Revealing the spatial variation in biomass uptake rates of Brazil's secondary forests

Summary

Monitoring forest aboveground biomass (AGB) is essential for quantifying the carbon cycle and mitigating climate change. Tropical secondary forests are significant carbon sinks that sequester large amounts of carbon dioxide. While recent studies have attempted to estimate the AGB recovery rates in tropical forests, considerable uncertainty remains in the estimation of AGB recovery of secondary forests and the spatial variability of the effects that different environmental conditions and degrees of human use may have on AGB recovery. These knowledge gaps hinder further understanding of climate change mitigation potential of secondary forests. Remote sensing products provide spatially and temporally explicit information for understanding and monitoring secondary forest dynamics. To explore the local effects of different factors on AGB of secondary forests in Brazil, we used geographically weighted regression (GWR) models that account for spatial heterogeneity in geospatial data to estimate the AGB of secondary forests in Brazil. Secondary forest areas (29142 polygons) were extracted from Brazil's forest age maps between 1984 and 2019. The AGB of these areas was derived from the Climate Change Initiative Biomass maps. The effects of selected predictors such as forest age, climatic water deficit, the cation exchange capacity of soil and surrounding tree cover were analyzed. The two most influential factors, forest age and surrounding tree cover were utilized to estimate the AGB and the recovery rates per year. Our results show the high spatial variation of different predictors’ effects on the AGB of secondary forests. Also, the GWR model (with an adjusted R² of 0.74) showed considerable improvements regarding “goodness of fit” of models compared with the Ordinary Least Squares (with an adjusted R² of 0.53). Our estimated average AGB recovery rate across all Brazil's biomes is 7.5 Mg ha⁻¹ yr⁻¹ (using forest age) for the first 20 years. We presented the map of the spatial variation of AGB recovery rates in Brazil. The estimated AGB recovery rates range using forest age is 28.9 Mg ha⁻¹ yr⁻¹. Our estimated mean AGB recovery rates of different biomes are 17.7 % on average higher than IPCC default rates. Our results provide baseline information for reducing uncertainties related to carbon sink estimation of secondary forests in Brazil, hence assisting in developing sustainable forest management and ecosystem restoration strategies.