Comparing the Index of Biodiversity Potential (IBP) in managed and unmanaged temperate lowland forests

Key message The Index of Biodiversity Potential (IBP) can differentiate between management regimes in a context of semi-natural, mature, close-to-nature managed and set-aside temperate lowland forests. Under optimal tree-growing conditions, thresholds within the capped IBP are frequently and largely exceeded. Application of an uncapped IBP or adjustment of the scoring systems can increase the discriminating power of the index for maturity-related factors. Context The Index of Biodiversity Potential (IBP) is a tool used by forest managers for fast and cost-efficient assessment of the biodiversity potential in forest stands. The index is based on a set of easy-to-assess structural attributes linked to forest biodiversity. Aims In this study, we want to assess whether the IBP can discriminate between close-to-nature managed and recently set-aside mature lowland temperate forests. Methods We use the Benelux version of the IBP to assess differences between 42 managed and unmanaged forest patches in Belgium. The IBP was measured in at least one circular plot of 1 ha per 10 ha for each forest patch. Results We find that the IBP increases with time since set-aside in most forests. This can be mainly attributed to increases in deadwood, very large trees and tree-related microhabitats. Overshoot of the threshold leading to maximum scores for IBP factors occurred in both managed and unmanaged forests, but is larger in unmanaged forests, implying that the capped IBP underestimates the actual difference between management types. Conclusion The IBP can be used in mature lowland temperate forest. However, depending on the required level of detail or stand maturity, an uncapped IBP or adjustment of the scoring system is advised.