In human dominated ecosystems worldwide, simultaneous and successive stressors are a common but often overlooked occurrence. For ubiquitous anthropogenic stressors like air pollution, complex interactions with co-occurring environmental conditions can influence dynamic processes such as forest growth. By understanding responses to multiple environmental conditions, ecologists can describe the relationships amongst these factors and reveal underlying ecosystem functions. In this study, we examine how Sierra Nevada forest ecosystems respond to climatic conditions and chronic ozone pollution, both separately and in interaction. A gradient of pollution exposure on the western slope of the southern Sierra Nevada enables our comparison of annual tree growth under very low to very high summer ozone levels; these sites also share climatic conditions. Our tools for understanding response to interacting environmental conditions included the record of growth from Pinus Jeffreyitree rings, over 20 years’ monitoring of air pollution damage to trees in and around Sequoia and Kings Canyon National Parks, and air quality monitoring at co-located sites the region.
Results/Conclusions
We first established that climatic conditions in the year prior to growth had an important influence on annual growth. Total precipitation from the previous June through February and mean temperature from March through September were key to growth in the following year. Building on this understanding of climatic dependency, linear mixed-effects analysis showed that trees exposed to elevated ozone had slower annual growth rates than their counterparts in relatively unpolluted locations. Growth rates in severely polluted sites were 8.4-23% lower than predicted growth under conditions that meet standards set by the Environmental Protection Agency. This difference has narrowed in recent years, tracking improvements in air quality. Although the isolated effects of both ozone and water limitation were negative, we also found that an antagonistic interaction occurs between these environmental factors. When summer temperatures exceed 22 degrees C, the negative growth impacts of ozone pollution are attenuated and even reversed. A potential mechanism for this interaction amongst stressors is stomatal closure, which prevents harmful uptake of ozone into the leaf. These growth losses, attributable to a chronic anthropogenic stressor and modified by prevailing environmental conditions, may facilitate further change in forest processes, including predisposing trees and forests to catastrophic mortality.