Tree growth periodicity in the ever-wet tropical forest of the Americas

The occurrence of annual growth rings in tropical trees-the result of the seasonal activity of vascular cambium-has been explained by seasonal water deficit or flooding periods. However, little is known about the drivers of annual tree-ring formation under tropical hyper-humid conditions without clear seasonal dry periods or flooding (ever-wet conditions). Shelford's law states that the deficit or the excess of environmental resources limits plant growth. Accordingly, we hypothesize that excess soil moisture, a slight seasonal reduction of precipitation and a reduction in light availability determine rhythmic growth in ever-wet tropical forests.We first assessed the occurrence of rhythmic growth in 14 tree species from the Biogeographic Choco Region (annual rainfall 7200 mm) using three methods: Radiocarbon (C-14) dating (all studied species), tree-ring synchronization (4 species that have replicates) and automatic dendrometers (two species). 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.We present evidence of annual tree-ring formation in all 14 studied tree species. Depending on the tree species, we observed positive and negative correlations between growth, water availability and light availability. These relationships suggest that both excess or deficit of environmental resources may explain the seasonal pattern of tree growth. Although we cannot differentiate between excess soil water and low light availability by high cloudiness, we suggest that cloudiness frequency could affect tree growth in these forests.Synthesis. We reveal the annual formation of growth rings in the unexplored wetter-end tropical forests, where seasonal growth depends on either high soil moisture and hypoxia or light limitations by cloudiness and photosynthesis constraints.