Tropical rainforest: carbon sink or carbon source? Heraclis Victoros & Honar Mahdi

Carbon Cycle Carbon follows a continuous cycle from the atmosphere into the biome and vice versa. Carbon Sink: storage of atmospheric carbon into the rainforest biome exceeds carbon release into the atmosphere. Carbon Source: carbon released into the atmosphere exceeds the rate of carbon stored into the rainforest biome.

Tropical Rainforest Tropical rainforest contain close to half of the global total of carbon biomass Important role in global carbon budget Multiple studies over the past years suggesting tropical rainforest are significant carbon sinks On the other hand rising factors suggesting tropical rainforest turning into carbon sources

Rainforest Distribution Total tropical rainforest area is about 56 million km² Three main continents South and Central America:32% Africa: 52% South and South East Asia : 17% (Malhi, 2010) South America tropical forests are a moderate net sink of carbon, with very high rates of deforestation. Asian tropical forest is a large net source of carbon because of direct deforestation and from oxidation and burning of peatlands. African tropical rainforest is a large net sink of carbon, with low carbon dioxide emissions (Malhi, 2010).

Estimates of carbon sources and sinks in tropical forest regions 2000–2005 (Malhi, 2010). Grey arrows indicate biomass carbon sink White arrows deforestation net carbon source Black arrows the net balance.

Factors affecting carbon balance Factors Increasing carbon sink Affected by elevated atmospheric carbon dioxide concentration. Concentration of atmospheric carbon dioxide is currently increasing by 1.5ppm year-¹ (Malhi et al., 2000) Now is close to 380ppm (Malhi & Phillips, 2005). Rate of photosynthesis increases Faster tree growth

Factors affecting carbon balance Increase carbon source Fire Bugs Logging/deforestation Drought Non traditional agriculture

Deforestation changing the carbon balance (Malhi, 2010) Net increase of carbon storage is decreasing throughout the years due to deforestation

Global distribution of carbon sink

Factors affecting carbon balance Rise in temperature carbon sink or carbon source? Photosynthesis rates are going to increase in combination with increased levels of carbon dioxide in the atmosphere, resulting in faster rates of carbon storage in the rainforest biomass. Respiration rates, decomposition rates will increase, resulting in faster rates of carbon release back into the atmosphere. The most likely that global prospect is that the sink will persist, but perhaps gradually decrease in magnitude. (Malhi, 2010).

Conclusion Intact Tropical forests are a carbon sink because of their average growth exceeded mortality In the future, the growth response of remaining old-growth mature amazon forest will saturate and the ecosystem may switch from carbon sink to carbon source driven by higher respiration (increase in temperature), higher mortality due to disturbances from humans. Any changes from carbon sink to carbon source would have profound implications for global climate, biodiversity and human welfare. Acceleration of tree growth and mortality may already be affecting the interactions among millions of species.

References Pan, Y., Birdsey, R. A., Fang, J., Houghton, R., Kauppi, P. E., Kurz, W. A., ... & Hayes, D. (2011). A large and persistent carbon sink in the world’s forests. Science, 333(6045), 988-993. Phillips, O. L., Lewis, S. L., Baker, T. R., Chao, K. J., & Higuchi, N. (2008). The changing Amazon forest. Philosophical Transactions of the Royal Society B: Biological Sciences, 363(1498), 1819-1827. Malhi, Y. (2010). The carbon balance of tropical forest regions, 1990–2005. Current Opinion in Environmental Sustainability, 2(4), 237-244. Malhi, Y., & Grace, J. (2000). Tropical forests and atmospheric carbon dioxide. Trends in Ecology & Evolution, 15(8), 332-337. Malhi, Y., & Phillips, O. L., (2005). Tropical forest and Global atmospheric change. Oxford University Press Clark, D. A. (2004). Sources or sinks? The responses of tropical forests to current and future climate and atmospheric composition. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 359(1443), 477-491.

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