Annual growth rhythm evidence of trees from the rainiest neotropical region
The occurrence of annual growth rings in tropical trees—the result of the seasonal activity of vascular cambium—has been explained either by seasonal periods of water deficit or flooding. However, little is known about the drivers of annual tree-ring formation under tropical hyper-humid conditions without evident seasonal dry periods or flooding (ever-wet conditions). Shelford's law states that both the deficit and the excess of environmental resources limit plant growth. Accordingly, we hypothesize that reduced solar radiation, excess soil moisture, and a slight reduction of precipitation control annual growth rings in ever-wet tropical forests (lacking seasonal droughts and flooding). We first tested the occurrence of rhythmic growth in several tree species from the Biogeographic Choco Region (annual rainfall 7,200 mm) using three different methods: Radiocarbon (14C) dating, tree ring synchronization, and dendrometer bands. Then, we assessed the effect of environmental drivers (rainfall, short wave radiation, temperature, and soil moisture) on tree growth based on tree ring and dendrometer observations. Depending on the tree species, we observe both positive (n = 2) and negative correlations (n = 2) between growth and water, and growth and light availability. This relationship suggests that both excess or deficit of environmental factors may explain seasonal cyclicity of growth rhythms in some trees, but not in others. We showed for the first time the occurrence of annual growth rhythms in several tropical tree species under extreme ever-wet conditions in the neotropics. Our analysis opens a new frontier for tree ring science, helping to disentangle multiple climatic influences on growth rhythms in the tropical forest.