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Abstract

Release of CO₂ from woody tree tissue was compared between two common tree species of central Amazonian white water inundation forests ("Várzea”) that have contrasting leaf phenology. The deciduous species Pseudobombax munguba and the evergreen species Laetia corymbulosa were selected for this study. Várzea forests are believed to have a high carbon release since net wood productivity is low in spite of favorable nutrient supply and climate conditions. Daily courses of carbon release were measured starting in March 1999, using stem and branch cuvettes in an open system with an integrated infrared gas analyzer running in differential mode. The results were analyzed with respect to the four distinct seasons of flooding conditions in the Várzea defined as: ‘up’ during increasing water level (February-April), ‘high’ during maximum water level (May- July), ‘down’ during decreasing water level (August- October), and ‘dry’ under conditions of no flooding (November-January). From March until July, the water level of the Amazon River rises constantly by several meters and inundates the adjacent forests. The trees showed maximum rates of woody tissue CO2 release during this season and rates varied from 2 to 10 mmol CO2 m-2 s-1 in the lower part of the main stem during the day. These values are higher than those reported in the literature for other broad-leaved species. For the remaining seasons daily variation of main stem CO2 release was smaller with a maximum of 4 mmol CO2 m-2 s-1 between night and midday hours. The high daily variation observed in the lower stem during early flooding was not observed in the upper stem where CO2 release rates remained rather constant throughout the year. Branch CO2 release was high during new leaf development indicating a tight relationship to leaf phenology. The response of CO2 release to phenology was expressed in the high values of the coefficient of temperature variation Q10 for branches during leaf flush. During leaf shedding, the CO2 release doubled during midday hours. This effect was not found for the CO2 release from stems. Carbon release rates at the lower stem locations were 3 to 3.5 times higher during early flooding compare to values recorded later during flooding. Mass transport of CO2 by xylem flux was considered as a factor causing the high CO2 release in the lower stem during early flooding. Since the high carbon release observed could not be explained by temperature variation, the relationship with xylem flux density was investigated. Additional carbon efflux by convective transport of CO2 from the low stem location during early flooding was higher in Pseudobombax munguba than in Laetia corymbulosa. The results of this study show that a close relationship exists between apparent carbon release during flooding in branches and tree leaf phenology. Acurrently non-described but quantitatively important source of CO2 release was found in additional CO2 release from upward advective transport with the xylem sap