The Amazon in peril: The life and death of the world’s most diverse biome

Brazil’s Amazon rainforest is one of the world’s most extensively researched ecological zones. In addition to its rich biodiversity and economic importance to Brazil, the Amazon is also one of the largest land-based sinks of atmospheric carbon in the world. A significant body of ecological research regarding the Amazon focuses on the impact of growing levels of atmospheric carbon dioxide on the growth and decay of biomass. Plant biomass has an ability unique among living things to absorb carbon dioxide during photosynthesis and create nutrients in the form of complex carbon molecules. An increasing trend in biomass growth points to higher absorption of carbon dioxide by the forest, while a net decay (or death) of biomass indicates a slow release of carbon into the atmosphere.

Herein lies a key bridge from the world of science to environmental policy. The Intergovernmental Panel for Climate Change considers the capture and storage of atmospheric carbon dioxide, also referred to as carbon sequestration, one of the primary global mitigation strategies in constructing its emission reduction scenarios. Academic research on this subject is dominated by mathematical models that simulate biological interactions. These models vary widely in their predictions. While earlier studies focusing on the Amazon predicted a large-scale decay of plant matter (indicating a poor carbon sink), newer models and the current prevailing model forecast robust growth (and a potent carbon sink) well into the 21st century.

But in a new paper in Nature, a RAINFOR team led by R. J. W. Brienen argue that the reliability of mathematical models is challenged by changes in forest dynamics due to unforeseen factors—namely, climate change.

The authors take an empirical approach to the question of the Amazon’s performance as a carbon sink. Brienen and their team achieve this by using observed growth in living biomass in their study area as a proxy for the level of sequestration. They estimate the quantity of biomass from the dimensions of trees. Individual dimensions were recorded from 1985 to 2010 in an area covering over 343 hectares of forest land and around 189,000 individual trees. This study was the longest and largest study of forest dynamics ever in South America.

Based on their measurements and analysis, the authors suggest that trees in the Amazon are ageing faster and experiencing shorter life spans. Experimental research shows that increasing carbon dioxide levels leads to early-onset plant reproduction, while faster growth exposes trees to size-related decline sooner, including decreased physical stability and photosynthetic decline. That is, living biomass is trending downwards, and there is increasing accumulation of dead plant matter in the Amazon, which releases carbon slowly into the atmosphere. In fact, the authors estimate that there has been a 30 percent increase in dead plant matter since 1983. This increase in dying plant biomass is nearly double the cumulative decline in live biomass in the Amazon during the study period.

The implications of these results are significant. As levels of carbon dioxide in the atmosphere rise, Brienen et al.’s research indicate that the Amazon is getting less efficient in sequestering carbon because it is now home to more dead plant matter than living. While related research posits that overall forest area and the global land carbon sink have been growing since the 1990s, the assumption that tropical forests will remain “the lungs of the world” can be legitimately questioned as birth-death cycles in trees are affected by global warming.

For policymakers, nature-based solutions that leverage the ability of forests to absorb carbon are extremely prudent and a low-cost counter to global warming. However, technical challenges exist in correctly predicting the impact that changing carbon dioxide levels have on natural environments. The most important takeaway from this paper is the emerging value of investing in large scale and unbiased monitoring of the world’s most diverse biomes: tropical forests like the Amazon.

Article source: R. J. W. Brienen et al., “Long-term decline of the Amazon carbon sink,” Nature,  Vol. 519, (2015).

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