Sustained high elevation ozone in the northeastern US: Physiological indicators in red spruce-dominated forests

Background/Question/Methods

Seasonal tropospheric ozone is a powerful photooxidant known to attack vegetation in natural, urban and agro-ecosystems.  Ozone pollution is especially persistence at remote higher elevations as characterized by nocturnally stable diel exposures, unlike typical diurnal ozone formation and fate observed at lower elevations.  Recently the 2009 and 2011 ozone seasons represented positive air quality anomalies, but elevated ozone events associated with more typical summer weather patterns continue to occur.  Montane red spruce forests represent a small and susceptible portion of forested ecosystems in the Northeast, so it is essential to understand all risk factors that may threaten their long-term viability. Bi-monthly measures of the physiological status of current and 2^nd year foliage were made at select high-elevation sites.  Stomatal conductance to water vapor (g_s) was converted to ozone conductance (g_O3).  Ozone flux to the leaf interior (F_O3) was derived from g_O3 and co-occurring [O₃].  Ozone flux, versus exposure, or potential flux, is the most accurate risk assessment metric available to quantify phytotoxic ozone dose.  Study goals were i) to assess long-term trends in ozone exposure based on up to 20 years of archival data, and ii) to quantify and compare the environmental drivers of ozone uptake in alpine environments from those at lower elevation.  

Results/Conclusions

Long-term air quality at surveyed high-elevation sites is non-trending (neither improving nor deteriorating).  Monthly average exposures are geographically similar, showing early season maxima. A substantial proportion of exposure in the highest concentration ranges (≥80 ppb) occurs nocturnally at high elevation, frequently exceeding that of valley locations. Highest average monthly exposures tend to occur close to flush of current-year red spruce needles.  Seasonal variability in simultaneous physiological condition of foliage of red spruce saplings mitigated some ozone uptake, e.g., where current-year needles had not achieved their seasonal maximum stomatal conductance (g_max).  Lower elevation needles showed higher average conductances.  Nevergtheless, stomatal ozone uptake potential in montane red spruce was determined to be significant.  Red spruce do not display signature foliar ozone injury symptoms, but phytotoxic ozone is a likely player in the multiple biotic and abiotic stressors implicated in the ecology of red spruce decline.