, overall mortality was 5.3%, and more spring transplants died than fall transplants. All plants that died did so in the summer season [11]. In the spring of 1988, total mortality for fall and spring transplants was 16% and 19%, respectively, suggesting that transplant timing was not critical. Fifty-two percent of dead plants occurred at the base of the hill where the density of associated trees and shrubs was greatest. Researchers located Knowlton's miniature cactus plants buried in as much as 2 inches (5 cm) of associated plant litter. Upslope sediment accumulations also likely contributed to burial. Other sources of mortality were rodent and insect damage and fungal disease. Transplants at the mid-slope position produced the most flowers and fruits, suffered the least mortality, and had greatest growth. Growth of Knowlton's miniature cactus transplants averaged 5 mm/year. No recruitment had occurred by the second transplant year [12].

Growth: Knowlton's miniature cactus has a unique growth pattern that facilitates avoidance of desert droughts and high temperatures. Generally there are 2 growth periods in a year. In the early spring (April-May), plants grow, bloom, and set seed. From June to September, aboveground growth stops, and plants shrink into the ground. Summer dormancy is considered "very definite". Cultivated plants receiving supplemental water also experience summer dormancy. Root growth continues when aboveground plant parts recede into the ground. After late summer rains, Knowlton's miniature cactus grows again. "Offsets", or new stems, are often produced at this time. Often flower buds are produced in the fall; buds remain unopened through the winter [5,11]. For more on Knowlton's miniature cactus phenology, see Seasonal Development.

Vegetative regeneration: Asexually generated stems are typically produced following summer dormancy [5].

Knowlton's miniature cactus occupies gravelly sites [3,10] where other vegetation is often lacking [6].

Climate: Pediocactus habitats are characterized by little precipitation, extreme summer heat, low humidity, and very cold winters [5]. In Knowlton's miniature cactus habitats of northwestern New Mexico, the annual precipitation is approximately 12 inches (300 mm), and most comes in the summer and winter months [6].

Elevation: The Knowlton's miniature cactus population just south of La Boca, Colorado occurs between 6,808 and 6,873 feet (2,075-2,095 m), and the population in Reese Canyon in San Juan County occurs at 7,500 feet (2,300 m) [6].

Soils: Knowlton's miniature cactus occurs on gravelly red-brown clay soils derived from tertiary alluvial deposits [2,6,11].

Early and late seral conditions are reported in Knowlton's miniature cactus habitats. However, direct investigations of Knowlton's miniature cactus recovery following disturbances of known severity and timing are lacking, making assertions about its disturbance tolerance tentative at best.

Knowlton's miniature cactus grows in full sun or in partial shade and is largely restricted to areas where other plants are scarce [6]. One of the first to collect Knowlton's miniature cactus, Fred G. Knowlton, reported finding many plants in an area where soils had been disturbed by a bulldozer. Later botanists report finding Knowlton's miniature cactus in an undisturbed area deemed "untouched wilderness" [2]. These reports suggest that soil disturbance may be tolerated though it is not likely required.

Short-lived Knowlton's miniature cactus flowers are produced in the spring (April or May) [3,10]. Flowers last just 2 to 3 days and are open for approximately 3 hours in mid-morning and early afternoon [5,11]. Fruits are produced from late May to early June and dehisce in mid- or late June. Flower buds are set in the fall after a period of summer dormancy and remain unopened through the winter [5,6,11]. Additional information on seasonal growth of Knowlton's miniature cactus is available in Growth.


SPECIES: Pediocactus knowltonii
Information on the effects of fire on Knowlton's miniature cactus are lacking, and the endangered status of the species suggests that experimental fire studies in the near future are unlikely. However, inferences regarding Knowlton's miniature cactus and fire can be made given what is reported about its seasonal development and habitats. Plants are most typical on gravelly sites that may not burn, or would burn at a lower severity than surrounding areas in the event of fire. Timing of fire would likely affect recovery. Since aboveground plant material is lacking in the summer season, fires during this time would probably be least damaging. Summer fires would have to produce lethal belowground temperatures to damage Knowlton's miniature cactus. The recovery of plants burned while actively growing is unknown.

Further research is needed on Knowlton's miniature cactus.


SPECIES: Pediocactus knowltonii
The following table provides fire regime information that may be relevant to Knowlton's miniature cactus habitats. Follow the links in the table to documents that provide more detailed information on these fire regimes. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".

Fire regime information on vegetation communities in which Knowlton's miniature cactus may occur. This information is taken from the LANDFIRE Rapid Assessment Vegetation Models [9], which were developed by local experts using available literature, local data, and/or expert opinion. This table summarizes fire regime characteristics for each plant community listed. The PDF file linked from each plant community name describes the model and synthesizes the knowledge available on vegetation composition, structure, and dynamics in that community. Cells are blank where information is not available in the Rapid Assessment Vegetation Model.
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
Minimum interval
Maximum interval
Southwest Grassland
Plains mesa grassland with shrubs or trees Replacement 76% 20    
Mixed 24% 65    
Southwest Shrubland
Low sagebrush shrubland Replacement 100% 125 60 150
Mountain sagebrush (cool sage) Replacement 75% 100    
Mixed 25% 300    
Southwest Woodland
Pinyon-juniper (mixed fire regime) Replacement 29% 430    
Mixed 65% 192    
Surface or low 6% >1,000    
Pinyon-juniper (rare replacement fire regime) Replacement 76% 526    
Mixed 20% >1,000    
Surface or low 4% >1,000    
*Fire Severities
Replacement: Any fire that causes greater than 75% top removal of a vegetation-fuel type, resulting in general replacement of existing vegetation; may or may not cause a lethal effect on the plants.
Mixed: Any fire burning more than 5% of an area that does not qualify as a replacement, surface, or low-severity fire; includes mosaic and other fires that are intermediate in effects.


SPECIES: Pediocactus knowltonii
1. Arp, Gerald. 1972. A revision of Pediocactus. Cactus & Succulent Journal. 44(5): 218-222. [22646]
2. Benson, L. 1961. A revision and amplification of Pediocactus--I. Cactus and Succulent Journal of the Cactus and Succulent Society of America. 33: 49-54. [65694]
3. Flora of North America Association. 2010. Flora of North America: The flora, [Online]. Flora of North America Association (Producer). Available: [36990]
4. Hann, Wendel; Havlina, Doug; Shlisky, Ayn; [and others]. 2008. Interagency fire regime condition class guidebook. Version 1.3, [Online]. In: Interagency fire regime condition class website. U.S. Department of Agriculture, Forest Service; U.S. Department of the Interior; The Nature Conservancy; Systems for Environmental Management (Producer). 119 p. Available: [2008, September 03]. [70966]
5. Heil, Ken; Armstrong, Barry; Schleser, David. 1981. A review of the genus Pediocactus. Cactus & Succulent Journal. 53(1): 17-39. [22564]
6. Heil, Kenneth D. 1985. Recovery plan for the Knowlton cactus, Pediocactus knowltonii L. Benson. Albuquerque, NM: U.S. Fish and Wildlife Service, Region 2. 32 p. [64738]
7. Kartesz, John T. 1999. A synonymized checklist and atlas with biological attributes for the vascular flora of the United States, Canada, and Greenland. 1st ed. In: Kartesz, John T.; Meacham, Christopher A. Synthesis of the North American flora (Windows Version 1.0), [CD-ROM]. Chapel Hill, NC: North Carolina Botanical Garden (Producer). In cooperation with: The Nature Conservancy; U.S. Department of Agriculture, Natural Resources Conservation Service; U.S. Department of the Interior, Fish and Wildlife Service. [36715]
8. LANDFIRE Rapid Assessment. 2005. Reference condition modeling manual (Version 2.1), [Online]. In: LANDFIRE. Cooperative Agreement 04-CA-11132543-189. Boulder, CO: The Nature Conservancy; U.S. Department of Agriculture, Forest Service; U.S. Department of the Interior (Producers). 72 p. Available: /downloadfile.php?file=RA_Modeling_Manual_v2_1.pdf [2007, May 24]. [66741]
9. LANDFIRE Rapid Assessment. 2007. Rapid assessment reference condition models, [Online]. In: LANDFIRE. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Lab; U.S. Geological Survey; The Nature Conservancy (Producers). Available: /models_EW.php [2008, April 18] [66533]
10. Martin, William C.; Hutchins, Charles R. 1981. A flora of New Mexico. Volume 2. Germany: J. Cramer. 2589 p. [37176]
11. Olwell, Peggy; Cully, Anne; Knight, Paul; Brack, Steve. 1987. Pediocactus knowltonii recovery efforts. In: Elias, Thomas S., ed. Conservation and management of rare and endangered plants: Proceedings of a California conference on the conservation and management of rare and endangered plants; 1986; Sacramento, CA. Sacramento, CA: California Native Plant Society: 519-522. [64737]
12. Owell, Peggy; Cully, Anne; Knight, Paul. 1990. The establishment of a new population of Pediocactus knowltonii: third year assessment. Mitchell, R. S.; Sheviak, C. J.; Leopold, D. J., eds. Ecosystem management: rare species and significant habitats. Museum Bulletin 471, Albany, NY: New York State Museum: 189-193. [65692]
13. Raunkiaer, C. 1934. The life forms of plants and statistical plant geography. Oxford: Clarendon Press. 632 p. [2843]
14. Spellenberg, Richard. 1993. Species of special concern. In: Dick-Peddie, William A., ed. New Mexico vegetation: Past, present, and future. Albuquerque, NM: University of New Mexico Press: 179-224. [21101]
15. U.S. Department of Agriculture, Natural Resources Conservation Service. 2010. PLANTS Database, [Online]. Available: /. [34262]
16. U.S. Department of the Interior, Fish and Wildlife Service. 2016. Endangered Species Program, [Online]. Available: [86564]
17. Weber, William A. 1987. Colorado flora: western slope. Boulder, CO: Colorado Associated University Press. 530 p. [7706]

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