Ozone exposure and nitrogen deposition lowers root biomass of ponderosa pine in the San Bernardino Mountains, California

Decreased root biomass in forest trees in response to anthropogenic pollutants is believed to be one of the first steps in forest health degradation. Although decreased root biomass has been observed in controlled experiments, ozone effects on mature tree roots in natural stands has not previously been documented. Here we report standing root biomass of ponderosa pine at three sites in the San Bernardino Mountains distributed along a known, long-term pollution gradient of ozone and nitrogen deposition. Trees at each site were assessed for foliar ozone injury and below-ground attributes, in addition to other environmental factors known to influence root growth. During the period of peak root growth in the spring, root biomass at the least polluted site was 6–14 times greater than that observed at the most polluted site. Known differences in climatic and edaphic factors among the sites potentially contributing to the observed response were discounted as primary contributors to the response since in most cases the site differences would have driven the patterns of root growth in the opposite direction to that observed. Differences in biotic competitive interactions, also known to affect root growth, did not explain the observed pattern for the same reason. The data suggests that elevated ozone, high nitrogen deposition, and possibly other contributing factors such as soil acidification are primarily responsible for lowering root biomass in ponderosa pine stands in the San Bernardino Mountains.