How does a biodiversity value impact upon optimal rotation length? An investigation using species richness and forest stand age

We have conducted one of the first attempts to integrate empirical biodiversity data with economic modelling of optimum rotation length for forest stands (the growth period required to derive maximum value from a stand of timber). We sought relationships between different taxonomic groups and stand age for a range of forest types based upon UK biodiversity assessment data. We then examined the impact of these relationships in combination with biodiversity monetary values on stand-level forest management – something which ecologists might anticipate would lead to extended rotations. Although we found some evidence of significant relationships between overall species richness and stand age for many of the forest types considered, with examples of minima at around 40 years for Sitka spruce in foothills and Corsican pine in lowlands and a maximum at around 50 years for Scots pine in lowlands, at individual taxonomic group level only about 4% of responses were different from constant. However, the overall results mask potentially diverse relationships for individual taxonomic groups. We found evidence of increasing and decreasing relationships (as well as no response) at individual group level which may result from the particular niche requirements and the species assemblages present. When incorporated into the economic Hartman model, the main relationships were found to make minimal difference (with two exceptions) to optimal rotation length under standard assumptions based upon biodiversity values from the literature. When we extended our modelling to incorporate a higher value afforded to biodiversity than published values, we saw significant changes in optimal rotation length in more cases. Our results suggest the need for further refinement of biodiversity estimation and valuation methodology – and we make recommendations for future developments.