Evaluation of health impacts arising from inhalation of pollutant particles <10 μm (PM10) is an active research area. However, lack of exposure data at high spatial resolution impedes identification of causal associations between exposure and illness. Biomagnetic monitoring of PM10 deposited on tree leaves may provide a means of obtaining exposure data at high spatial resolution. To calculate ambient PM10 concentrations from leaf magnetic values, the relationship between the magnetic signal and total PM10 mass must be quantified, and the exposure time (via magnetic deposition velocity (MVd) calculations) known. Birches display higher MVd (∼5 cm−1) than lime trees (∼2 cm−1). Leaf saturation remanence values reached ‘equilibrium’ with ambient PM10 concentrations after ∼6 ‘dry’ days (<3 mm/day rainfall). Other co-located species displayed within-species consistency in MVd; robust inter-calibration can thus be achieved, enabling magnetic PM10 biomonitoring at unprecedented spatial resolution.

Betula papyrifera trees were exposed to elevated concentrations of CO2 (1.4 × ambient), O3 (1.2 × ambient) or CO2 + O3 at the Aspen Free-air CO2 Enrichment Experiment. The treatment effects on leaf surface characteristics were studied after nine years of tree exposure. CO2 and O3 increased epidermal cell size and reduced epidermal cell density but leaf size was not altered. Stomatal density remained unaffected, but stomatal index increased under elevated CO2. Cuticular ridges and epicuticular wax crystallites were less evident under CO2 and CO2 + O3. The increase in amorphous deposits, particularly under CO2 + O3, was associated with the appearance of elongated plate crystallites in stomatal chambers. Increased proportions of alkyl esters resulted from increased esterification of fatty acids and alcohols under elevated CO2 + O3. The combination of elevated CO2 and O3 resulted in different responses than expected under exposure to CO2 or O3 alone. The combined effects of CO2 and O3 on birch leaf surface characteristics cannot be predicted on the basis of studies examining each of these gases separately.

Gene expression responses of paper birch (Betula papyrifera) leaves to elevated concentrations of CO2 and O3 were studied with microarray analyses from three time points during the summer of 2004 at Aspen FACE. Microarray data were analyzed with clustering techniques, self-organizing maps, K-means clustering and Sammon's mappings, to detect similar gene expression patterns within sampling times and treatments. Most of the alterations in gene expression were caused by O3, alone or in combination with CO2. O3 induced defensive reactions to oxidative stress and earlier leaf senescence, seen as decreased expression of photosynthesis- and carbon fixation-related genes, and increased expression of senescence-associated genes. The effects of elevated CO2 reflected surplus of carbon that was directed to synthesis of secondary compounds. The combined CO2 + O3 treatment resulted in differential gene expression than with individual gas treatments or in changes similar to O3 treatment, indicating that CO2 cannot totally alleviate the harmful effects of O3. Clustering analysis of birch leaf gene expression data reveals differential responses to O3 and CO2.

Recent evidence from novel phytotron and free-air ozone (O3) fumigation experiments in Europe and America on forest tree species is highlighted in relation to previous chamber studies. Differences in O3 sensitivity between pioneer and climax species are examined and viewed for trees growing at the harsh alpine timberline ecotone. As O3 apparently counteracts positive effects of elevated CO2 and mitigates productivity increases, response is governed by genotype, competitors, and ontogeny rather than species per se. Complexity in O3 responsiveness increased under the influence of pathogens and herbivores. The new evidence does not conflict in principle with previous findings that, however, pointed to a low ecological significance. This new knowledge on trees' O3 responsiveness beyond the juvenile stage in plantations and forests nevertheless implies limited predictability due to complexity in biotic and abiotic interactions. Unravelling underlying mechanisms is mandatory for assessing O3 risks as an important component of climate change scenarios.

The forest hydrologic budget may be impacted by increasing CO2 and tropospheric O3. Efficient means to quantify such effects are beneficial. We hypothesized that changes in the balance of canopy interception, stem flow, and through-fall in the presence of elevated CO2 and O3 could be discerned using image analysis of leafless branches. We compared annual stem flow to the results of a computerized analysis of all branches from the 2002, 2004, and 2006 annual growth whorls of 97 ten-year-old trees from the Aspen Free-Air CO2 and O3 Enrichment (Aspen FACE) experiment in Rhinelander, WI. We found significant effects of elevated CO2 and O3 on some branch metrics, and that the branch metrics were useful for predicting stem flow from birch, but not aspen. The results of this study should contribute to development of techniques for efficient characterization of effects on the forest hydrologic budget of increasing CO2 and tropospheric O3. Canopy architecture and stem flow are affected by elevated CO2 and tropospheric O3.