BIOGENIC ISOPRENE EMISSION - MODEL EVALUATION IN A SOUTHEASTERN UNITED-STATES BOTTOMLAND DECIDUOUS FOREST

Isoprene is usually the dominant natural volatile organic compound emi ssion from forest ecosystems, especially those with a major broadleaf deciduous component. Here we report isoprene emission model performanc e versus leaf and canopy level isoprene emission measurements made at the Duke University Research Forest near Chapel Hill, North Carolina. Emission factors, light and temperature response, canopy environment m odels, foliar mass, leaf area, and canopy level isoprene emission were evaluated in the field and compared with model estimates. Model compo nents performed reasonably well and generally yielded estimates within 20% of values measured at the site. However, measured emission factor s were much higher in early summer following an unusually dry spring. These decreased later in the summer but remained higher than values cu rrently used in emission models. There was also a pronounced decline i n basal emission rates in lower portions of the canopy which could not be entirely explained by decreasing specific leaf weight. Foliar biom ass estimates by genera using basal area ratios adjusted for crown for m were in excellent agreement with values measured by litterfall. Over all, the stand level isoprene emissions determined by relaxed eddy acc umulation techniques agreed reasonably well with those predicted by th e model, although there is some evidence for underprediction at ambien t temperatures approaching 30 degrees C, and overprediction during Oct ober as the canopy foliage senesced. A ''Big Leaf'' model considers th e canopy as a single multispecies layer and expresses isoprene emissio n as a function of leaf area rather than mass. This simple model perfo rms nearly as well as the other biomass-based models. We speculate tha t seasonal water balance may impact isoprene emission. Possible improv ements to the canopy environment model and other components are discus sed.