Sphagnum moss species often dominate in northern peatland plant communities, contributing to important ecosystem functions such as carbon storage. However, the high-latitude regions where peatlands are prevalent are expected to experience severe climate change conditions, potentially triggering plant community regime shifts that prompt Sphagnum moss decline. As the plant community represents an important aboveground-belowground linkage, regime shifts in northern peatlands are expected to alter belowground processes, including carbon storage. Of concern are changes in belowground dissolved organic carbon (DOC) quantity and quality as this carbon pool is a key energy source for microbial communities, which in turn determines microbial decomposition rates and carbon release. Changes in belowground phenolic compound concentrations are also of concern, as these compounds are purported to regulate microbial exoenzyme activity to drive carbon release. Accordingly, the main objective of our study was to determine if climate change factors can trigger a plant community regime shift that prompts changes in belowground DOC characteristics and phenolic compound concentrations. To complete this work we subjected intact, vegetated peat monoliths to elevated temperatures, increased atmospheric carbon dioxide (CO2) and two water table levels in a factorial design to monitor changes in the plant community composition, DOC quantity and quality, phenolic compound concentrations, and CO2 release.
Results/Conclusions
We identified multiple indicators of a regime shift occurring in our experimental system under climate change conditions including a non-linear decline of Sphagnum moss at temperatures 8°C above ambient conditions, concomitant increases in graminoid abundance at temperatures 4°C above ambient conditions, and increased variance of the plant community composition. We found that this regime shift coincided with a significant increase in belowground DOC quantity and quality prompted by increased graminoid biomass production. We also found that phenolic compound concentrations significantly increased with graminoid expansion. Unexpectedly, carbon release increased with rising phenolic compound concentrations, suggesting that phenolic compounds may cause the microbial community to respire a greater proportion of carbon instead of allotting it towards biomass production. Our findings indicate that plant community regime shifts in northern peatlands are likely under future climate conditions, with a demonstrated Sphagnum moss decline and graminoid expansion. Such a plant community shift has clear ramifications for belowground processes, increasing the quantity of readily decomposable DOC while increasing phenolic compound concentrations and carbon release. Clearly climate change conditions can trigger cascading impacts in peatland ecosystems that threaten the stability of important ecosystem functions like carbon storage.