Tree Growth
Growth of a tree occurs from division of cells in the meristematic tissue in the cambium and the tips of the branches and roots. Cambial cell division results in an increase in the tree diameter. The xylem cells added to the stem usually differentiate into thin walled springwood and thick walled summerwood that produce the annual rings visible in cross sections of most tree trunks from temperate areas. The meristems associated with the crown produce elongation of the branches, resulting in increases in tree height.
Threes are potentially immortal because meristematic tissue is retained throughout the life of the tree. Death usually result from some environment agent, such as fire, wind, lightning, drought, or human cutting, or from a biological cause, such as disease or insect attack. As a tree ages it may become more susceptible to insect, pest or diseases, such as bark beetles or fungal root rots, which contribute directly or indirectly to its death. Most trees actually die to several causes.
Ecology and Distribution
Moisture, temperature and nutrient condition are the most important environmental factor affecting the establishment and growth of tree species. Forest is widely distributed in the temperate and tropical regions of the world and are a reflection of favorable moisture and temperature regimes in these areas. Some notable forest regions in North America are the pine forests of the southeastern states, the deciduous hardwood firsts of the northeastern and Great Lakes region, the mixed coniferous forests of the Rocky Mountains and Sierra Nevada, and the dense coniferous forests of the Pacific Coast.
Tree species tolerate different environmental conditions. Trees grow more slowly, attain smaller dimensions, and are often more widely spaced in cold or arid regions. Cold temperatures, short growing seasons, and heavy snow prevent the growth of trees at high elevations and high latitudes. Moisture stress typically limits tree growth at lower timberlines, such as those adjacent to grasslands or deserts.