Sphagnum physiological responses to elevated temperature, nitrogen, CO2 and low moisture in laboratory and in situ microhabitats: a review

Sphagnum mosses are considered peatland engineers because of their ability to create conditions inducing carbon accumulation. Here, we report on a review of the effects of four environmental variables (elevated temperature, N and CO2 and reduced moisture) on the capitulum biomass, length increment, respiration, photosynthetic capability, N and P exchange and content of the 3 most studied Sphagnum subgenera (Acutifolia, Cuspidata, Sphagnum). Overall, we observe that, when compared to in situ experiments, laboratory experiments tend to exacerbate length increments and underestimate maximum photosynthesis in most of the studies inventoried. This review underscores some differences among results that can be associated with the used of different protocols (e.g. exposure time, instrumental analysis). Studies that investigated the impact of elevated temperature (2-5 degrees C) on Sphagnum reveal an increase in length, respiration and photosynthesis regardless of the experimental conditions and subgenus. Elevated N (3-23 g Nm(-2)y(-1)) on the other hand appears to reduce the length increment but had contrasting effects on photosynthesis. Some divergent responses are found with Cuspidata species because of their tolerance to high doses of N. Low moisture reduces the length increment and photosynthesis of species of the Cuspidata and Sphagnum subgenera but has different effects on species of the Acutifolia subgenus, which are relatively tolerant to water fluctuations. Responses to elevated CO2 have no clear trends reported. Allelochemical interactions between Sphagnum, their microbiome or surrounding mosses or other plants were found to be determinant to Sphagnum responses under those variables and reinforce the interest of such investigations.