NITROGEN SUPPLY, NITROGEN USE, AND PRODUCTION IN AN AGE SEQUENCE OF LODGEPOLE PINE

Forest productivity typically declines after reaching a peak early in stand development. Historically this pattern was attributed to constan t or declining stand photosynthesis and increasing stand respiration, but this explanation has been largely invalidated by field measurement s and modeling. Production in older stands might decline as a result o f declining soil nutrient supply. To test this idea, we examined patte rns of productivity and nitrogen (N) supply and use in four age classe s of lodgepole pine (Pinus contorta Dougl, ex. Loud.) ecosystems in so utheastern Wyoming. Aboveground net primary production (ANPP) increase d from 2200 kg ha(-1) yr(-1) at age 30 to 4600 kg ha(-1) yr(-1) at age 50,then declined to 2000 kg ha(-1) yr(-1) at age 200, Soil N supply ( indexed with ion exchange resin bags) was highest in the 30-yr-old sit es, but showed no further pattern with age in older sites. The ratio o f ANPP to the supply of N rose sharply from age 30 to age 50, and then declined slowly. The ratio of ANPP to the N actually taken up was hig h for the three youngest age classes, but declined substantially in th e old-growth (200 yr) site. We conclude that the low ANPP of the old-g rowth stands was probably not driven by changes in stand nutrition; th e decline in efficiency of N use indicates some other factor probably constrains productivity of old-growth stands.