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Contribution of the analysis of diurnal cycles for understanding the mean seasonal cycle of rainforest photosynthetic activity in Central Africa. [P-2215-10]

Philippon N., De Lapparent B., Gond V., Bigot S., Brou T., Camberlin P., Cornu G., Dubreuil V., Martiny N., Morel B., Moron V., Sèze G.. 2015. In : Our Common Future under Climate Change. International scientific conference Abstract Book 7-10 July 2015. Paris, France. Paris : CFCC15, p. 266-266. Our Common Future under Climate Change, 2015-07-07/2015-07-10, Paris (France).

Global carbon, water and energy cycles are substantially driven by vegetation phenology. In particular tropical rainforests have been shown to be a key component of the climate system as they act as major water vapor sources and carbon dioxide sink. For these reasons their evolution in response to both human pressure and climate change is critical. As compared to the Amazonian and Asian rainforests, the rainforest of Central Africa experiences slower deforestation rates, so that its main threat for the next decades might come from climate change. So far, the response and sensitivity of the Central Africa rainforest to the mean seasonal evolution and inter-annual variability of climate has attracted little interest. Indeed, most of the studies focus on its Amazonian counterpart and suggest that solar irradiation is the main driver of the annual and inter-annual variations of rainforest photosynthetic activity, and the Central Africa climate itself is not well documented. As a first step towards a better understanding of the Central Africa rainforest sensitivity to present-day climate variability and response to climate change, this study performs for a target region located between 0-5°N/12- 19°E (thus documenting forest areas from 5 countries) and using space borne observations, a detailed analysis of the rainforest photosynthetic activity mean seasonal cycle comparing it with those of climate variables considered as potential drivers, i.e. rainfall, cloudiness and solar irradiation. Several key points emerge from our study. First, the seasonal cycles of photosynthetic activity (EVI MODIS) and rainfall over our target region are both bimodal. However, the highest peak of EVI (March-May) coincides with the driest of the two rainy seasons while the lowest peak of EVI (September-October) coincides with the wettest of the two rainy seasons. Second, the two rainy seasons are not associated with two distinct lows in total solar irradiation and two distinct peaks in total cloudiness: the first rainy season (March-May) which is less rainy as compared to the second one (September-October), is also less cloudy and receives more total solar irradiation. This might explain the higher EVI values recorded. Third, the high total cloudiness recorded throughout the seasonal cycle actually hides marked seasonal variations in the frequency of the 5 main types of clouds analyzed. These cloud types have specific diurnal cycles which control those of solar irradiation (thus the daily light and energy available for photosynthesis), but also influence the remote sensed photosynthetic activity data (or index). Our results clearly show that (1) nor the two dry seasons, nor the two rainy seasons do compare in terms of mean rainfall, cloudiness, solar irradiation and temperature, and (2) water and light availability have a respective weight in the Central Africa rainforest photosynthetic activity which evolves throughout the seasonal cycle. They also suggest that any evolution, due to climate change, of the complex diurnal cycles of rainfall, nebulosity and solar irradiation which characterize the equatorial climate regimes might perturb the rainforest phenology and enhance these ecosystems vulnerability. (Texte intégral)...

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