Climatic and biotic controls on annual carbon storage in Amazonian ecosystems

1 The role of undisturbed tropical land ecosystems in the global carbon bud get is not well understood. It has been suggested that interannual climate variability can affect the capacity of these ecosystems to store carbon in the short term. In this paper, we use a transient version of the Terrestria l Ecosystem Model (TEM) to estimate annual carbon storage in undisturbed Am azonian ecosystems during the period 1980-94, and to understand the underly ing causes of the year-to-year variations in net carbon storage for this re gion. 2 We estimate that the total carbon storage in the undisturbed ecosystems o f the Amazon Basin in 1980 was 127.6 Pg C, with about 94.3 Pg C in vegetati on and 33.3 Pg C in the reactive pool of soil organic carbon. About 83% of the total carbon storage occurred in tropical evergreen forests. Based on o ur model's results, we estimate that, over the past 15 years, the total car bon storage has increased by 3.1 Pg C (+ 2%), with a 1.9-Pg C (+2%) increas e in vegetation carbon and a 1.2-Pg C (+4%) increase in reactive soil organ ic carbon. The modelled results indicate that the largest relative changes in net carbon storage have occurred in tropical deciduous forests, but that the largest absolute changes in net carbon storage have occurred in the mo ist and wet forests of the Basin. 3 Our results show that the strength of interannual variations in net carbo n storage of undisturbed ecosystems in the Amazon Basin varies from a carbo n source of 0.2 Pg C/year to a carbon sink of 0.7 Pg C/year. Precipitation, especially the amount received during the drier months, appears to be a ma jor controller of annual net carbon storage in the Amazon Basin. Our analys is indicates further that changes in precipitation combine with changes in temperature to affect net carbon storage through influencing soil moisture and nutrient availability. 4 On average, our results suggest that the undisturbed Amazonian ecosystems accumulated 0.2 Pg C/year as a result of climate variability and increasin g atmospheric CO2 over the study period. This amount is large enough to hav e compensated for most of the carbon losses associated with tropical defore station in the Amazon during the same period. 5 Comparisons with empirical data indicate that climate variability and CO2 fertilization explain most of the variation in net carbon storage for the undisturbed ecosystems. Our analyses suggest that assessment of the regiona l carbon budget in the tropics should be made over at least one cycle of El Nino-Southern Oscillation because of interannual climate variability. Our analyses also suggest that proper scaling of the site-specific and subannua l measurements of carbon fluxes to produce Basin-wide flux estimates must t ake into account seasonal and spatial variations in net carbon storage.