Development of forest biomass harvesting guidelines based on assessing changes in soil nutrient status

Samenvatting

The substitution of biomass for fossil fuels in energy consumption is a measure to decrease the emission of greenhouse gases and thereby mitigate global warming. Over recent years, this has led to an increasing interest to use tree harvest residues as feedstock for bioenergy. An important concern related to removal of harvesting residues is, however, the potential adverse effects on soil fertility caused by an increased nutrient removal, relative to conventional stem-only harvesting. In the Netherlands this is a major concern, because we have many forests on poor soils. Here we report the development of forest harvesting guidelines for the Netherlands, distinguishing seven major tree species, seven soil types (various forms of sand, loam, clay and peat soils) and nine regions, with clear variations in atmospheric deposition of the nutrients of nitrogen (N),
phosphorus (P), calcium (Ca), magnesium (Mg), potassium (K). The basis for the guidelines is a consistent modeling approach, assessing the change in soil nutrient status by comparison of nutrient inputs by deposition and weathering with nutrient outputs by harvesting and leaching for various timber harvesting scenarios, including both stem-only harvesting and additional removal of tree tops and branches. For each region-treesoil combination we calculated the maximum amounts that can be harvested such that the output of the nutrients Ca, Mg, K and P is balanced with the inputs. Results showed that at current harvesting rates a negative balance is hardly calculated for the richer loamy to clayey soil types, while depletion can occur for the poorer sandy soils, particularly in Ca and K. We discuss the uncertainties when translating the results to an advisory system for timber harvesting.