Nutrient limitation is often assumed to be similar among the species of a plant community. However, limitation can differ among ecosystems and among life forms and individual species within a particular ecosystem. Peatlands have some of the lowest nutrient availabilities and highest acidities among wetland types, but the relative roles of nutrient limitation and pH stress in structuring peatland plant communities are unknown. Accordingly, we measured changes in above-ground net primary production (ANPP) and percent cover of plants to additions of low levels of N, P, and calcium carbonate in a bog and fen in northern Minnesota, USA. Plots were treated for three years with a combination of 2 or 6 g N m−2 yr−1 as ammonium, 0.67 or 2 g P m−2 yr−1, and/or calcium carbonate to raise the pH of the bog from 3.8 to 4.9 and the pH of the fen from 4.9 to 6.4. In the bog, the low N treatment increased ANPP, whereas the high N treatment inhibited ANPP. Lime addition also stimulated ANPP. The whole-community bog response was largely due to bryophytes, which accounted for 76% of ANPP on average. However, the productivity of the shrub community (18% of total ANPP) increased with P additions but only during the third year of fertilizer application. Productivity of the bog graminoids did not respond significantly to any addition. Fen ANPP was stimulated by P addition, but the effect was isolated to graminoids (95% of total ANPP), and this was largely due to the response of Carex exilis. Our results suggest that low nutrient availability does not necessarily imply nutrient limitation of peatland plant communities. Furthermore, life forms and individual species responded differently, indicating that there are several levels of nutrient limitation within each peatland community. In particular, bog Sphagnum mosses appear to have a very low tolerance for N. Production and community structure were controlled by N-availability and pH in the bog and by P-availability in the fen.
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Vol. 24 • No. 1