Annual linear growth, net primary production, and decomposition of Sphagnum fuscum, Sphagnum magellanicum, and Sphagnum angustifolium were measured under experimental acidification and natural conditions in a poor fen at the Experimental Lakes Area (ELA), Ontario, Canada. Acidification increased growth and production of most species (2 out of 3 in the oligotrophic zone, and 2 out of 3 in the minerotrophic zone) in the first 2 yr. After 2-3 yr of artificial acidification, growth and production were not stimulated to the same extent in experimental areas as in earlier years. After 4 yr, growth and production in the experimental area declined so that they were the same as controls for 4 of the 6 treatments. Therefore, the effect of the acid treatment changes over the years. With 4 yr of simulated 'acid rain,' decomposition was unaffected. Our results suggest that the fertilizing effect of [SO.sub.4.sup.2-] and [NO.sub.3.sup.-] in North American acid precipitation as suggested previously in the literature is a very short-term one, at least for this type of peatland. Under natural conditions, in the oligotrophic central zone of mire 239, production for hollow species was somewhat greater than for hummock species for the 4 yr studied. Since the decomposition rate ratio between hummock-top, mid-hummock, and hollow species is roughly in the ratio 7:9:13, the rate of peat accumulation should be higher in hummocks. Hence hummocks appear to be maintained in this poor fen due to low decomposition rates rather than relatively high production. For 3 of the 4 yr studied (1985-1987) in the minerotrophic edge zone, hummock production (303, 175, and 156 [g.m.sup.-2].[yr.sup.-1], respectively) was greater than hollow (198, 100, and 97 [g.m.sup.-2].[yr.sup.-1]) and mid-hummock production (103, 59, and 52 [g.m.sup.-2].[yr.sup.-1]). The relative decomposition rates of the species had the same ratio as in the oligotrophic zone. Thus hummocks appear to be expanding in this minerotrophic edge zone at a faster rate than in the oligotrophic zone.