Atmospheric Deposition to a Low-Elevation Spruce-Fir Forest, Maine, USA
- James W. McLaughlin,
- Ivan J. Fernandez * and
- Karen J. Richards
A spruce-fir forest in central-Maine, USA, has been the site of dry and wet atmospheric deposition measurements since 1988. Precipitation flux of hydrogen ions (H+) and sulfate (SO2−4) decreased by 24 and 42%, respectively, during the study period, but no temporal trends occurred for precipitation H+ or SO2−4 concentrations. Decreased precipitation fluxes were likely due to a 33% decrease in precipitation amounts during the latter years of the study. Both H+ and SO2−4 concentrations and fluxes in throughfall decreased, corresponding to decreases in dry deposition of those constituents. No nitrate (NO−3) concentration or flux trends occurred in precipitation, throughfall, or dry deposition. Hydrogen, NO−3, and NH+4 had a negative net canopy exchange (NCE) attributable to N deficiency at this site and acid neutralization in the canopy. Sulfate and magnesium (Mg2+) were conservative and had no NCE. Calcium (Ca2+) and potassium (K+) had a positive NCE due to canopy leaching. A comparison of bulk and wet-only precipitation collectors showed that SO2−4, Ca2+, and K+ concentrations were greater in solutions from a bulk collector compared with a wet-only collector. The differences were partially attributable to dry deposition. Throughfall H+, Ca2+, and dissolved organic carbon (DOC) concentrations were higher in weekly wet-only collections composited monthly, compared with weekly bulk collections composited either weekly or monthly. Nitrate concentration, however, was lowest in the wet-only throughfall collections. Differences likely resulted from spatial variability. Ongoing research is attempting to further define trends apparent to date, and to determine the mechanisms controlling these characteristics.
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