Getting a virtual forester fit for the challenge of climatic change.
Abstract
The empirical study of forest ecosystem dynamics is difficult because of the longevity of trees. Many types of models were developed to assist with this problem, all of them with advantages and disadvantages. The strengths of gap models are that they are able to simulate forest dynamics under changing climatic conditions and are therefore suitable for exploring future forest dynamics. Most temperate and boreal forests are managed, making it important to incorporate harvesting functions depicting a wide range of silvicultural practices into the models and to test them under different climatic conditions. This is a necessary prerequisite to the application of these models under climatic change scenarios. Most gap models, however, do not feature such submodels, which disqualifies them as decision support tools. We implemented a management submodel in the gap model ForClim that is able to simulate a wide range of cutting and thinning techniques, including continuous cover forestry ('plentering'). We tested the new submodel against long-term data (72-111 years) from eight growth and yield research plots across climatic conditions ranging from warm-dry to cold-wet. Stem numbers were simulated accurately in nearly all cases, basal area showed a good fit on Quercus-dominated plots, but an over/underestimation on Fagus sylvatica-dominated and Picea abies-dominated plots. The diameter distributions simulated for the time of the most recent inventory did not differ significantly from empirical data except for two cases. Harvested basal area and stem numbers mostly agreed well with empirical data, but showed the same deviation from reality as simulated basal area. Simulations run with an accurate management plan taken from foresters' reports for the plots yielded nearly the same results as those run with a generic management setting. Synthesis and applications. We have demonstrated that (i) the management submodel adequately depicts silvicultural treatments, including continuous cover forestry; (ii) a generic harvesting setting can be substituted for a very detailed one, thus eliminating a major source of uncertainty in assessments of future forest dynamics; and (iii) as the new version of ForClim is able to deal with widely differing current climates, it can be employed with reasonable confidence to simulate future management strategies under climatic change. Overall, this modelling work is a major step towards the use of succession models as decision support tools in forest management.