EXPRESSO flux measurements at upland and lowland Congo tropical forest site

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 Nouabalé-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, difference in isoprene emission between the two seasons are difficult 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 affect 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.

Comparing three vegetation monoterpene emission models to measured gas concentrations with a model of meteorology, air chemistry and chemical transport

“…Increasing evidence shows that a great part of seasonal variations in emissions results from long-term plant phenological responses, such as budding, flowering, fruiting, defencerelated metabolism, leaf senescence and dormancy (Robertson et al, 1995;Hakola et al, 1998;Niinemets et al, 2002a;Lappalainen et al, 2009;Monson et al, 2012). Growth conditions, such as past and current temperature, light intensity, nutrition status, and water availability, are linked to the variations as well (Sharkey and Loreto, 1993;Bertin and Staudt, 1996;Serca et al, 2001). Table 3 summarizes the modelled relative contributions of individual monoterpenes to concentrations at the top of the canopy in different seasons.…”

Improved model of isoprene emissions in Africa using Ozone Monitoring Instrument (OMI) satellite observations of formaldehyde: implications for oxidants and particulate matter

“…Figure 2 compares OMI-derived isoprene emissions to REA measurements over equatorial evergreen trees Serça et al, 2001), woody savannas , and savannas (Harley et al, 2003) in central and southern Africa (sites 1-4 in Fig. 1 and Table 1a).…”

“…(1) with MEGANv2.1 emission factors, combined Terra and Aqua MODIS LAI (Yang et al, 2006), and the Goddard Earth Observing System (GEOS-5) assimilated meteorological data. The Serça et al (2001) and Harley et al (2003) No correction is applied to the REA flux measurements to account for interannual variability between 1996 (sites 1-3), 2001 (site 4), and the satellite observation period (2005)(2006)(2007)(2008)(2009)). As will be discussed in Sect.…”

Strong correlation between isoprene emission and gross photosynthetic capacity during leaf phenology of the tropical tree species Hymenaea courbaril with fundamental changes in volatile organic compounds emission composition during early leaf development

“…, 2004Harley et al . 2004), and long-term seasonal variations of isoprenoid flux rates in tropical regions are hardly characterized (Serca et al . 2001).…”