Long-term human impact on alluvial peatland dynamics in temperate climates

Peatlands across the globe are experiencing external pressures such as land use change, drainage and climatic changes, but are also directly impacted e.g. through peat harvesting. As a result, the dynamics of these peatlands, and their role in long-term carbon storage, has changed. In contrast to many other regions around the globe, temperate Europe has known a long history of human impact. In the northwest European lowlands, peat growth occurs mostly in floodplains under the form of alluvial peatlands. In central Belgian river valleys, alluvial peatlands developed since the Early Holocene, with a typical peat thickness between 1.5 and 2.5 metres, but reaching values of more than 6 metres at some locations.

Alluvial peatlands therefore are an important store of soil organic carbon reaching values of up to 2754 t ha^-1, thus providing an important ecosystem service. However, the fate of this carbon reservoir is challenged through many different types of human actions since at least the Middle Ages including peat cutting for fuel, drainage for land reclamation and changes in catchment hydrology through land use change. For instance, a comparison of field-based peatland carbon budgets for different river valleys indicates that floodplains where cutting of topsoil peat has been important in the Late Holocene, store significantly less carbon (729 ± 397 t ha^-1) than floodplains where Early to Mid-Holocene peat has been buried by mineral sediments originating from agricultural erosion on hillslopes (1991 ± 877 t ha^-1). Adequate modelling can provide a powerful tool to study peatland dynamics and the interaction between internal and external processes in peatlands, but unfortunately, there are currently no available modelling tools to study the long-term dynamics of alluvial peatlands.

A long-term peatland model (Digibog) was adapted to be applicable to the context of alluvial peatlands. Changes were made to both the hydrological and biological modules to include variations in the river water level, flooding, anthropogenic peat cutting and a wide variety of vegetation types, ranging from open meadows to carr forests. In a first step, the Holocene evolution of an alluvial peatland was simulated under the conditions which were typical for lowland Belgium to provide a Holocene peat sequence with an annual resolution. In a second step, this peatland was subjected to a wide set of alternative management scenarios that have been in place since the Middle Ages. The simulations allow to estimate the effect of these scenarios on the peatland dynamics in terms of peatland hydrology, productivity and carbon storage. Based on this modelling study, the sensitivity of these systems to human activities can be quantified. The resultant magnitudes and rates of change under different scenarios can provide useful information for future management of alluvial peatlands and a better understanding of long-term peatland dynamics in general.