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Nutrient Losses from Timber Harvest in the Idaho Batholith¹

ABSTRACT

Nutrient budgets for a mature ponderosa pine/Douglas-fir [Pinus ponderosa Dougl./Pseudotsuga menziesii (Mirbel) Franco var. glauca (Beissn.) Franco] forest in the southwestern Idaho batholith are used to evaluate the potential for accelerated nutrient losses associated with timber harvesting. Soils formed from granitic rock in the Idaho batholith are commonly shallow, coarse textured, and highly erodible following disturbance. Cycling and annual budgets for K, Ca, Mg, S, P, and N are described for an undisturbed control watershed, and accelerated losses assoicated with clearcutting, helicopter yarding, and broadcast burning of slash are evaluated in an adjacent treated watershed. There were small but statistically significant increases in dissolved N losses of approximately 10 times preharvest rates for a 4-yr period following treatment. Dissolved transport of other elements was not increased. Small but significant increases in sediment nutrient transport of all elements occurred 3 yr following harvest. The largest losses in nutrients were due to bole removal and ranged from 4% of total ecosystem N to 21% of ecosystem K. Rates of replacement from precipitation, primary mineral weathering, and N₂ fixation would restore the ecosystem to the nutrient status prior to harvest in 50 yr. Based on these findings, logging systems that minimize erosion should not cause unacceptable nutrient loss over a normal timber stand rotation.

¹ Contribution from the U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station, Ogden, UT 84401.

² Principal Research Soil Scientist and Chemist, respectively, stationed at the Forestry Sciences Laboratory, 316 E. Myrtle St., Boise, ID 83702.