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Isoprene emission capacity for US tree species


Isoprene emission capacity measurements are presented from 18 North American oak (Quercus) species and species from six other genera previously found to emit significant quantities of isoprene. Sampling was conducted at physiographically diverse locations in North Carolina, Central California, and Northern Oregon. Emissions from several sun leaves of each species were measured at or near standard conditions (leaf temperature of 30°C and photosynthetically active radiation of 1000 μmol m-2s-1) using environmentally controlled cuvette systems and gas chromatography with reduction gas detectors. Species mean emission capacity ranged from 39 to 158 μg Cg-1h-1 (mean of 86), or 22 to 79 nmol m-2s-1 (mean of 44). These rates are 2-28 times higher than those previously reported from the same species, which were summarized in a recent study where isoprene emission rates were assigned based on published data and taxonomy. These discrepancies were attributed to differences in leaf environment during development, measurement technique (branch or plant enclosure versus leaf enclosure), and lack of environmental measurements associated with some of the earlier branch enclosure measurements. Mass-based emission capacities for 15 of 18 oak species, sweetgum (Liquidambar styraciflua), and poplars (Populus trichocarpa and P. deltoides) were within ranges used in current biogenic volatile organic compound (BVOC) emission models, while measured rates for the remaining three oak species, Nyssa sylvatica, Platanus occidentalis, Robinia pseudoacacia, Salix nigra, and Populus hybrids (Populus trichocarpa × P. deltoides) were considerably higher. In addition, mean specific leaf mass of the oak species was 30% higher than assumed in current emission models. Emission rates reported here and in other recent studies support recent conclusions that isoprene emission capacities for sun leaves of high emitting species may be better represented by a value of 100 ± 50 μg Cg-1h-1 during hot summer conditions. We also find that intermediate isoprene emission rates previously suggested for some tree species may not represent their true emission capacities, and that broadleaf plant species may have either low (< 1.0 μg Cg-1h-1) or very high (∼ 100 μg Cg-1h-1) genetic capacity to emit isoprene when mature foliage is exposed to a high ambient temperature and light environment.

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