Causal links between drainage and forest growth response in boreal peatlands

Drainage is considered as an essential pre-requisite in management of peatland forests, and it generally increases stand growth. So far, the primary reasons behind the growth response are not fully understood. The explanation must be linked to direct or indirect growth factors, such as the supply of radiation, water, oxygen, and nutrients. Applying an empirical dataset consisting of 18 drained Scots pine (Pinus sylvestris L.) stands we constructed a causal network linking meteorology and climate variables, site and stand properties, organic matter decomposition, growth regulating factors and biomass growth. The network was analysed using piecewise structural equation models (SEM). The SEM analysis indicated that the stand growth response to drainage is mainly caused by increased supply of nutrients, especially potassium.  Based on this causal model, we constructed a dynamic simulation model called Peatland simulator SUSI. SUSI describes hydrology, stand growth, site carbon balance and stand nutrient supply and demand under different management schemes and under different site types and weather conditions. The simulator was tested against a large independent dataset consisting of 69 stands and 207 plots. SUSI was parameterized according to measured stand and site data and run using daily meteorological data. The simulation revealed that SUSI can predict five-year volume growth of the stand with good accuracy. Because SUSI links the drainage and the growth response in a process level, the model facilitates cost-benefit analyses of the drainage, helps in avoiding unnecessary drainage operations and their adverse environmental effects such as increased carbon emissions, peat subsidence and nutrient leaching. Thus, it can guide in the search for optional, more acceptable management schemes for drained forested peatlands.