Low Phosphorus Availability Decreases Susceptibility of Tropical Primary Productivity to Droughts

International audienceLarge uncertainties in the susceptibility of tropical forest productivity to precipitation changes hamper climate change projection. Interactions between the availabilities of water and phosphorus could theoretically either increase or decrease the susceptibility of tropical gas exchange to variation of precipitation. The inclusion of phosphorus-water interactions in a land surface model reduces the coefficient of variance, a measure of variability, of biweekly gross primary productivity by a factor of 1.5-2.3 at three tropical forest sites in Brazil, bringing it closer to estimates from eddy covariance measurements and remote sensing. Soil drought conditions are attenuated due to 8-30% lower water consumption during wet periods in presence of phosphorus limitation. When soils are dry, plant phosphorus acquisition is impaired by reduced ion mobility, despite an increase in net phosphorus mineralization. We conclude that water-phosphorus interactions cannot be omitted in analysis of the resilience of tropical ecosystems to precipitation changes. Plain Language Summary Phosphorus is an essential plant nutrient and limited availability of phosphorus in soils can negatively affect plant growth and photosynthesis. Therefore, a low soil supply of phosphorus can limit the capacity of ecosystems to draw down carbon dioxide emitted by human activities. Using model simulations for three tropical sites, we show that besides the negative effect on plant growth, low phosphorus availability can have a positive effect on photosynthesis during the dry season. The positive effect is caused by lower water consumption of plants during the preceding wet season due to an overall reduced plant growth. As the interactions between water and phosphorus availability are commonly overlooked in climate change assessment, we conclude that the susceptibility of tropical ecosystem to future changes in precipitation, and thereby the risk of a catastrophic die back of tropical vegetation, could be lower than previously estimated