Seasonal variations in isoprene emissions from a boreal aspen forest

The primary objective of this study was to understand the environmental and seasonal controls over isoprene emissions from a boreal forest ecosystem w hose isoprene source came from trees of the same species and age. A further objective was to establish an annual budget of isoprene emitted from a rem ote boreal forest and thus assess uncertainties associated with seasonal is oprene emission inventories. The onset of isoprene emissions occurred two w eeks after the forest attained its maximum leaf area. Scaled to the foliage level, averaged isoprene fluxes approached 10 +/- 5 nmol m(-2) s(-1) in th e spring. During the middle of the growing season averaged isoprene emissio ns amounted to 28 +/- 4 nmol m(-2) s(-1), whereas late summer values reache d 16 +/- 2 mmol m(-2) s(-1). These isoprene capacities were normalized to 2 5 degrees C and photosynthetically active radiation of 1000 mu mol m(-2) s( -1). Given the strong seasonality observed in isoprene emissions, the autho rs propose to include seasonally adjusted emission rates to derive isoprene inventories for the entire foliage growing cycle. With an active biomass o f 144 g m(-2), using a seasonally adjusted emission rate in a one-dimension al multilayered model it is estimated that during 1994 the boreal aspen for est emitted 32 mu mol of isoprene per square meter. Such isoprene source st rength represented approximately 1% of the photosynthetically fixed carbon by the aspen forest. In addition to the seasonal controls dictated by the i nherent plant metabolic activity, low temperatures (<10 degrees C) strongly reduced the amplitude of diurnal isoprene emissions.