Deciduous conifers: High N deposition and O-3 exposure effects on growth and biomass allocation in ponderosa pine

Ponderosa pines (Pinus ponderosa Dougl. ex. Laws) 21 to 60 yr old were used to assess the relative importance of environmental stressors (O-3, drought ) versus an enhancer (N deposition) on foliar retention, components of abov eground growth, and whole tree biomass allocation. Sites were chosen across a well-described gradient in ozone exposure (40 to 80 ppb per h, 24 h basi s, 6 month growing season) and nitrogen deposition (5 to 40 kg ha(-1) yr(-1 )) in the San Bernardino Mountains east of Los Angeles, California. A high level of chlorotic mottle indicated high O-3 injury at sites closest to the pollution source, despite potential for the mitigating effects of N deposi tion. At the least polluted site, foliar biomass was evenly distributed acr oss three of the five needle-age classes retained. At the most polluted sit e, 95% of the foliar biomass was found in the current year's growth. High N deposition and O-3 exposure combined to shift biomass allocation in pine t o that of a deciduous tree with one overwintering needle age class. Based o n whole tree harvests, root biomass was lowest at sites with the highest po llution exposure, confirming previous chamber exposure and field studies. A boveground growth responses in the high-pollution sites were opposite to th ose expected for O-3 injury. Needle and lateral branch elongation growth, a nd measures of wood production increased with increasing proximity to the p ollution source. An enhancement of these growth attributes suggested that N deposition dominated the ponderosa pine response despite high O-3 exposure .