Widespread decline of Congo rainforest greenness in the past decade

Abstract

Tropical forests are global epicentres of biodiversity and important modulators of climate change, and are mainly constrained by rainfall patterns. The severe short-term droughts that occurred recently in Amazonia have drawn attention to the vulnerability of tropical forests to climatic disturbances. The central African rainforests, the second-largest on Earth, have experienced a long-term drying trend whose impacts on vegetation dynamics remain mostly unknown because in situ observations are very limited. The Congolese forest, with its drier conditions and higher percentage of semi-evergreen trees, may be more tolerant to short-term rainfall reduction than are wetter tropical forests, but for a long-term drought there may be critical thresholds of water availability below which higher-biomass, closed-canopy forests transition to more open, lower-biomass forests. Here we present observational evidence for a widespread decline in forest greenness over the past decade based on analyses of satellite data (optical, thermal, microwave and gravity) from several independent sensors over the Congo basin. This decline in vegetation greenness, particularly in the northern Congolese forest, is generally consistent with decreases in rainfall, terrestrial water storage, water content in aboveground woody and leaf biomass, and the canopy backscatter anomaly caused by changes in structure and moisture in upper forest layers. It is also consistent with increases in photosynthetically active radiation and land surface temperature. These multiple lines of evidence indicate that this large-scale vegetation browning, or loss of photosynthetic capacity, may be partially attributable to the long-term drying trend. Our results suggest that a continued gradual decline of photosynthetic capacity and moisture content driven by the persistent drying trend could alter the composition and structure of the Congolese forest to favour the spread of drought-tolerant species.

A conceptual model for assessing rainfall and vegetation trends in sub‐Saharan Africa from satellite data

“…A long-term drying trend was also observed over the northern part of the tropics by Kawase et al (2010), which was mainly caused by an increase in anthropogenic aerosols influencing both atmospheric dynamics and thermodynamic effects. A recent study by Zhou et al (2014) also confirmed a decline in rainfall across the large area of central and northern Democratic Republic of Congo. A reduction in rainfall across tropical ecosystems could have a significant impact on the climatic condition at the global and regional scale considering that the Congo Basin is the second largest rainforest on earth.…”

“…The condition of evergreen vegetation across this area remained mainly constant or showed a small but statistically significant increase in NDVI values by <0.005 and <0.01 in contrast to negative rainfall trends of <−30 mm over 10 years. This discrepancy in vegetation patterns may be explained by the fact that Zhou et al (2014) used the Enhanced Vegetation Index (EVI) in their study, which is more sensitive to dense forest than other vegetation indices such as the NDVI used here. EVI has been used widely in recent studies of tropical rainforest in the Amazon (Samanta et al, 2012).…”

“…This may be explained by the fact that a reduction in the annual rainfall sum does not limit the vegetation growth in these areas due to the generally very wet conditions. However, a recent study by Zhou et al (2014) also presented evidence for a widespread decline in forest greenness particularly in the northern Congo Basin over the decade 2000-2012. The authors explained that this decline of vegetation greenness is related to a decrease in rainfall and increase in photosynthetically active radiation and land surface temperature.…”

Age, extent and carbon storage of the central Congo Basin peatland complex

“…However, they face two threats: changes in land-use, particularly drainage for agricultural use, as is occurring extensively across tropical Asia; and a regional reduction in precipitation via a changing climate, which may already be occurring 32 . While modelled projections of Central African rainfall are not consistent, some suggest declining annual precipitation 7 and more intense dry seasons 8 …”

Increased vegetation growth and carbon stock in China karst via ecological engineering

“…T he impacts of climate change and anthropogenic activities on Earth's vegetation and ecosystems have been in the spotlight of science in the past decades [1][2][3][4][5][6] . With increasing climate variability and more frequent occurrences of extreme events expected in the future 7 , research has targeted the sensitivity of ecosystems 8,9 .…”