All trees grow in response to the conditions in their environment. A record of those conditions can be found by examining the growth rings of any tree, which vary in width according to the growing conditions in a particular year.
For every year of any tree's existence, another ring or layer of wood is created, as seen in a cross-section of the trunk of the tree. The new rings are actually developed as part of the xylem of the tree. In years when water is plentiful and temperatures are conducive to growth, trees will grow relatively more quickly and will form wide growth rings. In years of drought, the rings will be much narrower, reflecting the difficult growing conditions of those years.