Ecosystem Consequences of Prolonged Ozone Pollution in the Southern Sierra Nevada

Abstract:

While acute O₃ exposure is widely known to damage plant tissues, the chronic effects on long lived organisms such as trees remain unclear. In the southern Sierra Nevada, O₃ pollution has afflicted pine-dominated forests for over 40 years. Here we report the results of a long-term study of O₃ impact on tree injury, growth, and mortality. Our study employed a network of forest plots along a gradient of O₃ pollution with recurring measurements from 1991-2012. Over the same period and locations, summer O₃ was monitored via partnership with USNPS and USFS, making this one of the longest known ecosystem studies of O₃ pollution and its effects. We found that exposure at the most polluted sites declined 33%, from a W126 index of 20.12 ppm-hrs in 1992 to 13.5 ppm-hrs in 2012. The severity of foliar pollution damage at these sites also declined, from 43.9 on the 0-100 Ozone Injury Index (OII) scale to 34.2, a drop of 22%. At locations with lower O₃ exposure, damage declined from OII of 16.9 to 9.2. Mean annual tree mortality rates over the 20 year period, calculated with a profile likelihood approach, were 0.5%/yr (95% CI 0.3 to 0.8 %/yr). This rate is similar to that of healthy canopy trees in similar unpolluted stands. However, low and declining tree growth rates reveal possible ecosystem impacts of prolonged exposure to pollution. Across affected sites, mean relative growth rates were 1.1%/yr in 1991-2000, and just 0.9%/yr in 2000-2011, a decline of 15.6% in the second decade. Initial analyses suggest that tree damage is positively correlated with June-October O₃, as indicated by previous studies. Further analysis will explore the drivers of ecosystem impacts and roles of other natural and anthropogenic stressors, including variation in climatic water deficit. Understanding the consequences of prolonged O₃ exposure on both individual trees and complex forest ecosystems helps identify the hidden environmental costs of tropospheric O₃ and potential benefits of cleaner air.