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 whose 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 remote boreal forest and thus assess uncertainties associated with seasonal isoprene emission inventories. The onset of isoprene emissions occurred two weeks 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 the spring. During the middle of the growing season averaged isoprene emissions amounted to 28 ± 4 nmol m^-2 s^-1, whereas late summer values reached 16 ± 2 mmol m^-2 s^-1. These isoprene capacities were normalized to 25°C and photosynthetically active radiation of 1000 μmol m^-2 s^-1. Given the strong seasonality observed in isoprene emissions, the authors propose to include seasonally adjusted emission rates to derive isoprene inventories for the entire foliage growing cycle. With an active biomass of 144 g m^-2, using a seasonally adjusted emission rate in a one-dimensional multilayered model it is estimated that during 1994 the boreal aspen forest emitted 32 μmol of isoprene per square meter. Such isoprene source strength represented approximately 1% of the photosynthetically fixed carbon by the aspen forest. In addition to the seasonal controls dictated by the inherent plant metabolic activity, low temperatures (<10°C) strongly reduced the amplitude of diurnal isoprene emissions.