Growth, Cover, and Productivity of Sphagnum Decline Sharply with Experimental Warming in a Forested Peatland

Peatlands hold a substantial stock of carbon in organic matter (peat) that accumulated over millennia and may now be vulnerable to unprecedented environmental changes. The source of much of the accumulated peat is mosses of the genus Sphagnum. Hence, understanding and predicting the responses of Sphagnum to warming is essential for assessment of the responses of the peatland ecosystem to climate change and its contribution to global C budgets. We measured Sphagnum species and community responses in an ombrotrophic bog in northern Minnesota, where ten plots within 12.8-m diameter open-top enclosures are subjected to five air and soil differential warming treatments (+0 to +9◦C) in combination with ambient or elevated CO₂ (+500 ppm). The site is dominated by S. magellanicum, S. angustifolium, and S. fallax, which are distributed on hummocks and hollows. In the second year of sustained treatment, Sphagnum growth increased with moderate warming to a maximum at 19.4◦C (average growing season temperature, 4.8 ◦C above ambient) and declined sharply as temperature increased further. Warming caused a large reduction in Sphagnum cover due primarily to irreversible desiccation. Prior to treatments and in the +0◦C enclosures, Sphagnum cover was >90%. However, cover of S. angustifolium and S. fallax declined from 66% to 36% in the two +9◦C enclosures, and S. magellanicum declined from 31% to a complete disappearance. Combining growth and cover data to produce estimates of plot-level productivity, we saw a significant linear decline with temperature, indicating a loss of 10 g C m^-2 per degree temperature increase. The effect of temperature occurred primarily through changes in water status of the ecosystem—both lowering of the water table and increased evaporation from Sphagnum capitula. Given its central role in the bog ecosystem, the decline of the Sphagnum community can be expected to have many follow-on consequences to the structure and function of this and similar ecosystems as they are subjected to rising temperatures.