Measured isoprene emission rates of plants in California landscapes: comparison to estimates from taxonomic relationships

Abstract

Isoprene emission rates of 64 plant species found in California's urban and natural landscapes were measured using a dynamic flow-through chamber enclosure technique. Species were selected to provide data for previously unmeasured species and to test estimates of isoprene emission rates based upon taxonomic relationships developed for compilation of biogenic emission inventories as proposed by Benjamin et al. (1996, Atmospheric Environment 30, 1437–1452). Branch-level isoprene emission rates ranged from undetectable for 47 species, to 54 μg g⁻¹ h⁻¹ for Quercus kelloggii, California black oak. Isoprene emission rate estimates based on taxonomy agreed well with our measurements for species within the same genus, with the exception of the Quercus genus for which a wide range of isoprene emission rates have been reported. As expected, family-level estimates based on taxonomy showed greater deviation from our measured values than did genus-based estimates. The data developed in the present study support use of a taxonomic predictive methodology, especially if previous measurements within specific families, sub-families, and genera are extensive, and the results of such assignment are treated with proper caution. A taxonomic approach may be most useful where plant species in natural and urban landscapes are numerous, such as in California, where no experimental measurements are available for thousands of species.

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.