Opuntia macrorhiza Engelm.
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Basis for Listing
Opuntia macrorhiza is a common plant in parts of the Great Plains, but it is rather rare in Minnesota where it reaches the edge of its geographic range. There are approximately 30 records of O. macrorhiza from the southwestern portion of the state, primarily in Redwood, Renville, and Yellow Medicine counties. The number of occurrences belies the rarity of this species in Minnesota since most populations seem to be quite small and widely separated from each other. The record from Washington County in eastern Minnesota is based on a single introduced plant that has since disappeared. There is no authenticated occurrence of this species east of Mankato.
In Minnesota, O. macrorhiza occurs on the margins of bedrock exposures and in dry prairies on shallow soils over bedrock, both of which are rare habitats prone to severe drought during the growing season. This raises some concerns about the ability of populations to maintain gene-flow between small and isolated populations. Bedrock would seem a safe habitat for a plant, especially compared to the surrounding prairies which are so easily plowed and planted to crops, but history is teaching us differently. Rock outcrops are attracting a lot of attention from quarrying companies. The rock is mined, crushed, then used in road construction and by the railroad industry. The demand for crushed rock and the technology for producing it are increasing every year, which does not bode well for O. macrorhiza. Given its limited geographic range in the state and restrictive habitat requirements, O. macrorhiza was listed as a special concern species in Minnesota in 1984.
Please note there is considerable uncertainty about the taxonomy of O. macrorhiza, and it is often confused with a Midwestern species named O. humifusa (devil's-tongue). It is clear that we have only one of the two species in Minnesota, but it is not entirely clear which one it is. The information included in this profile is specific to the entity that occurs in Minnesota regardless of its name.
Opuntia macrorhiza is usually classified as a clump-forming or mat-forming shrub, reaching a height of 1 or 2 stem segments, which is 8-16 cm (3.1-6.3 in.). The stem segments are dark, dull green, fleshy, flattened, obovate to circular in outline, 5-11 cm (2.0-4.3 in.) long, and 3.5-7.5 cm (1.38-2.95 in.) wide. A diagonal line through the widest portion of each stem segment would typically intersect 5-6 and up to 8 areoles. Areoles are specialized spine producing structures which are present on the surface of each stem segment. Each areole has 1-4 spines, or no spines at all. The spines are erect to spreading, white to red-brown, with a maximum length of 6 cm (2.4 in.). The tepals of the flowers are yellow with red bases and 2.5-4 cm (0.98-1.57 in.) long. The fruits are fleshy, green, yellowish or dull red, elongate-obovoid in shape, 2.5-4 cm (0.98-1.57 in.) long, and 1.5-2.8 cm (0.59-1.10 in.) wide.
The only other Opuntia in Minnesota is O. fragilis (brittle prickly pear), has and it has smaller stem segments.
In Minnesota, O. macrorhiza occurs on the margins of bedrock exposures and in associated dry prairie communities, specifically in thin, dry soil over granite, quartzite, and gneiss. Opuntia macrorhiza often roots directly in the rock crevices where a small amount of coarse, wind-borne soil accumulates. It is also found where the bedrock is covered by a somewhat thicker layer of soil a few to several inches deep. Even when the bedrock is not visible at the surface of the ground, it still has a profound influence on the surface vegetation.
Biology / Life History
Some years, but not every year, O. macrorhiza will produces large, yellow flowers that are consistently pollinated by flying insects. The pollinated flowers develop into large fleshy fruits that contain numerous seeds. The mode of seed dispersal is not well documented, but it appears that the fruits are eaten by animals that spread the seeds in their droppings.
If the stem of O. macrorhiza is broken apart by some mechanical force, the individual stem segments, often called pads, can survive independently for some period of time. If they come into contact with a suitable substrate they can even send down roots (McClain and Koelling 1992). These rooted segments can then grow additional segments and establish a new independent, free-living plant. Unlike the more common O. fragilis, which breaks apart easily, the stems of O. macrorhiza do not. So it is difficult to know how often this form of vegetative reproduction actually occurs.
The large, tuberous root of O. macrorhiza may contribute to its ability to store water internally and resist drought. Certainly the individual stem segments can store large amounts of water, which makes it nearly impervious to any degree of drought likely to be seen in Minnesota.
Opuntia macrorhiza flowers from about the middle of June through the end of July. However, it is not necessary to limit searches to this period. The stems are green all year, and the species can easily be recognized even in the winter.
Conservation / Management
Opuntia macrorhiza occurs in the prairie biome, and habitats in the prairie were historically exposed to wildfire every few years. The fires were typically started by lightning strikes during dry storms in the spring. A fire started by one lightning strike could burn thousands of acres before it encountered a natural fire break of some kind. Observation tells us that O. macrorhiza is vulnerable to fire, but in previous times it probably evaded fire because the bedrock outcrops on which it occurs did not carry a fire well. Growing conditions on the outcrops were not particularly conducive to the build-up of dry grasses which were the primary fuel of prairie fires. For this reason, prairie fires burn less often and less hot across exposed bedrock.
The situation is somewhat different now. Because wildfires are so aggressively suppressed today, fuels in O. macrorhiza habitats have more years to accumulate and may consist of brush as well as dried grasses. Land managers might be tempted to conduct a prescribed burn to restore the habitat by ridding it of accumulated fuels. Such a burn may have beneficial effects on some aspects of the habitat, but O. macrorhiza may be severely damaged or even killed. It seems O. macrorhiza has no adaptations to survive fire.
It is widely assumed, and with good reason, that Opuntia, with its long, sharp spines is avoided by cattle. However, because Opuntia occasionally produce stems that lack spines, they can become very palatable to cattle (Schifferdecker 1986). How often Minnesota O. macrorhiza occur without spines is not known, but it is perhaps wise to not assume that populations of O. macrorhiza are completely immune to the effect of cattle grazing.
Conservation Efforts in Minnesota
There are currently believed to be occurrences of O. macrorhiza at one State Park, two state Wildlife Management Areas, one National Wildlife Refuge, and two state Scientific and Natural Areas. While these public lands impart some level of protection, no known conservation efforts have been undertaken to specifically manage for the species within these areas.
In 2007, the Renville and Redwood County Soil and Water Conservation Districts in association with state and federal agencies coordinated an initiative to compensate interested landowners for permanently protecting the most ecologically intact rock outcrops and associated wetlands in those two counties. A total of 86 ha (212 ac.) were enrolled into perpetual conservation easements as a result of this initiative, and O. macrorhiza is one of the rock outcrop species most likely to benefit from the program. In 2009, the program was expanded to protect an additional 214 ha (530 ac.) within a five county area of the Upper Minnesota River valley. Conservation plans, which outline conservation practices such as removal of invasive species and livestock exclusion, are being developed for each of the easement sites. Habitat maintenance and improvement measures such as prescribed burning, fencing, rotational grazing, and/or seeding are also included in the plans.
McClain, W. E., and A. C. Koelling. 1992. The Plains Prickly-Pear (Opuntia macrorhiza Engelm.) in Illinois. Castanea 57(4):287-290.
Pinkava, D. J. 2003. Opuntia. Pages 123 -148 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 4. Oxford University Press, New York, New York.
Schifferdecker, R. C. 1986. Opuntia macrorhiza Engelmann: The Bigroot Prickly Pear of the Great Plains grasslands. Cactus and Succulent Journal 58:57-59.