Response of pioneer plant communities to elevated ozone exposure

Considerable research has documented the effects of ozone on crop plants, but little experimental work has examined the effects of ozone on natural vegetation. Our objective was to determine how a plant community responds over several generations to elevated ozone exposures. Seed bank soil collected from the Oregon State University Farm, containing a population of naturalized plants common to the Willamette Valley, was uniformly dispersed across the surface of nine modified open top chambers. Each chamber was randomly assigned one of three ozone treatments (0, 90 and 120 ppb episodic ozone). Sixty plant species from 22 families emerged in the chambers over the four year study. Seedling emergence was a highly sensitive period. High seedling mortality rates in the ozone treatments allowed more resistant individuals to persist. Communities not exposed to elevated ozone levels also had high rates of mortality probably due to density dependant mortality. No species were eliminated from the communities because of ozone. Plant species did not demonstrate ozone tolerance or susceptibility based on taxonomic relationships. Year-to-year environmental heterogeneity was a stronger influence on community composition than ozone exposures. At the community level, there was a trend of decreasing biomass with increasing ozone exposure. In the ozone treatments, premature senescence of taller species increased light availability to understory species, changing competitive interactions and altering community dynamics in the understory. The effects of ozone on individual species, both direct and indirect, in a community may be detrimental, insignificant, or positive. Predication of a species performance in a community will be difficult due to the many environmental and biological interactions occurring simultaneously or at alternative times during a specific species life cycle. Studies are necessary to decrease the uncertainty in attempting to assess the impact of ozone on both managed and natural ecosystems currently determined from extrapolating the effects on individuals. Currently, plant communities remain protected at an unknown level from ozone exposure.