Key Tree-cactus

Pilosocereus robinii

Description

Key tree-cactus, Pilosocereus robinii, is the largest of the native Florida cacti. It is characterized by erect, branched stems that can reach a maximum height of 30 ft (10 m). These succulent stems are cylindrical, spiny, and light or bluish green, measuring up to 4 in (10 cm) in diameter. Attractive flowers vary in color from white to pale green or purple, average 2.5 in (6 cm) across, and open in late afternoon or early evening. The fruit is a dark red berry that measures 2 in (5 cm) in diameter. Key tree-cactus is the only native Florida cactus that stands erect at maturity and is sometimes considered a tree. Plants are either self-pollinated or pollinated by moths.

Habitat

The Key tree-cactus grows in the rocky tropical hammock habitat of the Florida Keys. These hammocks are isolated groups of hardwoods amid freshwater or saltwater wetlands. The upper Keys support high hammocks with tree canopies 100-132 ft (30-40 m) high, dominated by gumbo limbo, pigeon plum, poisonwood, mahogany, and other large trees. The hammocks of the lower and middle Keys have canopies in the 66-79 ft (20-24 m) range, and are sometimes called thorn scrub or thorn forest. Much of the flora of the Florida Keys is derived from tropical species of the West Indies rather than from temperate forms that usually dominate communities of the Florida peninsula.

Key tree-cactus is typically found on Key West oolite (a type of dolomite) and on the limestone soils of Key Largo. The soil layer consists of partially decomposed organic matter resting directly on a porous limestone substrate. This thin organic layer is necessary to support the plantlife of hammocks. Where the tree canopy is closed it forms an insulative environment, moderating weather extremes and reducing the loss of soil moisture.

Distribution

This unique cactus occurs in the Florida Keys, where historically it was known from 11 sites, and in northwestern Cuba from two sites. Plant communities associated with Key tree-cactus have largely disappeared from the Keys and Cuba due to development and urbanization, and the tree-cactus itself is near extinction.

Five Key tree-cactus populations survive in remnant habitat in the Florida Keys. The loss of hammock acreage is estimated at 80-90%. Twelve likely areas within the historical range were searched in June 1979, but the cactus was found in only four. One of these sites, on Layton's Hammock, was visited again in August 1979, and most of the hammock and its vegetation had been bulldozed for residential construction. The plants on this site were presumed eliminated, but several survivors were re-discovered in 1982. A fifth site was discovered on private property in 1982. The largest populations occur in the southeastern corner of Big Pine Key, most of which falls within the boundaries of the National Key Deer Wild Refuge.

In Cuba, housing and resort development have destroyed a large percentage of the species' historic habitat, and it is now considered endangered there by the International Union for the Conservation of Nature and Natural Resources.

Threats

Of the remaining tree-cactus populations in the Keys, three occur on privately-owned lands that are in imminent danger of being developed for housing.

A major problem for the Key tree-cactus is its restrictive habitat requirements. It grows only on lightly shaded upland sites. Rare in the Keys, this habitat type is typically covered by dense growths of tropical hardwoods, which produce too much shade for the tree-cactus. Furthermore, habitat conditions are continually changing in the Keys, and what may be suitable habitat in one year may evolve into less suitable habitat in a short time.

Like many other species of cacti, Key tree-cactus is vulnerable to collectors. Even on public lands, enforcing prohibitions against collecting has been difficult. Evidence of vandalism has been noted from one site, where deep and damaging initials were carved in the trunks of several plants. Natural factors, such as hurricane wind damage, also threaten this species.

Conservation and Recovery

One population occurs on federal land in the National Key Deer Refuge administered by the U.S. Fish and Wildlife Service. The habitat is managed to benefit the cactus. Another population is protected on Florida state lands, but the habitat is not actively managed.

The Recovery Plan recommends rehabilitating all five Key tree-cactus sites by controlling non-native vegetation; restricting agricultural practices; clearing land; application of herbicides; restricting off-road vehicles; and preventing salt water intrusion. The plan also recommends purchasing the habitat site on Big Pine Key.

Contact

Regional Office for Endangered Species
U.S. Fish and Wildlife Service
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
http://southeast.fws.gov/

References

Austin, D. F. 1980. "Endangered and ThreatenedPlant Species Survey in Southern Florida and the National Key Deer and Great White Heron Wildlife Refugees, Monroe County, Florida." Report. U.S. Fish and Wildlife Service, Jacksonville.

Little, E. L., Jr. 1975. "Our Rare and EndangeredTrees." American Forests 81(7):18.

U.S. Fish and Wildlife Service. 1986. "Recovery Plan for the Key Tree-Cactus." U.S. Fish and Wildlife Service, Atlanta.

Ward, D. B. 1979. Rare and Endangered Biota of Florida; Vol. 5, Plants. University Presses of Florida, Gainesville.