GENERAL INTRODUCTION TO TREES
Trees are defined as woody perennial plants, typically having a single stem or trunk growing to a considerable height, and bearing lateral branches at some distance from the ground. Yet trees seem to defy a simple a definition. They belong to many different plant families and are not a single scientific group of their own. Some trees are closely related like Scott’s Pine and Stone Pine. Others are not closely related at all, like Incense Cedar and Silver Maple.
A simple, yet important, classification scheme is based on reproductive characteristics. This scheme defines trees as either cone-baring (conifers) - no flowers or fruits, but with seeds typically found on the bottom of individual scales that make up the cone (gymnosperms) and flowering plants that have seeds that are enclosed within an ovary, and forming a fruit (angiosperms).
Trees carry out the life-processes that all plants share (e.g. photosynthesis, need for nutrition, reproduction, etc.); however, they have a variety of ways in which they meet their biological needs. In a practical sense, trees can be viewed in a functional manner (e.g. how do they survive and grow, what ecosystem services do they provide and their benefits to humans).
It is projected that there are just over three trillion mature trees in the world, of which 1.4 trillion (46%) are in the tropics or sub-tropics, 0.61 trillion (20%) in the temperate zones, and 0.74 trillion (24%) in the northern coniferous boreal forests. Also, it has been calculated that about 15 billion trees are cut down annually and about 5 billion are planted. It is estimated that in the 12,000 years since the start of human agriculture, the number of trees worldwide has decreased by 40-50%.
While the common life-spans of trees range from 50-250 years there are some that are very long-lived, some reaching several thousand years old. These extremely long-lived trees are among the oldest organisms now living. For example, a 16-foot spruce tree in Sweden is 9,550 years old; Methuselah, a bristlecone pine tree from California’s White Mountains, is thought to be almost 5,000; in North Wales there is Llangernyw, a lush, 4,000-year-old yew tree, and the Zoroastrian Sarv is a 4,000 year old cypress tree in central Iran.