In addition to having the widest geographic range and the greatest number of species among the gymnosperms, the cone-bearing trees—the conifers—include both the tallest and the oldest living organisms on earth (FIGURE 12-18). The four tallest trees in the world are conifers: a coast redwood that is 380 feet (116 m) tall, a Douglas fir and a Sitka spruce, each 318 feet (97 m), and a Sierra redwood at 311 feet (95 m). But not all conifers are big: there are also miniature species of conifers, such as the shore pine, which can be just 20 centimeters tall. The oldest tree trunk belongs to a Great Basin bristlecone pine, the Methuselah tree, which has lived for more than 4,800 years (a slightly older tree was cut down in 1964). There’s more to a tree than its trunk, though, and trunks can be replaced. That is what a Norway spruce in Sweden has done: the current trunk is about 600 years old, but the roots are 9,550 years old. This tree has persisted by sending up a new trunk each time the old one died.

Figure 12-18Towering and ancient gymnosperms.

Trees can grow large and live to great ages because woody plants can be exceptionally strong and resistant to attack by herbivores. A cross section of a tree trunk shows the structural characteristics that allow a tree to stand erect and transport water to leaves more than 100 meters above the ground. Rigidity is provided primarily by the heartwood, which is a core of dead tissue that contains complexly cross-linked molecules.

Bark covers the outside of the tree trunk and branches. The outer layers of bark are dead tissue that can be shed without damage to the tree, protecting the living tissue from attack by plant-eating insects. Part of the success of conifers can also be traced to their ability to defend themselves by exuding a sticky pine pitch that can engulf and smother insects.