Simulating the response of mature yellow poplar and loblolly pine trees to shifts in peak ozone periods during the growing season using the TREGRO model

Abstract

Multiple TREGRO simulations were conducted with meteorological data files containing different growing season peak ozone (O₃) episodes at O₃ exposures of 1.0 and 2.0 × ambient O₃ to assess the relationship between O₃ response and the phenology of mature yellow poplar (Liriodendron tulipifera L.) and loblolly pine (Pinus taeda L.) trees. Regardless of O₃ exposure and peak O₃ episode occurrence, a peak O₃ episode in August caused the greatest reduction in carbon (C) gain in yellow poplar, whereas a peak O₃ episode in July caused the greatest reduction in C gain of loblolly pine. In both species, timing of the greatest simulated O₃ effect corresponded with the completion of the annual foliage production phenophase. Simulated C gain of yellow poplar (total tree, coarse root, and total nonstructural carbohydrate) was reduced by O₃ to a greater extent than the corresponding compartments in loblolly pine, but the opposite was true for fine roots. This differential sensitivity to O₃ reflects the fact that both C assimilation and the O₃ response of the species were parameterized according to observed field measurements of each species. The differential sensitivity to O₃ of these species may have long-term implications for species composition in southeastern USA forests.