No tree can live its whole life underwater. Trees are land plants, built to breathe air through their roots and leaves. Even so, many trees can live through floods that last weeks or months, and a few spend their entire lives with their roots in tidal or swamp water. Whether a tree can take being underwater depends on how it gets oxygen, not on how much water it can handle.
Why Water Suffocates Most Trees
Roots breathe. They draw oxygen from air pockets in the soil and use it to burn sugar for energy, and that energy powers nutrient uptake, new root growth, and the upkeep of healthy root cells.1Arboriculture & Urban Forestry (AUF). Soil Aeration, Flooding, and Tree Growth Without oxygen, roots switch to a backup process that releases too little energy to keep nutrient uptake going.2Arboriculture & Urban Forestry (AUF). Soil Aeration, Flooding, and Tree Growth It is a kind of fermentation, and it produces alcohol inside the roots.3PubMed. Effect of flooding on C metabolism of flood-tolerant (Quercus robur) and non-tolerant (Fagus sylvatica) tree species
Trees need water and oxygen from different places. Healthy soil is part water and part air pockets. Flooding fills every pocket, so the roots are wet but airless.
Fermentation gives roots only a small share of the energy they normally make, so stored sugar runs out much faster. Weakened roots can’t feed the tree, and the leaves yellow and wilt.
Water makes things worse by slowing gases to a crawl. Oxygen and carbon dioxide move through water about 10,000 times more slowly than through air.4PubMed Central. Underwater Photosynthesis of Submerged Plants – Recent Advances and Methods A submerged root or leaf can run short of oxygen even when the water holds some. Within about an hour of flooding, a gas the plant makes as a stress signal rises to full strength and starts its flood responses.5PubMed Central. Ethylene-Mediated Acclimations to Flooding Stress
Leaves underwater struggle too. They can barely take in carbon dioxide, so they make far less food.6PubMed Central. Underwater Photosynthesis of Submerged Plants – Recent Advances and Methods Muddy floodwater blocks light as well, cutting food-making even more.7PubMed Central. Response of underwater photosynthesis to light, CO2, temperature, and submergence time of Taxodium distichum, a flood-tolerant tree Some wetland leaves hold a thin film of trapped air that can boost underwater food-making up to sixfold.8PubMed. Underwater photosynthesis and respiration in leaves of submerged wetland plants: gas films improve CO2 and O2 exchange
Above ground, a flooded tree often looks like a drought-stricken one. Leaves wilt, curl, yellow, or drop even though the ground is soaked, because damaged roots can’t move water up to the crown. Symptoms often lag the flood by weeks or a whole season, so a tree that looks fine right after the water drops is not necessarily safe.
Do Any Trees Grow Fully Submerged
When plants moved from water to land, they evolved roots, water-carrying tubes, waxy leaf coatings, leaf pores, and stiff woody tissue.9Encyclopedia of Life Support Systems (EOLSS). Tropical Aquatic Plants: Morphoanatomical Adaptations The very parts that make a tree a tree are built for air, not water.10PubMed Central. Underwater Photosynthesis of Submerged Plants – Recent Advances and Methods
Several things get mistaken for underwater trees. Kelp forests are giant algae, not trees. Seagrasses are flowering plants but not woody. Mangroves stand in water, yet their crowns and breathing roots stay in the air. And photos of flooded forests show trees surviving a season of high water, not growing while submerged.
The closest thing to it is in the Amazon, where some tree seedlings can start growing while fully underwater. Some adult floodplain trees can be covered for months, but they rest rather than grow.
A tree can’t simply switch to water-plant life. Pondweeds have thin leaves with little wax, so they absorb carbon dioxide and minerals straight from the water. A tree’s thick waxy leaf coating stops water loss on land, but it also blocks gas uptake underwater. And a tall woody body needs light above the water to feed its bulk. A tree that stayed submerged would slowly run down its food reserves, even if its roots survived.
How Cypress and Mangroves Breathe in Water
Flooding makes bald cypress grow spongy air channels running through its roots, stems, and leaves.11South African Journal of Botany. Formation and function of aerenchyma in baldcypress (Taxodium distichum (L.) Rich.) and Chinese tallow tree (Sapium sebiferum (L.) Roxb.) under flooding Oxygen can then travel down inside the tree to roots sitting in airless mud.12South African Journal of Botany. Formation and function of aerenchyma in baldcypress (Taxodium distichum (L.) Rich.) and Chinese tallow tree (Sapium sebiferum (L.) Roxb.) under flooding These trees even grow new roots during flooding and leak oxygen into the soil around them.13South African Journal of Botany. Formation and function of aerenchyma in baldcypress (Taxodium distichum (L.) Rich.) and Chinese tallow tree (Sapium sebiferum (L.) Roxb.) under flooding Not every tree can do this. Of six trees tested from Brazil’s dry savanna, only one formed these air channels.14Acta Botanica Brasilica. Anatomical and morphological modifications in response to flooding by six Cerrado tree species Because cypress builds them in response to flooding, a tree that has grown up in wet soil copes better than one suddenly flooded.
Cypress knees, the woody bumps sticking up around a cypress, act like snorkels and carry air to drowned roots.15International Journal of Plant Sciences. Root Aeration Function of Baldcypress Knees (Taxodium distichum) Roots hold much more oxygen when their knee stays above water than when both are covered.16International Journal of Plant Sciences. Root Aeration Function of Baldcypress Knees (Taxodium distichum)
Grey mangroves send up pencil-like breathing roots dotted with tiny pores.17Annals of Botany. Horizontal Structures on Pneumatophores of Avicennia marina (Forsk.) Vierh.–A New Site of Oxygen Conductance18South African Journal of Botany. Ultrastructure of the pneumatophores of the mangrove A vicennia marina Air enters the pores and flows through air channels to the buried roots.19South African Journal of Botany. Ultrastructure of the pneumatophores of the mangrove A vicennia marina After the tide goes out, the pores dry and start working again within about three minutes.20Annals of Botany. Horizontal Structures on Pneumatophores of Avicennia marina (Forsk.) Vierh.–A New Site of Oxygen Conductance Oxygen inside these roots drops only modestly when the tide covers them,21Trees. Gas exchange and oxygen concentration in pneumatophores and prop roots of four mangrove species because sunlit breathing roots make some oxygen themselves, even underwater.22Trees. Gas exchange and oxygen concentration in pneumatophores and prop roots of four mangrove species Breathing roots only help while their tips reach air, so a deep flood that covers the knees or breathing roots cuts the supply.
Trees That Survive Months Under Water
In the Amazon floodplains, seven of eight tree species tested sprouted and formed seedlings in water.23PubMed Central. Seed germination and seedling development in response to submergence in tree species of the Central Amazonian floodplains Depending on species, those seedlings survived 20 to 115 days submerged without root damage.24PubMed Central. Seed germination and seedling development in response to submergence in tree species of the Central Amazonian floodplains Some bent their roots upward toward the surface, likely to reach more oxygen.25PubMed Central. Seed germination and seedling development in response to submergence in tree species of the Central Amazonian floodplains In one species, every seed submerged in water still formed a seedling.26AoB PLANTS. Seed germination and seedling development in response to submergence in tree species of the Central Amazonian floodplains Long flood seasons favor large seeds with big food stores, because a seedling must live on that store until it can reach air and light.
Strategies differ. Some floodplain trees from nutrient-poor black-water forests rest during flooding and hold on to long-lasting leaves.27Aquatic Botany. Seed germination and early establishment of 12 tree species from nutrient-rich and nutrient-poor Central Amazonian floodplains A floodplain population of one Amazon tree kept its backup energy system running for four months of flooding, but an upland population of the same tree died after its backup energy faded about 15 days in.28Aquatic Botany. Adaptive strategies to tolerate prolonged flooding in seedlings of floodplain and upland populations of Himatanthus sucuuba, a Central Amazon tree
Species matter too. Flood-tolerant oak keeps its roots’ backup energy going, while flood-sensitive beech roots run out of fuel for it.29PubMed. Effect of flooding on C metabolism of flood-tolerant (Quercus robur) and non-tolerant (Fagus sylvatica) tree species
How Mangroves Handle Salt Water
Waxy barriers in one mangrove’s roots keep out 90 to 95 percent of the salt.30PubMed. Role of root hydrophobic barriers in salt exclusion of a mangrove plant Avicennia officinalis The outer root layer also carries an electric charge that holds back sodium at the surface.31PubMed Central. Novel water filtration of saline water in the outermost layer of mangrove roots Some mangroves push leftover salt out through their leaves.32PubMed. Role of root hydrophobic barriers in salt exclusion of a mangrove plant Avicennia officinalis Most other trees can’t do this, so saltwater kills them.33Canadian Journal of Earth Sciences. A large earthquake occurring 700–800 years ago in Aialik Bay, southern coastal Alaska
Salt can even replace freshwater forests. In a Florida river, reduced freshwater flow let salt creep in, and mangroves replaced bald cypress swamp.34PubMed. Linking river, floodplain, and vadose zone hydrology to improve restoration of a coastal river affected by saltwater intrusion
Flood tolerance and salt tolerance are separate traits. Cypress tolerates floods but not much salt, while mangroves have both. After a storm surge, salt damage shows up as browning leaf edges or sudden needle loss in evergreens. Flushing with fresh water can help, especially in sandy soils, but salt-sensitive trees may still die slowly over several seasons.
What Flooding Does to a Yard Tree
Flooded trees are also prone to Phytophthora root rot, caused by a water mold that thrives in soggy soil.35Bartlett Tree Experts. Phytophthora root rot disease Its spores swim through water to infect fine roots, then it can spread into larger roots, the trunk base, and the stem, eventually killing the tree.36Bartlett Tree Experts. Phytophthora root rot disease In oaks, it kills root tips and damages bark at the trunk base.37European Journal of Forest Pathology. Isolation, identification and pathogenicity of Phytophthora species from declining oak stands
Longer floods mean much more disease. In young apple trees flooded weekly, trunk-base rot hit about 3 percent with no flooding but 50 percent with 3-day floods each week.38Journal of the American Society for Horticultural Science. Incidence and Severity of Crown and Root Rots on Four Apple Rootstocks Following Exposure to Phytophthora Species and Waterlogging
Large floods kill trees, especially young ones. After a major Amazon flood worsened by dams, tree deaths rose almost fivefold, from about 3 to 15 percent.39Journal of Applied Ecology. Interaction between extreme weather events and mega‐dams increases tree mortality and alters functional status of Amazonian forests Most of those deaths were smaller trees.40Journal of Applied Ecology. Interaction between extreme weather events and mega‐dams increases tree mortality and alters functional status of Amazonian forests Even flood-adapted trees died after several years in a row of flooding near one dam.41Forest Ecology and Management. Tree mortality of a flood-adapted species in response of hydrographic changes caused by an Amazonian river dam Species matter here too: bald cypress and many oaks handle wet feet, while beech and many fruit trees do not.
Once the water drops, avoid walking, driving, or parking on the soggy root zone, because wet soil compacts easily and squeezes out the air roots are trying to regain. Don’t fertilize a stressed tree, since damaged roots can’t use the nutrients and fertilizer adds stress. Over the next months, watch for wilting, yellowing, early leaf drop, or dark, damp bark at the trunk base, since damage can appear late. Fix drainage where water pools by regrading or diverting runoff, so the next rain doesn’t repeat the problem. If the trunk base looks sunken or oozing, call a certified arborist.
Why Drowned Forests Last for Centuries
The fast wood-rotting fungi that break down logs on land need oxygen.42IAWA Journal. MICROMORPHOLOGICAL CHARACTERISTICS OF WOOD BIODEGRADATION IN WET ENVIRONMENTS: A REVIEW Wet, low-oxygen wood is eaten only by slower fungi and bacteria, so wood buried in ocean mud can last hundreds or thousands of years.43IAWA Journal. MICROMORPHOLOGICAL CHARACTERISTICS OF WOOD BIODEGRADATION IN WET ENVIRONMENTS: A REVIEW Wood kept in a place that dries out and floods again rots faster, since the fungi get air back each time.
Earthquakes can sink whole forests. In Alaska, a coastal conifer forest was buried by beach sediment about 800 years ago after the land dropped 1 to 3.5 meters.44Canadian Journal of Earth Sciences. A large earthquake occurring 700–800 years ago in Aialik Bay, southern coastal Alaska On Washington’s coast, buried forest soils show land dropping into tidal water up to three times in 2,000 years.45Journal of Geophysical Research: Solid Earth. Geologic evidence for earthquakes during the past 2000 years along the Copalis River, southern coastal Washington Ghost forests of dead standing trees can therefore be a record of past quakes and tsunamis.