Foliar C-abundance (δC) was analyzed in the dominant trees of a temperate deciduous forest in east Tennessee (Walker Branch Watershed) to investigate the variation in foliar δC as a function of time (within-year and between years), space (canopy height, watershed topography and habitat) and species (deciduous and coniferous taxa). Various hypotheses were tested by analyzing (i) samples collected from the field during the growing season and (ii) foliar tissues maintained in an archived collection. The δC-value for leaves from the tops of trees was 2 to 3%. more positive than for leaves sampled at lower heights in the canopy. Quercus prinus leaves sampled just prior to autumn leaf fall had significantly more negative δC-values than those sampled during midsummer. On the more xeric ridges, needles of Pinus spp. had more positive δC-values than leaves from deciduous species. Foliar δC-values differed significantly as a function of topography. Deciduous leaves from xeric sites (ridges and slopes) had more positive δC-values than those from mesic (riparian and cove) environments. On the more xeric sites, foliar δC was significantly more positive in 1988 (a dry year) relative to that in 1989 (a year with above-normal precipitation). In contrast, leaf δC in trees from mesic valley bottoms did not differ significantly among years with disparate precipitation. Patterns in foliar δC indicated a higher ratio of net CO assimilation to transpiration (A/E) for trees in more xeric versus mesic habitats, and for trees in xeric habitats during years of drought versus years of normal precipitation. However, A/E (units of mmol CO fixed/mol HO transpired) calculated on the basis of δC-values for leaves from the more xeric sites was higher in a wet year (6.6±1.2) versus a dry year (3.4±0.4). This difference was attributed to higher transpiration (and therefore lower A/E) in the year with lower relative humidity and higher average daily temperature. The calculated A/E values for the forest in 1988-89, based on δC, were within ±55% of estimates made over a 17 day period at this site in 1984 using micrometeorological methods.

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We have modified recently published dry deposition models to estimate deposition velocities (Vd) for Hg in both fine aerosol and vapor form to forest canopy surfaces. Aerosol and total vapor phase Hg concentrations in air previously measured at Walker Branch Watershed in Tennessee have been used with model results to estimate dry deposition to a deciduous forest. The concentration data confirm that airborne Hg is dominated by vapor forms at this site and exhibits concentrations moderately above continental background levels. The modeled Vd values reflect published data which suggest that dry deposition of Hg vapor is strongly controlled by surface transport processes, notably stomatal and mesophyll resistances, the latter dominating. Weekly mean Vd values ranged from 0.006 (winter) to 0.12 (summer) cm s−1. We have also measured concentration gradients of Hg vapor in air above this forest to estimate air‐surface exchange during short‐term experiments. While the model results indicate that the canopy is a sink for Hg vapor, the concentration profiles suggest that the forest soils are a source during some periods, the combined effect of which is net Hg fluxes in the upward direction. Application of a detailed canopy turbulence model yielded soil emission rates of the order of 50 ng Hg m−2 h−1, ∼10% of which is deposited in the canopy. Our modeled dry deposition estimates plus limited measurements of wet deposition in this area suggest that dry and wet deposition may be comparable in magnitude.

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