The question of whether trees can grow underwater yields a nuanced answer, separating them from truly aquatic plants like kelp or seagrass. Most terrestrial trees cannot survive prolonged submersion. However, a specialized group of tree species has evolved unique biological mechanisms that allow them to thrive in saturated, flooded, or brackish environments. These trees do not grow under the water, but rather in soil that is perpetually or periodically covered by it.
Why Terrestrial Trees Cannot Survive Waterlogging
Most terrestrial tree species cannot tolerate waterlogged conditions because their root systems require oxygen. Tree roots rely on aerobic respiration to generate the energy necessary for growth, nutrient uptake, and maintenance. This process combines sugars with oxygen, which is typically drawn from the air pockets within the soil.
When soil becomes saturated with water, the oxygen-rich air pockets are displaced entirely, leading to anoxia, or a complete lack of oxygen. Water holds significantly less dissolved oxygen than air, and oxygen diffuses through water about 10,000 times slower than through air. Unable to perform aerobic respiration, the root cells of most terrestrial trees quickly suffocate and die.
The lack of oxygen triggers a metabolic shift from efficient aerobic respiration to less efficient anaerobic fermentation. This process produces toxic byproducts, primarily ethanol and lactic acid, which accumulate within the root tissues. These toxic compounds, combined with decay caused by anaerobic soil bacteria, lead to root rot and prevent the tree from absorbing water and nutrients, resulting in the collapse of the organism.
Biological Adaptations for Root Oxygen Access
Flood-tolerant trees overcome the challenge of anoxia through structural and metabolic adaptations focused on oxygen transport and toxin management. A widespread structural change is the development of aerenchyma, a spongy tissue composed of large, air-filled cavities. These channels run longitudinally throughout the plant, acting as a low-resistance internal pathway for gases to diffuse from the above-water parts of the tree down to the submerged roots.
Gas exchange with the atmosphere is often facilitated by specialized pores called lenticels, found on the bark of the trunk and stems above the water line. In response to flooding, these lenticels often become hypertrophied, or enlarged, to maximize atmospheric oxygen intake. The oxygen collected is then directed through the aerenchyma tissue to sustain the respiration of the underground roots.
Some species utilize specialized root structures that physically protrude above the water or mud surface, functioning like natural snorkels. These aerial roots, known as pneumatophores, are characteristic of wetland species, such as mangroves. Pneumatophores are equipped with their own lenticels, allowing them to directly capture atmospheric oxygen and transport it downward to the submerged root system.
Flood-tolerant species also exhibit a higher tolerance for the toxic byproducts of anaerobic metabolism. Some have a greater capacity to convert the toxic ethanol produced in the roots into less harmful compounds. Alternatively, they can transport the ethanol to the stems and leaves, where it is metabolized or vented as a gas. This metabolic tolerance, combined with oxygen transport, allows them to maintain basic energy generation and survive prolonged root submersion.
Examples of Water-Tolerant Tree Species
Mangroves represent a highly adapted group of water-tolerant trees, thriving in coastal, brackish, and saltwater tidal zones globally. Species like the Black Mangrove (Avicennia germinans) rely on numerous, pencil-like pneumatophores that emerge vertically from the mud to acquire oxygen during low tide. Red Mangroves (Rhizophora mangle) use large, arching prop roots or stilt roots that lift the trunk above the water. Lenticels on these aerial structures serve as the primary entry point for gas exchange.
Another prominent example is the Bald Cypress (Taxodium distichum), which dominates freshwater swamps and floodplains across the southeastern United States. This species produces distinctive woody projections known as cypress knees that grow vertically from the root system. While the exact function of these knees remains debated, evidence suggests they may assist in gas exchange, serve as structural anchors in the soft, anoxic mud, or function as storage organs.
Riparian species, such as Willows (Salix spp.) and Alders (Alnus spp.), tolerate seasonal flooding along riverbanks and lake shores. These trees respond to prolonged inundation by rapidly sprouting adventitious roots—new roots that grow directly from the submerged trunk or stem above the original root crown. These newly formed roots contain aerenchyma and can utilize the trace amounts of oxygen dissolved in the floodwater, allowing the tree to continue absorbing water and nutrients until the water recedes.