Air quality in natural areas: interface between the public, science and regulation.

Natural areas are important interfaces between air quality, the public, science and regulation. In the United States and Canada, national parks received over 315million visits during 2004. Many natural areas have been experiencing decreased visibility, increased ozone (O(3)) levels and elevated nitrogen deposition. Ozone is the most pervasive air pollutant in North American natural areas. There is an extensive scientific literature on O(3) exposure-tree response in chambered environments and, lately, free-air exposure systems. Yet, less is known about O(3) impacts on natural terrestrial ecosystems. To advance scientifically defensible O(3) risk assessment for natural forest areas, species-level measurement endpoints must be socially, economically and ecologically relevant. Exposure-based indices, based on appropriate final endpoints, present an underused opportunity to meet this need. Exposure-plant indices should have a high degree of statistical significance, have high goodness of fit, be biologically plausible and include confidence intervals to define uncertainty. They must be supported by exposure-response functions and be easy to use within an air quality regulation context. Ozone exposure-response indices developed within an ambient air context have great potential for improving risk assessment in natural forest areas and enhancing scientific literacy.

New exposure-based metric approach for evaluating O(3) risk to North American aspen forests.

The United States and Canada currently use exposure-based metrics to protect vegetation from O(3). Using 5 years (1999-2003) of co-measured O(3), meteorology and growth response, we have developed exposure-based regression models that predict Populus tremuloides growth change within the North American ambient air quality context. The models comprised growing season fourth-highest daily maximum 8-h average O(3) concentration, growing degree days, and wind speed. They had high statistical significance, high goodness of fit, include 95% confidence intervals for tree growth change, and are simple to use. Averaged across a wide range of clonal sensitivity, historical 2001-2003 growth change over most of the 26 Mha P. tremuloides distribution was estimated to have ranged from no impact (0%) to strong negative impacts (-31%). With four aspen clones responding negatively (one responded positively) to O(3), the growing season fourth-highest daily maximum 8-h average O(3) concentration performed much better than growing season SUM06, AOT40 or maximum 1h average O(3) concentration metrics as a single indicator of aspen stem cross-sectional area growth.

Impacts of greenhouse gases on epicuticular waxes of Populus tremuloides Michx.: results from an open-air exposure and a natural O3 gradient.

Epicuticular waxes of three trembling aspen (Populus tremuloides Michx.) clones differing in O3 tolerance were examined over six growing seasons (1998-2003) at three bioindicator sites in the Lake States region of the USA and at FACTS II (Aspen FACE) site in Rhinelander, WI. Differences in epicuticular wax structure were determined by scanning electron microscopy and quantified by a coefficient of occlusion. Statistically significant increases in stomatal occlusion occurred for the three O3 bioindicator sites, with the higher O3 sites having the most affected stomata for all three clones as well as for all treatments including elevated CO2, elevated O3, and elevated CO2 + O3. We recorded statistically significant differences between aspen clones and between sampling period (spring, summer, fall). We found no statistically significant differences between treatments or aspen clones in stomatal frequency.

Air pollution and forest health: toward new monitoring concepts.

It is estimated that 49% of forests (17 million km(2)) will be exposed to damaging concentrations of tropospheric O(3) by 2100. Global forest area at risk from S deposition may reach 5.9 million km(2) by 2050, despite SO(2) emission reductions of 48% in North America and 25% in Europe. Although SO(2) levels have decreased, emissions of NO(x) are little changed, or have increased slightly. In some regions, the molar SO(4)/NO(3) ratio in precipitation has switched from 2/1 to near 1/1 during the past two decades. Coincidentally, pattern shifts in precipitation and temperature are evident. A number of reports suggest that forests are being affected by air pollution. Yet, the extent to which such effects occur is uncertain, despite the efforts dedicated to monitoring forests. Routine monitoring programmes provide a huge amount of data. Yet in many cases, these data do not fit the conceptual and statistical requirements for detecting status and trends of forest health, nor for cause-effect research. There is a clear need for a re-thinking of monitoring strategies.

Environmental influences on the development of spruce needle cuticles.

Seedlings of Norway [Picea abies (L.) Karst.], red (P. rubens Sarg.), black [P. mariana (Mill) B.S.P.] and white [P. glauca (Moench.) Voss] spruce were grown from bud break to bud set outdoors, in the greenhouse, and in controlled environment chambers. The outdoor and greenhouse treatments also included adjacent shaded (50%) seedlings. The effects of growth environment on needle epicuticular wax production and morphology, and on needle wettability were investigated. Red and black spruce exhibited free growth in addition to determinate growth, but only in the greenhouse. Although relative growth rate was significantly lower for seedlings grown outdoors, there were no differences in wax morphology between outdoor- and greenhouse-grown seedlings, Wax deposits on controlled environment seedlings were significantly more crystalline. Wax tubes were crystallized in 'tufts'. There were significant differences in wax amounts recovered between species and treatments. Wax amounts relative to needle dry weights decreased in the order greenhouse > outdoors > controlled environment. Shaded red and black spruce produced significantly more wax than unshaded. Needles from seedlings grown in controlled environment chambers had the largest needle/droplet contact angles. These data indicate that seedling growth environment affects needle epicuticular wax characteristics and wettability. Studies designed to investigate factors determined by needle surface phystcochemical characteristics should include a consideration of growth environment.

Effects of ozone and acidic fog on red spruce needle epicuticular wax production, chemical composition, cuticular membrane ultrastructure and needle wettability.

One-year-old red spruce (Picea rubens Sarg.) seedlings were exposed, from bud break, to ozone (O3 ) and acidic fog for 14, 42 or 77 days. Ozone was more damaging than acidic fog to epicuticular wax. Wax quantity on needles exposed to charcoal-filtered air (CFA) and pH 4.2 fog (control) increased from 14 to 42 days, but decreased by 77 days. Exposure of elongating needles to 70 ppb O3 and pH 4.2 fog reduced the rate of de novo wax synthesis. Red spruce wax is composed of secondary alcohols (50%), nonacosane diols (25%), alkyl esters (12%), primary alcohols (6 %), estolides (2 %), fatty acids (3 %), and hydroxy-fatty acids (2 %). Wax chemical composition varied temporally. Needles exposed to O3 had significantly less secondary alcohols, diols, alkyl esters, fatty acids, and hydroxy-fatty acids than needles exposed to CFA. Production of secondary alcohols was significantly reduced following needle exposure to fog at pH 3.0. Cuticular membrane thickness increased significantly following needle exposure to O3 . The increase in thickness at 250 ppb O3 was due to a new amorphous layer which appeared above the reticulate layer. Contact angles on needles exposed to CFA and pH 4.2 fog increased from 92° after 14 days to 96° after 42 days, then declined to 77° after 77 days. Needle exposure to O3 for 42 or 77 days significantly decreased contact angles.

Effects of simulated acid rain on leaf wettability, rain retention and uptake of some inorganic ions.

Leaves of Phaseolus vulgaris L., Vicia faba L., Pisum sativum L. and Brassica napus L. were exposed from emergence to full expansion to seven treatments of simulated acid rain at pH values between 5.6 and 2.6. Droplet leaf contact angles in all species decreased on leaves exposed to simulated acid rain at pH <4.6 relative to those exposed at pH 5.6. Retention of rain containing fluorescein increased on P. vulgaris leaves exposed to simulated acid rain at pH 4.6 and at pH < 3.8. Retention by B. napus leaves was increased at pH < 4.6. Uptake of the three ions studied was in the order 86 Rb+ ≤35 SO4 2- ≤63 Ni2+ . Uptake of 86 Rb+ increased into B. napus leaves and decreased into P. vulgaris leaves exposed at pH ≤ 3.4. Uptake of 35 SO4 2- and 63 Ni2+ by B. napus leaves increased after exposure to simulated acid ruin at pH 2.6 but it was unaffected in P. vulgaris. Up to 7%, of applied 35 SO4 2- , was found in the epicuticular wax layer on B. napus leaves 48 h after application, most being found after exposure at pH 2.6. The order of lateral movement of ions within leaves paralleled the order of uptake. Movement of all three ions was increased in B. napus leaves exposed previously to simulated acid rain at pH 2.6. The increased retention of fluorescein on leaves exposed to simulated acid rain and the reduced contact angles for water are attributed to decreases in surface roughness. The altered ion uptake pattern could also be related to changes in surface wax structure or could be associated with observed changes in properties of cuticular membranes. These results demonstrate that the interaction of plants with their atmospheric environment could possibly be affected by pre-exposure to acid rain in amounts and at pH values that occur in ambient rainfall.