Abstract
The Mid-Atlantic region of the United States ranks as one of the top polluted regions with respect to ground-level ozone. Species-specific responses of trees to ozone may contribute to shifts in forest community structure. This study systematically compared the ozone sensitivity of native and invasive tree species common to MidAtlantic forests. The response of these tree species to ozone was examined in terms of gas-exchange, antioxidant capacity, and shifts in modeled forest community structure. A common garden of 13 (8 native, 5 invasive) species was planted at Blandy Experimental Farm and exposed to three ozone treatments (20 ppb, 80 ppb, 160 ppb) during the summers of 2008-2009. Strong species-specific responses were measured in stomatal conductance, ozone uptake, and post-exposure net photosynthesis. Overall, native and invasive species did not differ in response to ozone. To assess antioxidant protection, ascorbic acid levels within leaf extracts were quantified by hydrophilic interaction liquid chromatography. Constitutive levels of ascorbic acid differed among species. Invasive species contained higher constitutive levels of this antioxidant than native species. After ozone treatment, the majority of species had decreases in ascorbic acid. High post-exposure ascorbic acid levels were found in plants that had increases in net photosynthesis. Species-specific ozone tolerance and life-history traits were used to parameterize an individual-based physiological tree model (TREGRO), which was linked to a spatially explicit stand gap model (ZELIG). Three forests were simulated in ZELIG, which varied in ozone level (low, medium, high). The highest abundance of native species was found in medium treatment forests, while low and high ozone treated forests had similar iv presence of native and invasive species. Species composition and forest structure were affected by ozone exposure, and the response of a forest related to the ozone sensitivity and shade tolerance of modeled species. The multispecies comparisons made throughout this study allowed for the assessment of species-specific physiological responses to ozone. These differences shifted competitive abilities, resulting in altered forest dynamics but not vast restructuring of the forest community. Future studies evaluating ozone stress on a forest community would benefit from quantifying physiological-based ozone tolerance for multiple species.
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