Greening and browning trends in a tropical forest hotspot: Accounting for fragment size and vegetation indices

Abstract

Greening is the increase in vegetation biomass linked to raises in CO2 emissions, nitrogen deposition, climate warming, and changes in land cover. Because greening implies land carbon storage, it can contribute to buffering climate changes. While tropical forests are responsible for an important amount of global greening, these environments have been increasingly fragmented, and fragments are thought to lose biomass over time. However, the interferences of forest fragmentation in greening and browning (decrease in vegetation biomass) balance have been an overlooked aspect of greening studies. Furthermore, the saturation of the vegetation indices often used for biomass assessment has been an important challenge for greening studies in dense tropical forests. Here we used Google Earth Engine to address greening and browning trends over the last 35 years for fragments of different sizes from a tropical hotspot, the Atlantic Forest of northeastern Brazil, and we contrasted the results obtained from two vegetation indices, the traditional NDVI, and the recently developed kNDVI. Despite the highly advanced fragmentation level, greening predominated over browning independently of fragment size (<10 ha, 10–100 ha, 100–1000 ha, and >1000 ha), occurring more frequently but with lower intensity in the larger patches. Although these tendencies did not change with the use of different vegetation indices, kNDVI proved to be more efficient to detect browning, to identify the different classes of intensity in both greening and browning, and for capturing the extreme greening and browning levels, confirming its lower saturation in relation to NDVI. Climate and anthropogenic effects on vegetation were found only for small and isolated patches of pixels. Our results contradicted the prediction of a continuous unidirectional trend of biomass loss in highly fragmented habitats and revealed that although tropical forest fragments may retain less biomass than continuous forest tracts they may act as carbon sinks, and this can be another important reason for their conservation.