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.

Automated summary

Isotope variation in wood provides new insights on growth rhythms in trees growing in tropical forests, even without seasonal droughts or flooding.