Seasonal variations in C and H 2 O cycling of a tropical transitional forest George L Vourlitis 1, Nicolau Priante Filho 2, José de Souza Nogueira 2, Luciana Sanches 2, Fernando Raiter 3, Wander Hoeger 3, Eduardo Jacusiel Miranda 2, George Sanches Suli 2, Carla Maria Abido Valentini 2 1 Department of Biological Sciences, California State University, San Marcos, CA 92096, USA. 2 Departamento de Fisica, Universidade Federal de Mato Grosso, Cuiaba, MT, Brasil 3 Instituto Universitario do Norte Matogrossense, Universidade Federal de Mato Grosso, Sinop, MT, Brasil

Research Objectives To quantify the season and interannual patterns of C and H 2 O cycling processes for a tropical transitional forest. To understand how these processes are altered by variations in: - meteorology - hydrology

Data from INPE (2003); Vourlitis et al. (2002); Eiten (1976); Miranda et al. (1996); Vourlitis et al. (2004); Malhi et al. (1998); Rocha et al. (2002). Rain forest Annual rainfall = 2.3 m Dry season length = 0-2 months Canopy height = m LAI = 5-6 m 2 /m 2 Transitional forest Annual rainfall = 2.0 m Dry season length = 3-4 months Canopy height = m LAI = m 2 /m 2 Cerrado Annual rainfall = 1.5 m Dry season length = 3-5 months Canopy height < 20 m LAI = m 2 /m 2 Characteristics of tropical transitional forest

WPL and u* corrections CO 2 storage Measurement period: August 1999-present Eddy covariance measurements of net CO 2 exchange (NEE) and evapotranspiration (ET) Sensors at 42 m (a.g.l.) 3d sonic, open-path IRGA 10 Hz sampling

Meteorological measurements Humidity and air temperature (30 and 40 m) Rainfall Radiation: Net, global, PAR (incident, reflected, transmitted (20, 10, and 1 m). Not shown: Soil heat flux; canopy/soil temperature profile; soil moisture; water table depth.

Ancillary measurements Leaf gas exchange: Seasonal, various species/ canopy heights Litter pool: Monthly, n = 20 plots Litter fall: Monthly, n = 20 plots Soil respiration: Monthly, n = 20 plots

Seasonal trends in rainfall and soil moisture Mean (+ 1SD) monthly precipitation and surface (0-30 cm) volumetric soil water content between January December From Miranda et al., (in review).

Seasonal trends in A max, R 25, and G c A max = estimated maximum rate of gross primary production. R 25 = estimated ecosystem respiration 25 o C. G c = mid-day ( h) canopy conductance. Data are means + 95% CI From Vourlitis et al., 2002 and 2004.

Seasonal trends in leaf and total litter fall Monthly leaf and total litter fall (+ 1SD; n = 20 plots) averaged over the last 3 years. From Sanches et al. (in prep.).

Maximum cross-correlation coefficients (r max ) for monthly C and H 2 O flux time series vs. rainfall and surface (0-30 cm) soil moisture time series. Lag (1) = rainfall or soil moisture of the previous month; Lag (0) = rainfall or soil moisture of the current month. All r max values are significantly different from r = 0 (p < 0.05). Modified from Vourlitis et al. (2004).

Comparison to other tropical ecosystems Data from Keller et al. (2004); Rocha et al. (2002); Saleska et al. (2003); Vourlitis et al. (2004) Cerrado Transitional Forest Rain forest Annual NEE range = Monthly maximum NEE - Monthly minimum NEE

Conclusions Seasonal variations in C and H 2 O cycling processes are highly correlated with rainfall and soil moisture. C and H 2 O cycling processes may lag behind rainfall and/or soil moisture because of lags in vegetation response. Rainfall sensitivity of NEE declines with an increase in annual rainfall.

Acknowledgements