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Atmospheric Deposition to a Low-Elevation Spruce-Fir Forest, Maine, USA

Dep. of Applied Ecology and Environ. Sci., Univ. of Maine, 5722 Deering Hall, Orono, ME 04469.

^* Corresponding author (rpt391{at}maine.maine.edu).

ABSTRACT

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 (SO^2–₄) decreased by 24 and 42%, respectively, during the study period, but no temporal trends occurred for precipitation H⁺ or SO^2–₄ concentrations. Decreased precipitation fluxes were likely due to a 33% decrease in precipitation amounts during the latter years of the study. Both H⁺ and SO^2–₄ 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 SO^2–₄, 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.

Sponsoring organizations are the USDA Forest Service Northeastern Forest Exp. Stn. and the Univ. of Maine.

Received for publication March 27, 1995.