Although growth rings are useful for tracing the past, not all tree-rings are suitable for dating. Specific limiting factors, both external and internal, affect the plant growth rings. Internal factors are growth regulators, enzymes, and water. These factors can be the result of external factors as well. Each ring is the result of both factors within a single year.
Growth begins in spring, and ends in summer or early autumn.
When one or more factors are limiting, distinct growth rings, or layers are formed. It is possible that no growth rings are formed under favorable conditions, or, one or more growth rings can be formed during an unfavorable year. Therefore, there are many species in different geographic areas thart are not suitable for applying dendrochronology, because their growth rings are inconsistent. However, recent study shows that tree-rings study can be applied at the tropical areas where there is a seasonal rainfall. Geographical areas where suitable trees are available, are shown in the map.