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

Physiological Controls on Tropical Forest CO 2 ExchangeM.Goulden 1 , H.da Rocha 2 , S. Miller 1 , H.C. Freitas 2 , M.Menton 1 , A.M.Figueira 3 , C.A.deSousa 4 , C. Doughty 1 , J. Elliot 1 , E. Read 11University of California, Irvine2Universidade de S. Paulo3Desenvolvimento Regional RHAE/LBA4Iniciação Científica CNPq/LBAAuthor address: Michael L. Goulden ,Department of Earth System ScienceUniversity of California, Irvine, CA 92697-3100, USAAuthor email: mgoulden@uci.eduWe used eddy covariance to measure the net exchange of CO 2 between the atmosphereand a primary tropical forest in Para, Brazil from June 20, 2000 to July 1, 2001. Themean air temperature and daily temperature range varied little year round, and the rainyseason lasted from late December to late June. Daytime maximum CO 2 uptake was ~20µmol m -2 s -1 and nocturnal CO 2 efflux was 6 to 7 µmol m -2 s -1 . The year-round growingseason and high rate of canopy photosynthesis (~25 µmol m -2 s -1 ) resulted in an annualGross Primary Production of ~26 tC ha -1 yr -1 . Light intensity was the main controller ofdiel CO 2 exchange, explaining 48% of the variance. CO 2 uptake increased with an initialslope of 0.045 µmol CO 2 µmol PAR -1 before saturating partially at 500 to 1000 µmolPAR m -2 s -1 . CO 2 uptake at a given light intensity was ~3 µmol m -2 s -1 lower in theafternoon than in the morning, possibly due to stomatal closure. The seasonal pattern ofdaily carbon balance was the opposite of what we expected, with greater carbonaccumulation during the dry season. Nocturnal CO 2 efflux was 2.1 µmol m -2 s -1 lower inthe dry season than the wet season. The surface litter became quite dry in the dry season,and we believe the seasonal pattern of respiration was a direct effect of reduced forestfloor decomposition caused by desiccation. CO 2 uptake at a given light intensity was 3.3µmol m -2 s -1 greater from October to April than from May to September. We believe theseasonal pattern of CO 2 uptake was a result of seasonal changes in leaf area or leaf-levelphotosynthetic capacity, rather than a direct effect of drought stress. The trees at the sitewere apparently sufficiently deeply rooted to escape drought stress, whereas litterdecomposition was curtailed by desiccation, resulting in an increase in daily carbonuptake during the dry season.