There is no single strongest tree, because “strongest” can mean the hardest wood, the toughest wood, or the tree best at staying upright. The champions for hard, heavy wood are a small group of dense, slow-growing species, led by an Australian tree most people have never heard of. Hard wood alone doesn’t keep a tree standing in a storm, which is why an ordinary-looking tree can outlast a rock-hard one. The densest woods also come at a cost, since the trees that build them grow slowly, and that trade-off shapes which trees fill a forest.
The Tree With the Hardest Wood
Wood hardness is usually ranked with the Janka hardness test, which measures the force needed to press a small steel ball halfway into the wood. A higher number means the wood resists dents and wear better, which is why it appears on flooring labels. Common oak flooring scores roughly 1,300 pounds-force. The test measures one thing, resistance to denting at the surface, so it suits flooring and worktops better than beams.
Australian buloke has a Janka hardness of 5,060 pounds-force, the highest commonly listed.1Ligna. The 10 hardest woods in the world That is close to four times harder than oak flooring. Buloke wood is extremely dense and tough as well as hard.2Ligna. The 10 hardest woods in the world Another contender is Borneo ironwood. Grown in plantations, it has dense wood of about 0.86 grams per cubic centimetre, somewhat lighter than water.3J-STAGE (Tropics). Basic wood properties of Borneo ironwood (Eusideroxylon zwageri) planted in Sarawak, Malaysia Its wood is also uniform from bark to centre, so it is similarly strong all the way through.4J-STAGE (Tropics). Basic wood properties of Borneo ironwood (Eusideroxylon zwageri) planted in Sarawak, Malaysia
If you’re shopping for wood, a high number means resistance to dents from furniture legs, pets and dropped objects. The price is that very hard woods are harder to cut, drill and nail, and they dull tools faster. Hardness says little about whether the wood bends or splits under load.
Hardness is only one kind of strength. Engineers test wood separately for bending, stiffness, crushing and pulling apart, and different species lead on different tests.5Journal of Testing and Evaluation. Measurement of Some Strength Properties and the Chemical Compositions of Seven Hardwood Species Grown in Northwest Egypt These differences come mainly from differences in wood density and chemical make-up.6Journal of Testing and Evaluation. Measurement of Some Strength Properties and the Chemical Compositions of Seven Hardwood Species Grown in Northwest Egypt A wood can top one ranking and sit lower on another, which is one reason lists of “strongest woods” disagree. Before comparing two woods, check which test a list uses.
Lignum Vitae, the Wood That Wears Like Metal
Lignum vitae is a slow-growing tropical tree whose wood sinks in water.7Composites Part B: Engineering. Effects of anisotropy of lignum vitae wood on its tribological performances Its wood is cross-grained, extremely hard and horn-like, and stays heavier than water even when fully dried.8Yale School of Forestry Bulletin. Lignum-vitae: A Study of the Woods of the Zygophyllaceae with Reference to the True Lignum-vitae of Commerce–Its Sources, Properties, Uses, and Substitutes Its heartwood is also soaked with a fragrant resin that gives it an oily look and feel.9Yale School of Forestry Bulletin. Lignum-vitae: A Study of the Woods of the Zygophyllaceae with Reference to the True Lignum-vitae of Commerce–Its Sources, Properties, Uses, and Substitutes
Among woods compared for wear, its cells have the thickest walls, the smallest openings and the tightest packing.10Journal of Cleaner Production. Insight into the influence of the anatomical properties of wood on the tribological properties Together with its high resin and wax content, this gave lignum vitae the best wear resistance of the woods tested.11Journal of Cleaner Production. Insight into the influence of the anatomical properties of wood on the tribological properties When its resins meet water they form a slippery film, so the wood lubricates itself.12Composites Part B: Engineering. Effects of anisotropy of lignum vitae wood on its tribological performances Buloke wins on the hardness number, lignum vitae on wear and self-lubrication, so which counts as strongest depends on whether you need dent resistance or wear resistance.
Lignum vitae was the first material used for bearings lubricated only by water.13Journal of Applied Polymer Science. Bioinspired Water‐Lubricated Bearing Materials of Marine Propulsion for Tribological Properties Advancements It was the only material that could last in the bearing around a steamship’s propeller shaft, where no oil could be used.14Yale School of Forestry Bulletin. Lignum-vitae: A Study of the Woods of the Zygophyllaceae with Reference to the True Lignum-vitae of Commerce–Its Sources, Properties, Uses, and Substitutes It is still known as the bearing material gentlest on steel shafts,15Composites Part B: Engineering. Effects of anisotropy of lignum vitae wood on its tribological performances and engineers now copy its structure to design modern ship bearings.16Journal of Applied Polymer Science. Bioinspired Water‐Lubricated Bearing Materials of Marine Propulsion for Tribological Properties Advancements
What Makes Wood Strong
Wood is made of long hollow cells, and its density depends on how thick their walls are and how wide the hollow centres are.17Forest Ecology and Management. Changes in wood density of Eucalyptus camaldulensis due to temperature – the physiological link between water viscosity and wood anatomy Some of those hollow channels carry water from the roots to the leaves.18Forest Ecology and Management. Changes in wood density of Eucalyptus camaldulensis due to temperature – the physiological link between water viscosity and wood anatomy Woods with thick walls and narrow openings pack more solid material into the same space. In spruce, wood with wider cell openings tested less dense.19PubMed Central. Relationships Between Wood-Anatomical Features and Resistance Drilling Density in Norway Spruce and European Beech The trade-off is that wide channels help move water, so a tree gives up some plumbing capacity when it builds thick walls.
Each cell wall is wrapped in tiny cellulose fibres wound in a spiral and glued together by lignin.20Journal of Building Engineering. Exploring wood micromechanical structure: Impact of microfibril angle and crystallinity on cell wall strength When those fibres run closer to straight up and down, the wood is stiffer.21Journal of Building Engineering. Exploring wood micromechanical structure: Impact of microfibril angle and crystallinity on cell wall strength Young saplings have steeply angled fibres, which makes their wood flexible enough to bend in high winds without snapping.22PubMed. Cellulose microfibril angle in the cell wall of wood fibres Taller species tend to have stiffer wood than shorter species of the same density, likely because their fibres run straighter.23PubMed. Do tall tree species have higher relative stiffness than shorter species?
A tree’s own wood changes as it grows. Young wood near the centre is more flexible to survive bending, while older growth on mature trees is stiffer to support height. So planks from different parts of one tree can differ in strength.
Stiffness rises almost in step with density across trees worldwide.24Journal of Arboriculture. Worldwide Correlation of Mechanical Properties and Green Wood Density for Conifers and Deciduous Trees Even when density is known, though, stiffness can still vary by around 14 percent.25Journal of Arboriculture. Worldwide Correlation of Mechanical Properties and Green Wood Density for Conifers and Deciduous Trees
Which Trees Stand Up Best to Wind
Among rainforest trees, denser and stiffer wood helped trees avoid buckling.26PubMed. Wood mechanics, allometry, and life-history variation in a tropical rain forest tree community But the tree’s proportions, its height compared with its trunk width, mattered most for staying upright.27PubMed. Wood mechanics, allometry, and life-history variation in a tropical rain forest tree community Using wood density alone to judge a tree’s safety is an oversimplification,28PubMed. Integrative biomechanics for tree ecology: beyond wood density and strength partly because denser wood also makes the tree heavier and costlier to build.29PubMed. Integrative biomechanics for tree ecology: beyond wood density and strength Hard-wooded trees can still fail if their trunks are hollow. A slender trunk acts like a tall pole: a gust applies a large bending force at the base compared with how much wood is there to resist it.
Rigid crowns catch more wind than flexible ones. In strong wind, branches and leaves fold back and shrink the area the wind can push on.30Canadian Journal of Forest Research. Wind tunnel measurements of crown streamlining and drag relationships for three conifer species At wind of 20 metres per second, about 72 kilometres per hour, paper birch crowns shrank to 20 percent of their normal face area.31Canadian Journal of Forest Research. Wind tunnel measurements of crown streamlining and drag relationships for several hardwood species Conifers folded less, with lodgepole pine shrinking its face area by 36 percent at the same speed.32Canadian Journal of Forest Research. Wind tunnel measurements of crown streamlining and drag relationships for three conifer species Flexible broadleaf crowns can shed much of a storm’s force.
Trees also straighten themselves. A tilted tree grows special wood to bring itself back toward upright.33PubMed. Reaction Wood: Its Structure and Function Broadleaf trees build it on the upper side and pull, while conifers build it on the lower side and push.34PubMed. Reaction Wood: Its Structure and Function
How Roots Hold a Tree Down
Many tropical trees grow buttress roots, wide fins that flare out from the base of the trunk. Buttressed trees resisted pulling almost twice as strongly as trees without buttresses.35Journal of Experimental Botany. The function of buttress roots: a comparative study of the anchorage systems of buttressed (Aglaia and Nephelium ramboutan species) and non-buttressed (Mallotus wrayi) tropical trees That is because buttresses work under both pulling and pushing and end in deep “sinker” roots.36Journal of Experimental Botany. The function of buttress roots: a comparative study of the anchorage systems of buttressed (Aglaia and Nephelium ramboutan species) and non-buttressed (Mallotus wrayi) tropical trees Buttresses supply about 60 percent of such a tree’s anchorage, around six times more than thin roots give unbuttressed trees.37Journal of Experimental Botany. The function of buttress roots: a comparative study of the anchorage systems of buttressed (Aglaia and Nephelium ramboutan species) and non-buttressed (Mallotus wrayi) tropical trees
A flat fin is far deeper and stiffer than a round root of the same wood, so it resists bending as a plank does when set on edge. When wind tips the tree, the fins on the windward side are stretched like ropes while those on the other side are squeezed like props. The sinker roots at the ends grip deeper soil, which tethers the whole structure. The tree gets a strong anchor without needing deep roots, which helps in tropical forests where soils are often thin.
Buttresses grow fastest along their top edges, where strain from swaying concentrates.38Wind and Trees. Development of buttresses in rainforest trees: the influence of mechanical stress39Wind and Trees. Development of buttresses in rainforest trees: the influence of mechanical stress Scientists still debate exactly what triggers buttresses to form.40Digital Commons @ University of South Florida. Structure and function of buttress roots in tropical oaks of Quercus spp. in windward and leeward cloud forest, August 2010
Why Not Every Tree Grows Super-Strong Wood
Trees with lighter wood grow faster,41Journal of Ecology. The role of wood density and stem support costs in the growth and mortality of tropical trees but they also die more often.42PubMed Central. A trait-based trade-off between growth and mortality: evidence from 15 tropical tree species using size-specific relative growth rates Wood that uses less material per unit volume lets the tree add height and leaf area quickly, so light-wooded trees race into sunny gaps and win early.
Dense-wooded trees add height little by little over many years. Their tough tissue makes them less likely to die from breakage, drought stress or attack, so a tree that survives long enough can still reach the canopy and reproduce for decades. Slow, dense-wooded trees take over in the shade over time.
Water adds another cost. Denser wood tends to carry water less easily, and it also stores less water in its trunk.43PubMed Central. Wood allocation trade‐offs between fiber wall, fiber lumen, and axial parenchyma drive drought resistance in neotropical trees Building very hard wood can limit how fast a tree moves water to its leaves. Slow growth also means decades before a tree yields usable timber, which helped make lignum vitae scarce.
Hard Wood Is Not Always Long-Lasting Wood
Resistance to rot is a separate trait. In tests against common rot fungi, conifer sapwood generally resisted decay better than broadleaf sapwood.44Research.fs.usda.gov. Relative in vitro wood decay resistance of sapwood from landscape trees of southern temperate regions Northern white cedar sapwood resisted every fungus tested, losing little weight after two years.45Research.fs.usda.gov. Relative in vitro wood decay resistance of sapwood from landscape trees of southern temperate regions A soft, light wood can outlast a harder one outdoors. Lignum vitae’s resin helps protect it as well as lubricate it.46Yale School of Forestry Bulletin. Lignum-vitae: A Study of the Woods of the Zygophyllaceae with Reference to the True Lignum-vitae of Commerce–Its Sources, Properties, Uses, and Substitutes
Hardness comes from how much solid wood a tree packs into its cells, while decay resistance depends largely on chemicals the tree deposits in the wood. Fungi that cause rot are discouraged by those compounds, not by density itself. A very hard wood without them can rot quickly, while a lighter wood rich in them can stay sound.
That is also why the part of the trunk matters. Sapwood is the pale outer layer that carries water and still holds stored food that fungi like. Heartwood is the older wood at the centre, where the tree deposits protective chemicals as it ages. In a rot-resistant species, heartwood boards outlast sapwood boards. Cedar heartwood is a common choice for fences and decks for this reason.