Validation of basal area growth functions for larch in Heureka DSS

Abstract: Larch is getting more common in Sweden. This highlights the need of reliable growth models for larch species in Heureka DSS. Precise and accurate growth models are essential for long-term forest planning. The risk of using under- or overpredicted basal area growth in forest planning is that longterm projections could get more and more imprecise over time. This could, in turn, lead to suboptimal forest management and decision-making, leading to non-optimal choice of tree-species, early- or late timing of silvicultural treatments and ultimately to economic loss. The aim of this thesis was to validate Heureka’s basal area growth function for Siberian larch (Larix sibirica), European larch (Larix decidua) and hybrid larch (Larix x eurolepis). To validate the growth function, field trials of larch from all over Sweden were used to compare basal area growth prediction errors between Heureka predicted growth and basal area measured in the field. A sample of plots were also chosen for simulation in Heureka StandWise for further analysis of basal area, height and volume growth. Age-related prediction errors along with ground vegetation type were tested and compared for the Heureka basal area function. The results showed that basal area growth of Siberian larch was underpredicted at early age and overpredicted at old age, regardless of vegetation type. European larch basal area growth was neither under- nor overpredicted for the vegetation types but showed random error at young age. Basal area growth of Hybrid larch showed a general underpredicted with vegetation type bilberry while no such trend was seen for vegetation type no field vegetation. Heureka simulations showed a slightly higher underpredicted basal area growth than predictions from the growth function. This could be explained by that the predicted growth gets more imprecise over time or due to a too small sample size. There are possibilities to increase the precision of Heureka’s growth predicted where one strategy would be to develop and apply species specific growth models in Heureka DSS.