Life cycle events and weather in the tropics (DR Congo)

Life cycle events in tropical forests, like their more temperate counterparts, are in part tuned to changes in the weather throughout the year. Unlike the more temperate regions the temperature in the tropics is not the main driver of phenology, as it stays more or less stable throughout the year. Below you see a summary graph of the temperature of Kisangani (120 km to the east of Yangambi), which averages around a ~25C or rather perfect growing conditions.

However, the region and most of the basin goes through two wet seasons. This is due to the fact that the intertropical convergence zone (ITCZ), a band of clouds and thunderstorms, moves back and forth across the equator following the sun’s zenith point. Given the location of the Congo Basin around the equator the ITCZ passes over the basin two times a year, due to tilt in the Earth’s axis, creating two rainy seasons. In the bar graph below you see the monthly totals, where values over 150mm are considered “wet” months.

If you compare an example of life cycle events of some of the intermediate Jungle Rhythm results you see that this particular tree flowers at the start of the wet season, while leaf senescence starts at the end of the dry season. Not all trees will show this pattern, as different species might take different environmental cues (which are or are not met depending on the seasonal changes or yearly variability). However, the example illustrates the relation between weather and life cycle events very clearly. The importance of these seasonal changes in the tropics can not be understated. A recent study also linked these seasonal changes to measurable differences in CO2 uptake from the atmosphere, a main incentive for me to document these life cycle changes in the Jungle Rhythms project.