Regressive and progressive scenarios of ’drying’ have opposite implications to ecosystem functions in northern peatlands

Climate warming may increase the water table depth (WTD) and alter ecosystem functions like carbon sequestration and greenhouse-gas fluxes in northern peatlands. There are two fundamentally different trajectories towards increase of WTD, however, that can have contrasting impacts: 1) a regressive process resulting from lowering of water level, and 2) a progressive process of increased growth above water level of mosses, both of which can result in the increase of WTD. The common expectation stated in peatland studies of drying of peatlands in response to climatic warming, and of the consequent threat to peatlands’ carbon sink, manifests the regressive drying scenario. In contrast, the general succession pattern of the historical development of contemporary raised bogs represents the progressive drying scenario of the ‘hydrosere’. The fact that both wet and dry peatland types and microforms are found over a wide range of climate conditions across the distribution range of peatlands underlines that peatland wetness is not strictly dictated by climate. Instead, vegetation dynamics, catchment and basin conditions, historical legacy of the peat formations, and climate interact to form different peatland states with their characteristic hydrological cycle and WTD. It is important to recognize the differences of regressive and progressive drying trajectories and events, as the implications to ecosystem functions are markedly different and partly opposite. At present, these contrasting scenarios are not recognized in the use of palaeoecological proxies, which limits our understanding of past climate responses of peatlands. Furthermore, most experimental treatments of ‘drying’ impacts on peatlands represent the regressive drying scenario and, hence, fail to indicate responses of potentially progressive development. I present alternative models of regressive and progressive ‘drying’ and demonstrate their different implications to ecosystem functions and assess verified example cases of both scenarios. Examples of progressive drying include the increase of WTD following the fen-bog transition in boreal aapa mires and in effect of moss growth with Sphagnum cultivation and peatland restoration.