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 polluta nts 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 b een documented. Here we report standing root biomass of ponderosa pine at t hree sites in the San Bernardino Mountains distributed along a known, long- term pollution gradient of ozone and nitrogen deposition. Trees at each sit e were assessed for foliar ozone injury and below-ground attributes, in add ition to other environmental factors known to influence root growth. During the period of peak root growth in the spring, root biomass at the least po lluted site was 6-14 times greater than that observed at the most polluted site. Known differences in climatic and edaphic factors among the sites pot entially contributing to the observed response were discounted as primary c ontributors to the response since in most cases the site differences would have driven the patterns of root growth in the opposite direction to that o bserved. Differences in biotic competitive interactions, also known to affe ct root growth, did not explain the observed pattern for the same reason. T he data suggests that elevated ozone, high nitrogen deposition, and possibl y other contributing factors such as soil acidification are primarily respo nsible for lowering root biomass in ponderosa pine stands in the San Bernar dino Mountains. Published by Elsevier Science Ltd.