Measured isoprene emission rates of plants in California landscapes: comparison to estimates from taxonomic relationships
Introduction
It is now well known that reactive volatile organic compounds (VOC) are emitted from vegetation, including urban landscapes, agricultural crops, and natural plant communities. More than 70 different biogenic VOC (BVOC) compounds are known to be emitted by plants (Isidorov et al., 1985; Winer et al., 1992) but only a few are emitted in large quantities. Isoprene is the BVOC emitted in greatest quantity by the plant kingdom worldwide (Guenther et al., 1995) and is the dominant BVOC emitted by deciduous forests (Geron et al., 1995), typically accounting for 2% of the carbon fixed during photosynthesis (Loreto and Sharkey, 1990).
Among the plant species previously measured, emission rates of isoprene differ by more than three orders of magnitude (Benjamin et al., 1996) and the resulting ozone-forming potential of individual trees and shrubs ranges over nearly four orders of magnitude (Benjamin and Winer, 1998). BVOC emissions are affected by many factors, including genetics, light, temperature, CO2 concentration, humidity, plant health, transpiration rate, stomatal conductance, leaf development, time of day, season and environmental stresses (Guenther et al., 1993, Guenther et al., 1995).
Given the enormous number of plant species in regions, a framework for understanding and categorizing emissions rates, and for assigning rates to unmeasured species is necessary for assembling species-specific BVOC emission inventories (Benjamin et al., 1998). Categorizations of BVOC emissions rates based on plant taxonomy have been made (Guenther et al., 1994; Rasmussen and Khalil, 1997; Klinger et al., 1998; Csiky and Seufert, 1999) and taxonomic approaches were used in developing BVOC emissions inventories for urban areas (Geron et al., 1995; Benjamin et al., 1997). An explicit link between isoprene and monoterpene emissions rates and plant taxonomic relationships was proposed for species found in southern California (Benjamin et al., 1996) and subsequently used with the available emission rate literature to compile a detailed BVOC emissions inventory for the California South Coast Air Basin (SoCAB) (Benjamin et al., 1997). Additional emission rate data have since become available from the Biogenic Emissions in the Mediterranean Area (BEMA) project (Owen et al., 1997, Owen et al., 1998; Csiky and Seufert, 1999) and at this time it may be possible to make general predictions of emission rates based on plant taxonomy, although important exceptions have been identified (Staudt and Seufert, 1995; Kesselmeier et al., 1996; Loreto et al., 1996).
California's flora is exceptionally diverse both in terms of numbers of species and species density. Exotic species have become naturalized, and a total of 173 plant families, 1222 genera, 5862 species and 1169 subspecies found in natural plant communities in California have been listed and described (Hickman, 1993). These numbers do not include hundreds of exotic species which have been introduced into urban landscapes. Clearly, it is not possible to measure experimentally the isoprene emission rates of even a small fraction of the plant species found. However, for BVOC inventory development, some systematic method of assigning emission rates to unmeasured species is needed. The present study produced experimental isoprene emission rate data to provide a test of estimates based upon taxonomic relationships (Benjamin et al., 1996), and expanded the measured isoprene emission rate database by more than 60 species.
Section snippets
Location and selection of plant specimens
Candidate plant species were selected to allow empirical evaluation of emission rate estimates based upon taxonomy. For intrafamily comparison, species were chosen to explore whether additional (not previously measured) genera within families had isoprene emission rates similar to the family mean. For intrageneric comparisons, species were chosen where others within the genus had been reported in Benjamin et al. (1996). Most urban tree and shrub species sampled in the study were found in
Isoprene emission rates
The isoprene emission rate data collected in this study were branch-level results, and were expected to be approximately 60% of leaf-level results obtained with a cuvette apparatus (Guenther et al., 1994; Harley et al., 1997). We followed the example of Guenther et al. (1994) in describing normalized isoprene emission rate ranges, with slight modifications: our ranges were based on branch-level data, we used units of micrograms of isoprene per gram dry leafmass per hour (μg g−1 h−1), and we
Isoprene emission rates within families
As seen from Table 1, species sampled in this study within 18 families had isoprene emissions BDL. These results are consistent with the literature cited in Benjamin et al. (1996) for nine families, including Anacardiaceae, Bignoniaceae, Caprifoliaceae, Compositae, Cupressaceae, Juglandaceae, Lamiaceae, Oleaceae, Rosaceae, and Taxodiaceae. Additional families not reported in Benjamin et al. (1996) but sampled in this study and found to contain only negligible emitters included Cornaceae,
Conclusion
Given the enormous number of plant species in both urban and natural landscapes, a system will be necessary for categorizing emissions rates and for assigning rates to unmeasured species for assembling BVOC emission inventories. As Benjamin et al. (1996) emphasized, the taxonomic relationship they proposed to meet this need is possible because there is a range of almost four orders of magnitude in emission rates, allowing resolution between negligible, low, moderate and high emitters.
Within an
Acknowledgments
We thank C. Haas for conducting the GC analyses of isoprene samples, J. Arey and S. Aschmann for helpful suggestions regarding the GC protocol, W. Long and P. Newberg for design and fabrication of the enclosure and frame, and M. Gregg for conducting the enclosure sampling and processing of leaf samples. E. Sampson, curator of the Mourning Cloak Botanic Garden, provided access to the plant collection of the Garden. The contributions of members of the California Air Resources Board staff,
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