Tree trunks and branches grow and thicken as new cells are added beneath the bark. These cells form two types of tissues, called xylem (wood) and phloem (bark), by which water and food are transported throughout the tree. The xylem transports water and nutrients from the roots to the leaves. The active xylem is called sapwood. The old xylem no longer carries water and forms the heartwood of the tree, which may be a different color than the sapwood. The phloem, also called inner bark, transports nutrients from the leaves to the branches, trunk, and roots. Enveloping the phloem is the outer bark, which protects the tree from injury. The cambium is a zone of extraordinary cellular activity, located between the phloem and xylem, whose mission is to produce new xylem cells, which will give rise to growth rings, and phloem, to the bark.
The rings in samples taken from the trunk of trees also make it possible to evaluate their age and to analyze the climate. In nature, when climatic conditions are favorable to growth and there is no interference from other factors, the rings will be wider. Conversely, they will be narrower in cold periods, if we are in mountain environments, or during dry spells, where the fundamental limiting factor for growth is humidity. Any environmental component that affects the growth of a tree can be studied from its rings. Thus, the recurrence of forest fires, avalanches, floods, slope movements, among others, that affect forest formations can be analyzed from the traces they have left in the growth rings.
Trees usually produce one growth ring per year, in which we can generally distinguish between earlywood, which is lighter, and latewood, which is darker. In the trunk of trees, cells usually begin to grow in the spring, with cells first forming that appear to the naked eye to have a basic tan color. Dendrochronology and light dendroclimatology (known as earlywood). As the growing season ends, usually in the fall, the cell walls thicken, producing a dark band (or latewood). Both earlywood and latewood mark the annual growth. The change from the dark color of the end of growth to the light color of the beginning of growth in the following year makes it easy to differentiate the annual ring. Dendrochronology is the science that, from the observation and analysis of the annual growth rings of tree trunks, is able to establish its age and the environmental changes to which it has been subjected, and which have occurred in the territory in which it lived. By overlapping the dendrochronological record of different trees that lived in the same area at different times, it is possible to compose time series spanning even several thousand years.
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