Technical Note: Nitrogen Supply, Nitrogen Use, and Production in an Age Sequence of Lodgepole Pine
Abstract:Forest productivity typically declines after reaching a peak early in stand development. Historically this pattern was attributed to constant or declining stand photosynthesis and increasing stand respiration, but this explanation has been largely invalidated by field measurements and modeling. Production in older stands might decline as a result of declining soil nutrient supply. To test this idea, we examined patterns of productivity and nitrogen (N) supply and use in four age classes of lodgepole pine (Pinus contorta Dougl. ex. Loud.) ecosystems in southeastern Wyoming. Aboveground net primary production (ANPP) increased 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 sites, but showed no further pattern with age in older sites. The ratio of 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 high for the three youngest age classes, but declined substantially in the old-growth (200 yr) site. We conclude that the low ANPP of the old-growth stands was probably not driven by changes in stand nutrition; the decline in efficiency of N use indicates some other factor probably constrains productivity of old-growth stands. For. Sci. 44(3):454-457.
Document Type: Technical Note
Affiliations: Professor, Department of Forest Sciences at Colorado State University
Publication date: 1998-08-01
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