Tree rings are created when then tree grows. They grow under the bark, which pushes it out whilst it is growing. Trees have a growing season, which usually starts in the spring. The inner part of the tree grows fast early on in the growing season the colour of this is lighter because the cells are larger. In summer the growth slows down and the wood becomes darker due to the cells being smaller. When the tree is cut you see the alternating layers of light and dark wood. The outer part of the tree is the later wood and is usually denser. Each ring shows how much wood has grown in one growing season. (typically a year) The wider the rings the more wood has been grown and therefore the more favorable the conditions were likely to have been. The tree rings can become a natural event history book. You can see if the tree had suffered droughts, excessive water, air pollution and even things like insect plagues and fire damage. This process of reading and dating trees is called Dendrochronology. Dendro (meaning using the growth rings of trees) Chronos (meaning time) and Ology (meaning the study of)
Right in the core of the tree is the heartwood. Heartwood as a rule is more durable than sapwood in damp locations and less subject to attack by certain insects and by stain- and mould-producing fungi so this is one of the reasons we use it to build with. Due to its more stable water content it is also the reason we use it to build with so that the wood is less prone to warping.
Trees have taught scientists a lot about the change in our environments over time and give us accurate data on issues like the effect of global warming and climate. Even trees that have died thousands of years ago can be dated and analysed. Records have been created from trees that were living over 9,000 years ago so we can learn a lot from those silent giants that surround us everyday and are so important to our lives in many different ways. Just another reason as to why we love trees so much!