Water and nitrogen outflow from lodgepole pine forest after two levels of tree mortality

Abstract

Water and nitrogen outflow beyond the rooting zone were estimated in three adjacent forest plots dominate by lodgepole pine ( Pinus contorta ssp. latifolia [Engelm. ex Wats.] Critchfield) in the Medicine Bow Mountains, southeastern Wyoming, USA. One of the plots was clearcut, one was thinned by tree girdling and fungal infection, and the third was monitored as a control. Water outflow was estimated using a stand-level simulation model, and N outflow was calculated as the product of N concentration in the soil-water and the volume of soil-water outflow. Tree thinning reduced stand leaf area by 43% and led to a 92% increase in water outflow; clearcutting the stand increased water outflow by 277%. The thinning treatment, which killed 60% of the trees, had an insignificant effect on nitrate and total N outflow 2 years later. However, clearcutting increased nitrate concentrations by 10–40 times and total N outflow by about 37 times. The marked differences in N outflow from the thinned and clearcut stands suggest that surviving trees in the thinned stand are important for nutrient immobilization. Nitrogen outflow is compared with N inputs, N losses via tree harvesting, and N outflow after disturbances in other forests.