The microclimatic response to increasing shrub cover, and its consequent control on Sphagnum carbon dioxide exchange in an ombrotrophic bog

Shifts in vegetation from a moss-shrub peatland to one more dominated by shrubs will influence the microclimate. I analyzed the seasonal effect of increasing shrub cover in response to two NPK treatments (5NPK and 20NPK, with addition rates of 3.2 g N m-2 y-1 and 6.4 g N m-2 y-1, respectively) compared to the control (0NPK) in triplicate 3 x 3 m plots in an ombrotrophic bog located in Ottawa, Ontario. Half-hour measurements of light, soil temperature and moisture were measured in each plot. During the growing season, increases in LAI resulted in a decrease in soil temperature by 0.3 to 1.5 °C in the top 2 cm. Differences in the average soil moisture amongst treatments were small, and it ranged between 0.20 to 0.33 m3 m -3. Variations in temperature between the plots decreased with increasing depth. During the winter, soils under the fertilized plots were warmer, where temperatures were above 0 °C for 2 +/- 1 day(s) longer in the 20NPK plots. An increase in LAI was negatively correlated with light transmission through the canopy, decreasing PAR transmission in the 20NPK plots by up to 66 %. A factorial lab study of the effect of changes in temperature, moisture and light on the CO22 exchange in S. capillifolium showed that there was a significant interaction between moisture and temperature, and its effect on CO2 exchange. The absence of moss in the 20NPK plots is in part the result of changes in the shrub canopy leading to decreases in light to the surface layers. Due to the sensitivity of Sphagnum to a changing microclimate, alterations to the competitive balance between the shrub and moss layer could lead to changes in C storage in these ecosystems.