EXPRESSO flux measurements at upland and lowland Congo tropical forest site

As part of the EXPRESSO program (EXPeriment for the REgional Sources and Si nks of Oxidants), biosphere-atmosphere exchanges of trace gases were invest igated in a ground-based forest site of the Republic of Congo. Experiments were carried out in March and November-December 1996. A 60-meter walkup tow er was erected in an undisturbed mixed tropical forest typical of upland ve getation in the Nouabale-Ndoki National Park. Eight belt transects radiatin g from the tower were used to characterize the species composition and stru cture of the upland mixed forest. As a comparison. and to investigate horiz ontal heterogeneity of the trace gases exchanges, additional measurements w ere made in a nearby monospecific forest stand characteristic of lowland Gi lbertiodendron dewevrei (Gilbert. dew.) forest. Micrometeorological data, t race gas concentrations and flux measurements were made from the tower. We report daily above-canopy variation in temperature and radiation. energy pa rtitioning into latent and sensible heat flux, volatile organic compound (V OC) mixing ratios, isoprene and CO2 fluxes. Fluxes of isoprene and CO2 were measured above the canopy using relaxed eddy accumulation and eddy covaria nce methods, respectively. These fluxes show a seasonal variation between t he two experiments, as does energy partitioning. However, difference in iso prene emission between the two seasons are difficult to reconcile with mete orological (T. PAR) data only, and more data such as plant water potential are needed to modeled the seasonal isoprene emission cycle. Isoprene emissi on at the leaf level was also determined for plant species at both upland a nd lowland sites using environmentally controlled leaf enclosures. Together with the ecological survey, the leaf level work suggests that lowland Gilb ert. dew. forests act as hot spots in terms of isoprene emissions. Future c limate and land use changes could greatly affect the isoprene regional emis sion estimate through changes in the respective proportion of the upland an d lowland forests, and the extent of dry versus wet season.