St. Thomas Prickly-Ash

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St. Thomas Prickly-Ash

Zanthoxylum thomasianum

ListedDecember 20, 1985
FamilyRutaceae (Citrus)
DescriptionEvergreen shrub or small tree up to 20 ft (6 m) in height with shiny pinnate leaves.
HabitatOpen deciduous forests in limestone soils.
ThreatsLimestone mining, road maintenance.
RangePuerto Rico, U.S. Virgin Islands


St. Thomas prickly-ash, Zanthoxylum thomasianum, is an evergreen shrub or small tree, often multi-stemmed, 6-20 ft (2-6 m) in height; stem diameter reaches about 4 in (10 cm) at breast height. Pinnate compound leaves, with five to nine shiny, stalkless, rounded leaflets, grow alternately along the stems. Each leaflet has several sharp spines at the base and several more on the mid-vein beneath. Flowers are minute, clustered, and unisexual with male and female parts on different plants (dioecious). Fruits consist of one to three egg-shaped follicles from each flower that split along one side to disgorge a single black, shiny seed.


Prickly-ash is found in coastal, mixed deciduous, subtropical, dry forests in soils derived from limestone or volcanic rock. The dry season extends from December through April; the other months receive rain but the hard easterly winds that are almost constant, keep the forest dry even during the rainy season. The St. Thomas Island population occurs in moderately sloping to steep, well-drained soils that are shallow over partially weathered, basic volcanic rocks.


St. Thomas prickly-ash is found on the island of St. Thomas in the U.S. Virgin Islands near Charlotte Amalie, where large tourist hotel complexes have recently been built or expanded. A few days before this species was federally listed in 1985, half of a population of 300 plants was bulldozed to make way for vacation cottages. The principal site on the island of St. John, with about 50 plants, is located in the Gift Hill area of Fish Bay Estates.

The plant is also found on the island of Puerto Rico but is considered nearly extinct there. A few plants still survived in 1986 at the summit of Piedras Chiquitas between Salinas and Coamo and along Road 155 north of Coamo. A third site, also consisting of a few plants, was located in the upper portion of the Guajataca Gorge near Isabela.

Its multi-stemmed growth habit makes estimating individual plants difficult, but the total range-wide population was probably fewer than 500 plants.


The major threat to this species is loss of habitat to residential development and to limestone mining. Road maintenance has also destroyed some plants. Population sites in both St. Thomas and St. John have been subdivided into building lots, and since all populations are on private land, habitat protection is difficult. The St. Thomas population is located near the capital, Charlotte Amalie, and some of the principal hotel complexes. A population in Coamo, Puerto Rico, was destroyed by road improvement and by illegal trash dumping and burning. Because most of the populations occur on private lands, federal regulations do not assist much in the plant's recovery.

St. Thomas prickly-ash populations are compact and localized so that reproduction results in loss of genetic viability. The three Puerto Rican populations are so small they may not be able to reproduce at all.

Because the populations are so small, hurricanes pose a serious threat; the Piedra Chiquitas population was virtually destroyed. Individual plants in exposed areas are also adversely affected by high winds.

Conservation and Recovery

Populations in Puerto Rico are probably too small to guarantee reproduction. Exposed plants have suffered severe wind damage from past hurricanes. Attempts to propagate the plant from cuttings or seed have so far been unsuccessful although an ongoing propagation program is carried out by Fairchild Tropical Garden in Miami.


U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd, Ste. 200
Atlanta, Georgia 30345

U.S. Fish and Wildlife Service
Boquerón Field Office
P.O. Box 491
Boquerón, Puerto Rico 00622


Vivaldi, J. L., and R. O. Woodbury. 1981. "Status Report on Zanthoxylum thomasianum (Krug and Urban) P. Wilson." U.S. Fish and Wildlife Service, Mayaguez, Puerto Rico.

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St. Thomas Prickly-Ash

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