Trees have existed since the Devonian period of the Paleozoic era (Geologic Time Scale)/ The oldest trees known to paleobotanists are those of the genus Cordaites, which originated in the early Devonian period and became extinct by the end of the Paleozoic era. The oldes known surviving order o f trees, the broad-leaved gymnospermous Ginkgoales, is now represented by a single species, the maidenhair tree, Ginkgo biloba (Ginkgo). Coniferous trees have existed since the middle of the Carboniferous period. Angiospermous trees first appeared in the lower Cretaceous period of the Mesozoic era, and by the beginning of the Pliocene epoch of the Cenozoic era virtually all tree genera now in existence were growing profusely. The majority of fossil tree leaves found in Pliocene rocks are indistinguishable from leaves of present day trees.

Trees come in different shapes and sizes, from gnarled old bristlecone pines to prickly saguaro cactuses. Worldwide there are over 20,000 different species of trees. Over 800 grow in North America. Trees are classified according to how they reproduce, what types of flowers and seeds they have (if any), how they grow, and how they are structured inside. (Sometimes trees that look very much alike are not closely related.)

GROUPS: Most trees fall into these two major plant groups: Gymnosperms (jim-no-sperms) and Angiosperms (An-gee-oh-sperms): Gymnosperms have “naked” seeds, which means the seeds are enclosed in flowers and later fruit. (Gymnosperms do not produce true flowers or fruit.) In most gymnosperms, the seeds are produced on the surface of the scales of female cones. Most gymnosperms are pollinated by the wind. Conifers are the most common types of gymnosperms. Pines, hemlocks, redwoods, spruces, and firs are all types of conifers. Coniferous trees are also called needle-leaved trees because most have thin, needlelike leaves. There are about 500 species of conifers in the world.
Angiosperms are the only types of plants that have true flowers and bear their seeds in fruits. There are over 235,000 species of angiosperms. Oaks, willows, maples, birches, palms, and all other broad-leaved trees (trees with flat, broad leaves) are in this plant group. So are all the flowering non-trees, such as tulips, blackberries, poppies and so on.

CLASSIFICATION: All trees are seed bearing plants, either gymnosperms, mostly cone-bearing plants commonly called softwoods, or angiosperms, which are flowering plants, the trees of which are commonly called hardwoods (Angiosperm; Gymnosperm). The angiosperms are further divided into two classes, the Liliopsida (monocots) and the Magnoliopsida (dicots), depending on seed structure. Of the 60,000 to 70,000 species of trees, all are dicotyledonous except a few hundred monocotyledonous species and less than 1,000 gymnospermous species.

All five living orders of gymnosperms consist primarily of tree species; the most important Gymnosperm orders are the Pinales and Taxales, comprising the conifers (Conifer). Among the angiosperms, few tree species are monocots. The only monocotyledonous family containing a preponderance of tree species is the palm family, Areceae, the genera of which are native to tropical and subtropical regions throughout the world. The dicots include most of the broad-leaved trees, which are distributed throughout the world.

In the U.S., native trees belong to about 850 species, which are classified in 222 genera and 69 plant families. Of this total, about 300 species belong to the oak genus, Quercus (Oak). Other large genera of American trees are Crataegus, the hawthorns; Pinus, the pines, and Salix, the willows (Hawthorn, Pine, Willow). About 110 other species native to the U.S. are tropical or subtropical trees restricted to Florida. In addition, about 60 species of trees from Europe and Asia are naturalized in the U.S. after centuries of cultivation, and more than 200 species of foreign trees are commonly grown in the U.S. as ornamental shade and fruit trees. Tree species make up about 3.5 percent of the plant species found in the U.S.

There are all kinds of general tree terms floating around that can make learning about trees pretty confusing. We’ve already talked about needle-leaved trees and broad-leaved trees. Here are a few more tree terms that you’ll often see:

Hardwoods: Foresters often call broad-leaved trees “hardwoods: because most broad-leaved trees have harder wood than do needle-leaved trees. For example, maples and oaks are known for their tough, hard wood and are often used to make high-quality furniture and floors. But the term hardwood is confusing because some broad-leaved trees, such as cottonwoods and magnolias, have very soft, lightweight wood.

Softwoods: Softwood is another confusing term because not all softwoods are soft. Foresters use the term softwood to describe needle-leaved trees, such as pines, spruces, and redwoods, because most have softer wood that do broad-leaved trees. But some softwoods, such as yellow pines and yews have very hard wood.

Although there are thousands of different kinds of trees in the world, most trees work in much the same way. Here’s a look at how the parts of a tree work together to help a tree get the food, water and minerals it needs to survive.

TRUNK: The trunk of a tree is important for two reasons: First, it acts as a support rod, giving the tree its shape and strength. Second, it acts as the central “plumbing system” in a tree, forming a network of tubes that carries water up and down to the branches, trunk and roots.
The easiest way to see how a tree works is to look at a cross section of a trunk. Here are the five main layers you would see, and what each layer does,

1. Barking Up the Right Tree: The outer layer of the trunk (and branches) is called the outer bark or just the bark. Tree bark can be smooth, scaly, rubbery, flaky, craggy, or bumpy. Its texture, thickness and flexibility depend on the type of tree. Although bark looks different from tree to tree, it serves the same purpose – to protect the tree from injury and disease. Some trees have very thick bark that helps prevent damage from fires. Others have bad-tasting chemicals in their bark that discourage hungry insects. And some bark is covered with spines or thorns that keep browsing mammals away.
The bark of large Douglas firs and sequoias may be more than two feet (.6 m) thick.

2. Food is for Phloem: The layer next to the outer bark is called the inner bark, or phloem (FLOW-em). The phloem is a thin layer that acts as a food supply line from the leaves to the rest of the tree. Sap (water containing dissolved sugars and nutrients) travels down from the leaves through channels in the phloem to the branches, trunk and roots, supplying all the living parts of the tree with food. At certain times during the year, the phloem also transports stored sugars up from the roots to the rest of the tree. (If you were to cut a band around the trunk, through the bark and phloem, the tree would probably due. That’s because the phloem would be severed and food could no longer flow to the lower trunk and roots.)

3. Keep’em Coming Cambium: Next to the phloem is a very thin layer called the cambium. (It is often only one or two cells thick, and you need a microscope to see it well.) The cambium is one of the growing layers of the tree, making new cells during the growing season that become part of the phloem, part of the xylem, or more cambium. The cambium is what makes the truck, branches, and roots grow thicker.

4. Up, Up and Away with Sapwood: The layer next to the cambium is called the sapwood or new xylem. The sapwood is made up of the youngest layers of wood. (Each year the cambium adds new layers of woody tissue.) The sapwood is a network of thick-walled cells that forms a pipeline, carrying water and minerals up the tree from the roots to the leaves and other parts of the tree. The sapwood also stores nutrients and transports them across the tree, from one part to another.

5. A Dead Heart: Most of the trunk in an old tree is dead wood called heartwood, or just plain wood. The heartwood is old xylem that no longer transports water and minerals up a tree. (After a few years the sapwood in most trees gets filled in with resin-like material and slowly changes into heartwood. The new xylem is the only part of the wood that works as a transport system.) The heartwood is often much darker in color than the sapwood. The heartwood gives the tree support. But sometimes it rots away, leaving a hollow, living tree. Hollow trees often topple over or split apart in storms because they are very weak after the heartwood has decayed.

Note: Palm trees have a different type of structure than most other trees. They do not have real branches and produce no annual rings. Instead, they grow taller without growing thicker.

ROOTS: A trees’ roots are long, underground branches that spread out to help anchor the tree and to absorb water and nutrients from the soil. Some trees have long taproots that reach straight down for 15 feet (4.5 m) or more. Other trees have more shallow root systems that lie closer to the surface of the ground. Large taproots and lateral roots branch into smaller and smaller roots. An average tree has millions of these small rootlets, each covered with thousands of fine root hairs. The root hairs make it easier to soak up water and dissolved minerals from the soil. (Most of the rootlets lie very close to the surface of the ground where most of the water and nutrients are located.)

LEAVES: From skinny pine needles to broad palm leaves, all tree leaves serve the same purpose – to make food for the tree. Leaves use carbon dioxide from the air, water from the roots, and sun’s energy (in the form of sunlight), to make sugar (glucose). This food-making chemical reaction is called photosynthesis. Photosynthesis can take place only in the presence of chlorophyll – the green pigment that is found in all green plants. Chlorophyll absorbs the sunlight needed for photosynthesis. During photosynthesis the leaves release oxygen that becomes part of the air that we and other animals breathe

Trees, like all living things, could not survive without water. Here are some of the reasons that water is so important to a tree:

1. A large percentage of each living cell in a tree is made up of water
2. Water helps move dissolved minerals and gases from cell to cell
3. Water pressure inside a leaf’s cells helps maintain the leaf’s shape
4. Water is needed in order for photosynthesis to occur (water for photosynthesis is carried up through the xylem network from the roots)
5. Water carries dissolved sugars (made during photosynthesis) down through the network of phloem to the branches, trunk and roots

Although trees use a lot of water every day, they also lose a lot of water. About 99% of the water the roots absorb from the soil evaporates from the leaves through a process called transpiration. (Water evaporates through tiny pores in the leaf (stomata) as carbon dioxide -also needed for photosynthesis - rushes in. During photosynthesis there is a trade-off between water loss and carbon dioxide gain.) As water evaporates, it pulls up more water from the roots to the leaf. This “transpiration pull” is one of the things that help move water and minerals through the tree and help keep trees cool in hot weather.

Trees grow wherever adequate groundwater is available for the major portion of the year. Trees do not grow profusely in desert areas or in areas in which the groundwater table is sufficient only for grassland vegetation; in such areas trees grow successfully only under careful cultivation, in desert oases, or along the banks of rivers and streams. Moreover, in areas bordering a grassland or desert, trees are frequently stunted and gnarled in growth. In high mountains or at the edge of the northern coniferous forests, such scattered, stunted, twisted trees are called krummholz. Under optimum conditions, however, trees grow in large aggregations called forests.

The climatic and soil requirements of trees are somewhat different for each species. Most tree species grow over large areas, of which only a small portion permits optimum growth of the plant. The most common tees species in a given area is called the dominant species of the area. In eastern U.S., for example, spruce and fir are dominant in Maine and northern New York; beech, birch and maple in the southern portions of New York, Michigan and Wisconsin; longleaf, loblolly and slash pines in the Gulf States; and oak and hickory in most of the remaining areas of the eastern U.S.

Trees live longer than most other plants. In fact, some of the oldest living things on earth are trees. For example, bristlecone pines can live for over 4500 years! Unlike annuals (pants that sprout, reproduce and die in one season) and biennials (plants that sprout, reproduce and die in two seasons), trees are a type of perennial. Perennials live for many seasons. Non-woody perennials, such as lilies and irises, die back each year and pass through the dormant season as underground roots, stems, bulbs or tubers. But trees don’t die back. Many do become dormant during the winter, but the stems, branches and twigs (as well as the roots) are still alive and will continue to grow taller and thicker each year. And because of their unique “plumbing” and their strong, woody support system, trees can survive much longer than other perennials.
The world’s most massive living organism in the world is the Quaking Aspen grove that covers 106 acres and is made up of 47,000 tree trunks.