Why can the density of tree rings indicate the cold and warm weather? Why?

Tree rings After the tree is cut down, many concentric rings can be seen on the stump, which are called tree rings in botany. Tree rings are formed by the influence of seasons in the process of tree growth, and one ring is produced every year. Every spring, the climate is mild, the rainfall is abundant, the trees grow rapidly, and the cells formed are large in volume and quantity, thin in cell wall, loose in material and light in color, which are called early wood or spring wood; In autumn, the temperature is getting colder, the rainfall is scarce, the trees grow slowly, and the cells formed are small in size, few in number, thick in cell wall, dense in material and dark in color, which is called late wood or autumn wood. Spring wood and autumn wood in the same year are collectively called annual rings. There is an obvious line between the autumn timber in the first year and the spring timber in the second year, which becomes the annual ring line, marking the turning point of the annual growth alternation of timber and wood. Therefore, from the number of rings at the base of the trunk, you can know the age of the tree. Trees growing in temperate and tropical areas with alternating rainy and dry seasons have annual rings, while trees growing in areas with little climate change in four seasons have no obvious annual rings. Tree rings contain a lot of historical information about climate, astronomy, medicine and environment. At the same time, tree rings also play an important role in historical archaeology, forestry research, geology and public security.