The effect of pests and pathogens on forest harvesting regimes: A bioeconomic model

Pests and diseases are an existential threat to trees in forests and woodlands. There is, therefore, a pressing need to use ecological and bioeconomic models to inform forest managers on control and mitigation strategies. For example, the incidence of Dothistroma needle blight in the UK has increased rapidly since the 1990s, and it is a significant threat to the productivity of commercial forestry. Climatic changes are expected to exacerbate this problem further. Control of the disease in the UK primarily focuses on good stand management through pre-commercial thinning; similar practices are widely used in commercial forests worldwide. Forest managers would benefit from evidence on the effectiveness of this precautionary strategy (in comparison to its alternatives) to reduce disease impacts and increase the value extracted from timber. In this paper, we develop a bioeconomic model to determine the economically optimal harvesting regime – in terms of thinning and rotation – of an even-aged plantation under the risk of an invading pest. We extend a Schaefer–Faustmann model to include a compartmental epidemiological system that governs timber growth and disease spread. We analyse a set of management regimes, including the timing of the final clear-felling of the forest and the timing and level of earlier thinning. Thus, in our approach, forest managers decide whether and when to thin and must balance (i) harvesting before infection destroys the timber’s value and (ii) exploiting the forest’s density-dependent growth. We use a sensitivity analysis with respect to the disease spread and impact on the tree dynamics to demonstrate that, in the presence of disease, thinning can significantly improve the net present value of the plantation if applied correctly. Furthermore, if thinning reduces the transmission rate significantly, the priority is to protect the final harvest, and rotations extend while the thinning time shortens. Our study provides a framework to help design appropriate forest management strategies in the presence of disease.