The Chinese brake fern, Pteris vittata, is a unique species that has garnered scientific attention. This interest stems from its remarkable biological capability, which distinguishes it from most other plants. Its distinctive features and widespread presence make it a subject of ongoing study.
Identification and Natural Habitat
Pteris vittata is identified by its long, ladder-like fronds, known as pinnae, which are typically dark green and divided once. Spores form noticeable lines along the edges of each pinna, a distinguishing characteristic. Individual fronds can reach lengths between 0.15 and 0.8 meters, with pinnae extending up to 25 centimeters.
This fern is naturally widespread across the paleotropics, with its native range encompassing tropical and subtropical regions of Asia, Africa, southern Europe, and Australia. Beyond its indigenous habitats, Pteris vittata has become widely naturalized as an introduced species in various parts of the world, particularly in the southeastern United States, including Florida, Hawaii, Cuba, and Brazil. The fern frequently colonizes disturbed environments, thriving on calcareous substrates such as limestone, roadsides, and old masonry. It can also be found growing in pavement cracks, on building walls, and in pinelands in South Florida.
The Phenomenon of Arsenic Hyperaccumulation
Pteris vittata is recognized as the first discovered plant capable of arsenic hyperaccumulation. A hyperaccumulator is a plant that can absorb and concentrate a specific element in its tissues at levels hundreds or thousands of times higher than the surrounding soil. The fern’s root system actively draws arsenic from the soil, a process facilitated by specific transport mechanisms.
Once absorbed, the arsenic is transported upwards through the plant and stored primarily in its fronds. This leads to high concentrations of the metalloid within the fern’s aerial parts. In controlled hydroponic experiments, Pteris vittata has been observed to accumulate up to 27,000 milligrams of arsenic per kilogram of dry weight in its fronds. Such concentrations are significantly higher than the levels that would be toxic to most other plant species, which typically show phytotoxicity at 5 to 100 milligrams per kilogram of dry weight. The ability of this fern to tolerate and sequester large amounts of inorganic arsenic, often in the arsenite form, represents a biological adaptation.
Practical Uses in Environmental Cleanup
The ability of Pteris vittata to absorb and concentrate arsenic has led to its application in phytoremediation. Phytoremediation is an environmental cleanup technique that uses plants to remove, degrade, or contain contaminants in soil, water, or air. Pteris vittata is employed for removing arsenic from contaminated land and water bodies.
The process involves planting the ferns directly in an area affected by arsenic contamination, such as former industrial sites, mining waste areas, or agricultural lands with elevated arsenic levels. Once the fronds have accumulated sufficient arsenic, they can be harvested and safely disposed of, effectively removing the contaminant from the site. This harvesting can occur multiple times a year without negatively impacting the fern’s health or its capacity to continue accumulating arsenic. Field studies have demonstrated that Pteris vittata can reduce mean surface soil arsenic concentrations from 190 to 140 milligrams per kilogram at sites contaminated with chromated copper arsenate.
Cultivation and Invasive Potential
For those cultivating Pteris vittata, understanding its preferred growing conditions is beneficial. The fern thrives in alkaline soils and prefers calcareous substrates, such as limestone. It requires good drainage to prevent waterlogging and tolerates a range of light conditions from partial shade to full sun. It is hardy in USDA zones 8a through 11.
Despite its beneficial uses, Pteris vittata is recognized as a potentially aggressive plant in some regions. It is categorized as an exotic species in Florida, where its abundance has increased, and is considered invasive in Hawaii, Cuba, and Brazil. Its robust nature and prolific spore production enable it to spread rapidly, with spores dispersed by wind, water, or human and animal activity. This vigorous growth allows it to outcompete native plant species, potentially altering local ecosystems. If found on a property, containing its spread can involve mechanical methods such as mowing or pulling fronds, which helps reduce spore production and manage populations.