The light-deficient climates of western Central African evergreen forests

Abstract

Rainfall thresholds under which forests grow in Central Africa are lower than those of Amazonia and southeast Asia. Attention is thus regularly paid to rainfall whose seasonality and interannual variability has been shown to control Central African forests' water balance and photosynthetic activity. Nonetheless, light availability is also recognized as a key factor to tropical forests. Therefore this study aims to explore the light conditions prevailing across Central Africa, and their potential impact on forests' traits. Using satellite estimates of hourly irradiance, we find first that the four main types of diurnal cycles of irradiance extracted translate into different levels of rainfall, evapotranspiration, direct and diffuse light. Then accounting for scale interactions between the diurnal and annual cycles, we show that the daily quantity and quality of light considerably vary across Central African forests during the annual cycle: the uniqueness of western Central Africa and Gabon in particular, with strongly lightdeficient climates especially during the main dry season, points out. Lastly, using an original map of terra firme forests, we also show that most of the evergreen forests are located in western Central Africa and Gabon. We postulate that despite mean annual precipitation below 2000 mm yr −1 , the lightdeficient climates of western Central Africa can harbour evergreen forests because of an extensive lowlevel cloudiness developing during the June-September main dry season, which strongly reduces the water demand and enhances the quality of light available for tree photosynthesis. These findings pave the way for further analyses of the past and future changes in the light-deficient climates of western Central Africa and the vulnerability of evergreen forests to these changes.

Rare ground data confirm significant warming and drying in western equatorial Africa

“…Reduced light availability during the long dry season in the Gabon region is most strongly associated with seasonal low-level cloud cover (Philippon et al, 2019). Aerosol load may also have a seasonal influence on light availability as aerosol optical depth and solar radiation appear to cycle in anti-phase although we are not able to tease apart their relative importance in this analysis (Fig.…”

“…Convection clouds develop into storms late in the day or night leaving clear skies during the daytime (Gond et al, 2013). By contrast, light is least available during the long dry season due to persistent low-lying cloud cover throughout the day (Philippon et al, 2019). The seasonal synchrony between light and precipitation in western equatorial Africa is in contrast to the central Congo Basin and the neotropics where dry seasons tend to coincide with peak irradiance Philippon et al, 2019).…”

“…By contrast, light is least available during the long dry season due to persistent low-lying cloud cover throughout the day (Philippon et al, 2019). The seasonal synchrony between light and precipitation in western equatorial Africa is in contrast to the central Congo Basin and the neotropics where dry seasons tend to coincide with peak irradiance Philippon et al, 2019). In western equatorial Africa the long dry season is also the coolest time of year (Munzimi et al, 2015;Tutin & Fernandez, 1993).…”

“…The humid forests of Central Africa make up 30% of the world's tropical forests (Malhi et al, 2013), are a major carbon store (Lewis et al, 2013) and influence weather globally (Bonan, 2008;Washington et al, 2013). Most African evergreen tropical forests are found in the western equatorial region where total annual rainfall is less than 2,000 mm (Philippon et al, 2019). Evergreen forests can be maintained in this relatively dry region due to reduced water demand during seasonal drought associated with extreme cloudiness (Philippon et al, 2019).…”

“…Most African evergreen tropical forests are found in the western equatorial region where total annual rainfall is less than 2,000 mm (Philippon et al, 2019). Evergreen forests can be maintained in this relatively dry region due to reduced water demand during seasonal drought associated with extreme cloudiness (Philippon et al, 2019). Long-term changes to climate and climatic variability in the region (James, Washington & Rowell, 2013) are likely to have far-reaching impacts on the functioning of these evergreen tropical forests (Asefi-najafabady & Saatchi, 2013;Zhou et al, 2014) with knock-on effects for the global carbon cycle (Mitchard, 2018) and local human livelihoods (Niang et al, 2014).…”