Gas-exchange and antioxidant response to elevated ground-level ozone of native and invasive deciduous tree species

Background/Question/Methods   Ozone is a phytotoxic greenhouse gas that accumulates in the troposphere due to the chemical interactions of the products of combustion, volatile organic compounds, and nitrogen oxides. Because of increased production of ozone precursor emissions, tropospheric ozone is increasing its geographic range. Twenty-five percent of all forests routinely experience hourly ozone level of 60ppb, the threshold considered harmful for plants. Ozone is predicted to increase in concentration by 60% in the next century, therefore magnifying the risk of forest community structure shifts in the future. The potential of ozone to alter community structure of a forest lies in the variance of ozone-tolerance among tree species. Sensitive species will suffer greater consequences of high ozone concentrations, creating greater opportunities for an increased presence of tolerant species. By quantifying the tolerance of native and invasive species, we can generate better predictions about future invasive species threats in a time when ozone concentrations are above 60ppb. This study focuses on eight native and five invasive species pervasive in mid-Atlantic temperate forests. Each species was planted into a common garden at Blandy Experimental Farm, where leaves were isolated for ozone exposure treatments during the summers of 2008-2009. Carbon assimilation (A), evapotranspiration, and ozone-uptake were measured during trials with a 3-chamber gas-exchange manifold. To examine the effects of ozone on A after trials, a QUBIT photosynthesis system was used two days following treatments. Control and treated leaves were collected after QUBIT measurements, and flash frozen to test for the antioxidant ascorbic acid (ASA). Levels of ASA were measure using hydrophilic interaction chromatography.

Results/Conclusions   Results show the influence of ozone on A differs by species (X2=5.41,p=0.02), as did cumulative ozone uptake (COU) (X2=6.62,p=0.01). All species showed significant negative correlations between A and COU. Post-exposure values of A show that two days after trials, A of exposed leaves is significantly reduced by ozone treatments (F=3.66,p=0.0425). These results indicate persistent effects of ozone damage not measured during exposure trials. Levels of ASA significantly differed among species in control (F=5.62,p<0.0001) and treated leaves (F=3.23,p=0.0113). Though ozone did not show an effect on ASA levels (F=0.26,p=0.7761), there was a positive effect of ASA on A (F=4.17,p=0.0448). Among the medium ozone treatment, plants with higher COU and ASA were able to maintain high rates of A (F=6.79,p=0.0143). Native and invasive species did not differ in response to treatments instilling the importance of species-specific variation rather than a native and invasive division.