Abstract
As part of the EXPRESSO program (EXPeriment for the REgional Sources and Sinks of Oxidants), biosphere-atmosphere exchanges of trace gases were investigated in a ground-based forest site of the Republic of Congo. Experiments were carried out in March and NovemberDecember 1996. A 60-meter walkup tower was erected in an undisturbed mixed tropical forest typical of upland vegetation in the Nouabale´-Ndoki National Park. Eight belt transects radiating from the tower were used to characterize the species composition and structure of the upland mixed forest. As a comparison, and to investigate horizontal heterogeneity of the trace gases exchanges, additional measurements were made in a nearby monospecific forest stand characteristic of lowland Gilbertiodendron dewevrei (Gilbert. dew.) forest. Micrometeorological data, trace gas concentrations and flux measurements were made from the tower. We report daily above-canopy variation in temperature and radiation, energy partitioning into latent and sensible heat flux, volatile organic compound (VOC) mixing ratios, isoprene and CO 2 fluxes. Fluxes of isoprene and CO 2 were measured above the canopy using relaxed eddy accumulation and eddy covariance methods, respectively. These fluxes show a seasonal variation between the two experiments, as does energy partitioning. However, diVerence in isoprene emission between the two seasons are diYcult to reconcile with meteorological (T, PAR) data only, and more data such as plant water potential are needed to modeled the seasonal isoprene emission cycle. Isoprene emission at the leaf level was also determined for plant species at both upland and lowland sites using environmentally controlled leaf enclosures. Together with the ecological survey, the leaf level work suggests that lowland Gilbert. dew. forests act as hot spots in terms of isoprene emissions. Future climate and land use changes could greatly aVect the isoprene regional emission estimate through changes in the respective proportion of the upland and lowland forests, and the extent of dry versus wet season.
Highlights
Tropical regions are of great significance in the study of global chemistry
First steps were taken towards determination of important parameters for trace gas emission modeling, such as species characterization, VOC emission characteristics and leaf area index
photoionization detector (PID) work conducted in parallel with the ecological survey allowed us to characterize the emission capacities of 82% of the biomass in the mixed forest, and of 85% of the biomass in the Gilbert. dew. forest
Summary
Tropical regions are of great significance in the study of global chemistry. They are of particular importance because of the intense biogenic activity. A number of papers published in previous special issues of the Journal of Geophysical Research (vol 93, 1988; vol 101, 1996) have demonstrated the impact of tropical sources of trace gases on atmospheric composition Biomes present in these regions, mainly grassland savanna, woodland savanna, and tropical rainforest, account for more than half of global net primary productivity (Rodin et al, 1975), which is an indicator of the ecological efficiency of carbon, nitrogen and sulfur exchanges through biogeochemical cycles. No similar work has been conducted on African tropical forests, the mixing ratios of some light hydrocarbons in air masses above the Central African forest were measured during the 1988 DECAFE experiment (Bonsang et al, 1991; Rudolph et al, 1992) This lack of data on the African continent was one of the motivations for the EXPRESSO program (see overview in Delmas et al (1999)). Isoprene emission data were incorporated into an isoprene emission model and canopy level fluxes were used to constrain that model in order to estimate hourly emissions on a spatial scale of about 1 km (Guenther et al, 1999)
Sign-in/Register to view highlights & summary 
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call