The rings are created by a cycle of dormancy and growth ... thereby saying a ring is 1 year.
But in reality some places (semi tropical) dont have a genuine dormancy period and can experience more than one growing period a year resulting in multiple rings for a years growth.
It has been found by both anecdotal and scientific methods that some trees in tropical type regions grow according to available water resources.
Where I live in winter it is common to have 24C days. However winter traditionally is our dry season and summer our wet. Now if it does rain a lot in winter (unusual but can happen) the trees grow, sometimes a darn lot! So what happens through a calendar year is trees might have 3 or 4 rings for various growing conditions pertaining to rain.
Many times I can tell when cutting trees or even palms for that matter how wet years were in the past ... currently we are experiencing drought conditions and palms have small heads and tapers and in real wet years the head region will fatten up ... trees similar but with growth rings.
But lets say down is some place where it's cold in winter, deciduous trees that lose their foliage and sit there dead looking for months there will be definate rings that YOU CAN COUNT ON. In those rings you'll also see bad years and good years by the width of the ring.
Hope that helps.
