WATER AND NITROGEN OUTFLOW FROM LODGEPOLE PINE FOREST AFTER 2 LEVELS OF TREE MORTALITY

Water and nitrogen outflow beyond the rooting zone were estimated in three adjacent forest plots dominated 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.