Microclimatic response to increasing shrub cover and its effect on Sphagnum CO2 exchange in a bog

We examined the effect of long-term nitrogen (plus phosphorus and potassium) fertilization, resulting in increased shrub cover, on seasonal changes in understory light, soil temperature, and soil moisture in an ombrotrophic bog. An increase in leaf area index (LAI) was negatively correlated with light transmission through the canopy, decreasing photosynthetically active radiation (PAR) reaching the peat surface by up to an average of 77% compared to unfertilized plots. Owing to the denser shrub canopy, near surface soil temperature was cooler in summer and less spatially variable within the fertilized plots. A laboratory study of the environmental controls on Sphagnum capillifolium carbon dioxide (CO2) exchange showed that there were significant interactions between moisture and temperature, but changes in CO2 exchange in response to temperature or moisture were small compared to the influence of light. These results suggest that the absence of moss in the fertilized plots may be, in part, the result of decreased light availability. Alterations to the competitive balance between the shrub and moss layer could lead to changes in C storage in these ecosystems.