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Effects of Chemical Defoliation of an Abies grandis Habitat on Amounts and Chemistry of Throughfall and Stemflow¹

ABSTRACT

Amounts and chemistry of gross precipitation, throughfall, and stemflow were measured before and after chemical defoliation of a coniferous forest stand in eastern Washington. Throughfall (T) in the natural stand is related to gross precipitation (P) by the equation T = 0.842P – 0.182 with a coefficient of determination (R²) of 0.972. Throughfall and stemflow increased significantly after defoliation but the actual amounts were small.

Gross precipitation before defoliation contained 0.47 mg/liter total nitrogen (T-N), 0.06 mg/liter total phosphorus (T-P), and 0.7 mg/liter total cations (Ca, Mg, Na, and K). Total inputs between 24 Aug. 1975 and 1 June 1976, were 0.84, 0.07, and 0.90 kg/ha for T-N, T-P, and total cations, respectively. Approximately 80% of the total annual precipitation is normally received during the period sampled.

Concentrations of chemical constituents were increased up to 15 times in throughfall and up to 48 times in stemflow compared to gross precipitation.

Ratios of nutrient delivery (kg/ha) before defoliation as gross precipitation/throughfall/stemflow were 1.00:1.01:0.02 for T-N, 1.00:3.78:0.15 for T-P, and 1.00:7.57:0.20 for total cations.

Defoliation reduced concentration of ammonium-N and increased concentration of T-P in throughfall. Concentrations of other measured constituents were not changed. Delivery of all nutrients to the forest floor, however, was greater after defoliation because of increased amounts of throughfall and stemflow.

Comparisons of summer with winter storms and dry fallout with precipitation indicated that (i) T-P and K are of local origin; (ii) N probably originates from passage of air masses over urban and industrial areas, automobile exhaust, and agriculture (fertilization); and (iii) Na and Mg are of marine origin.

Key Words: grand fir • precipitation chemistry • nutrients • nutrient cycling

¹ Contribution from USDA Forest Service, Pacific Northwest Forest and Range Exp. Stn., Forest Hydrology Lab., Wenatchee, WA 98801. The work leading to this publication was funded in part by the USDA Douglas-fir Tussock Moth Expanded Research and Development Program.

² Supervisory Range Scientist, Principal Hydrologist, and Chemist, respectively.

Received for publication January 31, 1979.