How are annual rings formed in trees?

When trees grow, they accumulate wooden fibres around their heartwood. These fibres are bright and porous in spring so that a lot of water can reach the fresh leaves through them. By autumn, they become darker and thicker so the tree becomes stable and resistant to frost in the winter rest period. This process forms a bright ring and a dark ring around the heartwood every year. If you count the dark rings of a horizontal cross section of the tree trunk, you can find out the age of the tree. In the tropics, where there are no distinct seasons, the trees do not develop any rings. They grow all round the year and do not have any rest period.