Why are tree rings not always annual in tropical and subtropical trees?

In tropical and subtropical regions, trees often do not form rings on a strictly annual basis because their growth is not governed by the same predictable, singular seasonal cycle found in temperate climates. Instead, these trees form growth rings in response to various cyclic environmental conditions.1

While temperate trees typically develop one ring per year due to distinct seasonal changes, tropical and subtropical trees may experience multiple growth spurts or periods of dormancy throughout the year, triggered by fluctuations in factors such as water availability or other environmental stressors. Consequently, the resulting growth increments are not always annual. Furthermore, in many tropical species, the variations in cell structure—which typically delineate rings—can be subtle or obscured, making it difficult to distinguish individual rings based on chronological time alone.1