Variability in Amazon rainforest gross primary productivity co-determined by fire emissions from the arc of deforestation

Tropical rainforests are an important sink of carbon (C). However, the ability of tropical rainforests to remove C from the atmosphere is constrained by nutrient availability. Specifically, phosphorus (P) has been identified as a limiting nutrient for tropical forest growth. One potential source of incoming P fluxes in tropical rainforests is the deposition of particles from distant wildfires and prescribed fires. Savannah and deforestation fires release substantial amounts of particles that can be transported towards the equator by trade winds and, subsequently, be deposited into tropical rainforests. 

In this study, we aim to quantify the impact of distant fire-emitted nutrients on the spatial variability of gross primary productivity (GPP) of the Amazon rainforests. To achieve this, we used data on black carbon deposition from MERRA-2, as a proxy for the deposition of fire-emitted nutrients, and an ensemble of solar-induced fluorescence (SIF) datasets, as a proxy for GPP. We fitted a Random Forest regression to predict the spatial variability in SIF using black carbon deposition along with climate and soil variables as input. Subsequently, we applied SHapley Additive exPlanations (SHAP) and other variable importance techniques to evaluate the relevance of black carbon deposition in predicting the spatial variability in SIF.

Our results show that trade winds transport fire emissions from the Amazon arc-of-deforestation towards the southern part of the Amazon rainforest, creating a north-south gradient in nutrient deposition across the undisturbed rainforest. Black carbon deposition emerged as the most relevant predictor of SIF, accounting for 21.9% of the total variable contributions. In addition, the spatial distribution of SHAP values revealed that the southern Amazon experiences the most substantial positive effect of black carbon deposition on SIF. These findings confirm earlier results from field measurements conducted in a tropical lowland forest in Africa and generalize the impact of distant savannah and deforestation fires on gross primary productivity across the Amazon rainforest. Our findings indicate that distant fire emissions can alleviate nutrient limitations in undisturbed tropical forests, with potential implications for global carbon budgets.