The Serpent's Garden
In the northern reaches of Myanmar's Sagaing Region, the Htamanthi Wildlife Sanctuary protects a vast expanse of forest covering 2,151 square kilometers. The sanctuary's geology is part of the Naga Hills Ophiolite, a section of ancient oceanic crust and upper mantle pushed onto the continent during the collision of the Indian and Eurasian plates. This geological history created large areas of ultramafic rock, which weathers into a challenging soil known as serpentinite.
Serpentine soils are notoriously difficult for most plants to colonize. They are characterized by a toxic imbalance of minerals: high concentrations of heavy metals like nickel, chromium, and cobalt, coupled with a deficiency in essential plant nutrients such as calcium, nitrogen, and phosphorus. The very name "ophiolite" hints at the rock's nature, derived from the Greek word ophis for snake, referring to the serpent-like green, mottled texture of the serpentinite rock. In this inhospitable environment, a specialized group of plants has evolved to survive and thrive by embracing the metal.
Botanical Metal Miners
The flora adapted to Htamanthi's ultramafic terrain includes a remarkable group of plants called hyperaccumulators. These species have developed a physiological mechanism to absorb extreme amounts of heavy metals from the soil and store them in their tissues—leaves, stems, and sap—at concentrations that would be lethal to other organisms. Globally, there are around 750 known species of hyperaccumulators, and the majority of these specialize in nickel.
While specific hyperaccumulating species within Htamanthi are still under study, related families known for this trait, such as Phyllanthaceae and Salicaceae, are present in the region. These plants actively draw nickel ions up through their roots and sequester them in cell vacuoles, effectively isolating the toxic metal from their metabolic processes. The concentration can be extraordinary, reaching up to 3% of the plant's total dry weight. Some related species in Southeast Asia have sap that is green or blue-green due to the high nickel content. A tree in New Caledonia, Pycnandra acuminata, has latex that contains up to 25% nickel. This adaptation provides a powerful defense against insects and herbivores that have not evolved a tolerance to the metal-laced vegetation. This process of using plants to extract metals has led to a new field of research called phytomining, which explores using these "metal crops" to harvest valuable elements from the ground.