Hydrologic variability and its influence on long-term peat dynamics

Peatlands are carbon-rich ecosystems that are extensive in the northern high latitudes where significant 21st century climate changes are expected. In response to climate change, peatlands may become a net source of greenhouse gases, thereby inducing a positive climate feedback effect. In this paper, the impact of precipitation variability and the mean climate state on long-term peat accumulation is investigated with model simulations. The models couple peat accumulation with the hydrological cycle, which results in peatland bistability, where peatlands may take the physical characteristics from one of two possible alternative stable states. The models consider precipitation as a stochastic forcing variable, temperature-dependent functions and are parameterized with climatology and peat characteristics to represent the West Siberian Lowlands (WSL) between 55°N and 60°N. Observed WSL peat depths statistically imply bistability. Peatland bistability, however, is eliminated in model simulations with moderate-to-large precipitation variability and warmer and wetter climates. This suggests that projected late 21st century climate change would put the thick peatlands in WSL on a transition to thin peatlands. The loss of thick peatlands could significantly increase atmospheric carbon dioxide and provide a positive climate feedback effect. However, the impacts depend on the importance of unaccounted stabilizing factors. The study also shows that precipitation variability induces peatlands to switch between extended periods of accumulation and depletion even if the peatlands are in long-term equilibrium. Thus, short-term observations may see only natural fluctuations and new, longer-term observational strategies are necessary to diagnose if peatlands are undergoing fundamental changes.