A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Carbon balance and vegetation dynamics in an old-growth Amazonian forestLucy Hutyra 1 , Amy H. Rice 1 , Elizabeth Hammond Pyle 1 , Scott R. Saleska 1 , Kleber Portilho 2 ,Dulcyana F. Marques 3 , Plínio B. de Camargo 4 , and Steven C. Wofsy 11 Harvard University, Earth and Planetary Sciences20 Oxford St.Cambridge, MA 02138 USAluh@io.harvard.edu2 Universidade Federal do Para, Santarém, PA3 FIT, Santarém, PA4 CENA/USP, Piracicaba, SPAmazon forests could be globally significant sinks or sources for atmospheric carbon dioxide,but carbon balance of these forests remains poorly quantified. We surveyed 20 ha of welldrainedold-growth upland forest near the km 67 access road of the Tapajós National Forest nearSantarém, Pará, Brazil (54°58’W, 2°51’S) in order to assess carbon pool sizes, fluxes, andclimatic controls on carbon balance. Live trees with diameter at breast height (DBH) >10 cmhad a stem frequency of 467 ha -1 and accounted for 142.5 ± 6.5 Mg C ha -1 in 1999 while coarsewoody debris (CWD) accounted for 32.6 ± 2.9 Mg C ha -1 . Net flux to live wood biomass,estimated by resurvey after two years, was 1.44 ± 0.57 Mg C ha -1 yr -1 , the net result of highgrowth rate (3.23 ± 0.20 Mg C ha -1 yr -1 from a mean bole increment of 0.36 cm yr -1 ), recruitmentof new trees (0.61 ± 0.02 Mg C ha -1 yr -1 ), and high mortality (-2.4 ± 0.51 Mg C ha -1 yr -1 due toindividual stem mortality of 1.8% yr -1 ) . The measured net gain in live wood biomass wasexceeded, however, by estimated net loss (1.7 to 5.0 Mg C ha -1 yr -1 ) from the large stock ofCWD, resulting in an overall aboveground carbon balance that was negative (estimated flux: -0.3 to -4.0 Mg C ha -1 yr -1 ). Tree growth and litterfall were highly seasonal and correlatedstrongly with variations in precipitation, suggesting that climatic variations exert a stronginfluence on short-term carbon balance. Three observations – (i) the stock of CWD is large, (ii)all of the net gain in live biomass is due to small-tree growth and recruitment, and (iii) thedistribution of stem density is piecewise log-linear with a notable steeper slope for trees < 40 cmDBH – suggest that an episode of high mortality (possibly caused by the strong El NiñoSouthern Oscillation event of 1997-98) preceded study initiation. This hypothesis suggests thatvariations in medium- to long-term carbon balances are also strongly influenced by climatechanges, and that such variations may overwhelm the relatively smaller carbon balance effectspredicted from rising atmospheric carbon dioxide.