What is a tree? That's a question each of us probably feels we can answer almost instinctively. Images immediately come to mind, most likely of some old and massive, or very tall, trees that have meant something special to us. We probably also have positive feelings about trees as we have learned how they contribute to mankind by providing oxygen, food, shelter, heat, medicines and aesthetic beauty. We may also know that they help prevent soil erosion, clean our air and water, and serve as “carbon sinks” by reducing carbon dioxide in the atmosphere and converting the carbon into their wood. But do we have a definition that is clear and decisive? Would your neighbors define “tree” in the same way that you do?
Actually, there is no universally agreed upon definition of “tree” even in a strictly botanical sense. While it is initially a question of grouping plants with similar characteristics, it is ultimately a function of subjective criteria that change from context to context.
Usually, people think first of form and structure. A tree should be big, but certain species can vary dramatically in size in different environments. Bald cypress (Taxodium distichum) is one of Florida's largest and tallest residents, as well as its oldest. They have been known to reach heights of 118 feet, with a trunk circumference of 425 inches, in Florida, and they grow even bigger in some other states. Yet this same species is called 'dwarf cypress' in an area of Everglades National Park where they often do not even reach 5 feet. Are they not trees?
Some focus on the trunk, picturing a vertical pole of wood from which branches radiate. But must a plant have a main or central trunk to be properly classified as a tree? The Jacaranda genus is characterized by species that tend to have several trunks, often of equal magnitude, yet most of us consider them trees. Even the icon tree of the Gifford Arboretum, Psuedobombax ellipticum, tends to have multiple trunks rather than a clearly dominant leader.
If we define “tree” as having a trunk made of wood, then some plants commonly referred to as “trees” do not qualify. For example, bananas (Musa species) are commonly called “trees,” but their trunks contain nothing that could remotely be called wood. Not only is the substance inside a banana stalk soft and watery, but it consists of fiber that is not bound together like wood. Instead of strength derived from wood, the force that holds a banana stem erect is simply water pressure (called “turgor”) inside the stem. Accordingly, bananas are botanically characterized as large herbs rather than trees. These plants lack lignin, which is the chemical that makes cellulose strong and hard to form wood.
There are also many plants that have very soft wood, but are generally considered to be trees. Example are the baobabs (Adonsonia species) and balsa (Ochroma pyramidale), both of which become large trees. In particular, balsa wood is very soft and flexible, yet it is also one of the strongest woods in the world in terms of tensile strength.
If wood is a necessary component of trees, how should “wood” be defined? Must it have growth rings like many dicots and conifers? But that is a phenomenon primarily of temperate trees. On the basis of wood characteristics, some argue that dicots and conifers are the only true trees. But what about palms? We commonly say “palm tree,” yet the wood of palms contains no rings even though some palms (like the Borassus species) are valuable for timber.
Palms are monocots, but so are grasses, aloes, and lilies which we do not usually think of as trees. Yet some species within those groups should arguably be included as “trees” given their large size. Those include Joshua trees (Yucca breviflia), certain Dracena species that grow quite large, and certain of the larger screw pines (Pandanus species), which can reach more than 35 feet tall.
While grasses are generally considered to be the opposite of trees, bamboo are grasses that can reach very large sizes and those are sometimes referred to as “trees.” Bambusa oldhamii is called the timber bamboo because its stems are both big and strong.
Must plants have seeds to be trees? Tree ferns of the Southern Hemisphere can reach heights of more than 60 feet, but they reproduce through spores and don't consist of what most people would consider wood. Are tree ferns like Cyathea cooperi, which you can see in the Arboretum, “trees”?
Should a “tree” be thought of only as a single organism? Especially in the tropics, trees can literally constitute an entire city as they support a network of other plants and animals. When trees provide favorable conditions for life by other species, they are considered “keystone” species. Yet trees are often dependent on other species. Many trees could not live without other cooperative species with which they have relations, usually mutually advantageous ones. Nature is full of evolutionary co-dependencies in which particular tree species requires one particular pollinator to reproduce (for example, Ficus species). Many trees are dependent on animals for seed dispersal. Oaks and pines are known to be particularly dependent on mycorrhizae, a soil-borne fungi that significantly increases the absorptive area and efficiency of a tree's roots. A single fungal mycelium may extend for acres and also may interact with many plants of different species. Are these linked organisms a tree or many trees? Species evolution can certainly be described as a strange mix of collaborations as well as competitions!
While there is no universally accepted definition of a “tree,” we invite you to explore the many “trees” in the Gifford Arboretum, and consider the important roles these diverse plants play in our lives.