Possibilities to aggregate raster cells through spatial optimization in forest planning

This study divided the forest into raster cells and used these cells as calculation units in optimisation instead of predefined stand compartments. It was hypothesized that raster cells would result in feasible treatment units and more efficient utilization of the production potential of the forest when spatial optimisation is used to compile the plan. The optimization problems of this study included both ecological and economic objectives. The raster cells were hexagons (721 m²) and their data were derived from traditionally defined forest compartments. Three forest plans were developed by using the rasterized forest, and they were compared to the corresponding forest plans developed by using compartments. Cutting areas were aggregated in all plans, by maximizing the proportion of the boundaries between two adjacent calculation units that were both cut during the same management period, and by minimizing the proportion of cut–uncut boundaries. In the first plan, only cutting areas were aggregated. In the second and third plan, also old forests were aggregated by using two different spatial objectives. The first maximized the proportion of the boundary between adjacent calculation units that were both considered as old forest. The second objective maximized the mean of the neighbourhood minima of the calculation units' old forest indices. The neighbourhood included the calculation unit itself and all the adjacent calculation units. The growing stock volume targets at the end of the 60-year planning period and the cutting volume targets of the three 20-year management periods were set to the same levels in all plans. The results showed that the raster approach was able to aggregate old forest patches and cutting areas similar in shape and size as the conventional approach. When the aggregation of old forest was a management objective, the total old forest area was larger in the raster forest but the mean size of the old forest patches was larger with predefined compartments. The trade-off curve between harvested volume and old forest area was further from the origin for the raster forest.