Exploring the link between clouds, radiation, and canopy productivity of tropical savannas

The control of clouds on the canopy gross primary productivity (GPP) was examined at Howard Springs, a tropical savanna site in the Northern Territory, Australia. It was demonstrated in this study that cloudiness can increase the initial canopy quantum efficiency (α), midday light use efficiency (LUE) and water use efficiency (WUE), but decrease GPP in savannas. Thick clouds (clearness index of 0–0.3 in the wet season produced much more diffuse fraction of Photosynthetically Active Radiation (fDPAR > 80%), which caused increases in α by 24% and 62% compared to thin clouds (fDPAR between 30% and 80%) and clear sky (fDPAR < 30%) conditions respectively. The influence of environmental conditions shows that under similar vapour pressure deficit, temperature and soil water content classes, α values were significantly higher under thick clouds compared to thin clouds or clear skies. This indicates the importance of diffuse radiation in enhancing LUE even within similar environmental conditions. However, the enhanced LUE under cloudy skies is insufficient to increase GPP due to the dramatic decline in total radiation. Therefore, it can be concluded that the quantity of solar radiation is more critical than the quality of radiation in savannas. However, savanna ecosystems appear to be well adapted to the environment where a 63% decrease in PAR only reduced GPP by 26%. These findings highlight the importance of clouds as a critical factor in determining savanna productivity that has implications for savannas carbon cycle.