Intra-annual isotope variations in tree rings reveal growth rhythms within the least rainy season of an ever-wet tropical forest

Key message Isotope variation (delta O-18) in wood suggests new insights on growth rhythms in trees growing in tropical forest with extremely high precipitation, without seasonal droughts or flooding. It is well known that growth-limiting factors such as seasonal droughts can induce periodicities in woody tissue formation of tropical trees. In regions without seasonal droughts or flooding but sufficient water for photosynthesis (ever-wet tropical forests), rhythmic growth has been previously reported; however, triggering factors remain little explored. Our objective was to establish tree-ring frequency and probable growing season by analysis of the intra-annual variability of isotopic ratios in cellulose (delta O-18(cellulose) and delta C-13(cellulose)) and relationships with environmental variables in two tree species (Humiriastrum procerum and Virola dixonii) growing in an ever-wet tropical forest (Choco region of Colombia, precipitation 7200 mm year(-1), mean annual temperature 25.9 degrees C), located close to the Pacific Ocean at ca 3 degrees 57 ' 12.54 '' N-76 degrees 59 ' 27.96 '' W. Here, we report annual rhythmic growth evidenced by radiocarbon analysis, leaf phenology, dendrometer records, and stable-isotope variation in cellulose. All evidence points to the probable growing season occurring during the least rainy months for both species. While intra-annual delta O-18(cellulose) values follow a rhythmic variation, delta C-13(cellulose) variations show a less clear pattern, probably due to deciduity and remobilization of non-structural carbon stored in previous growing stages. Furthermore, delta O-18(cellulose) covary with relative humidity, vapor pressure deficit, short-wave solar radiation, and temperature during the least rainy months. In contrast, delta C-13(cellulose) values were not significantly correlated with environmental variables. Our results show that stable-isotope variations in tree rings, even under ever-wet conditions, are valuable for understanding drivers of tropical tree growth in such conditions.