In beech forest landscapes composed of different management systems biodiversity increases with the share of even-aged forests.

Abstract

The composition of forest landscapes in terms of tree age, developmental phases and the share of unmanaged forests is substantial for protecting biodiversity. However, the optimal composition in temperate forests is still under debate. Forest conservationists and policy nevertheless advocate increasing the share of unmanaged and uneven-aged forests at the expense of traditionally managed even-aged forests to preserve biodiversity.

We studied the biodiversity of forest landscapes composed of even-aged, uneven-aged and unmanaged (formerly managed) European beech forests for 14 organism groups from bacteria to vertebrates sampled in the largest contiguous beech forest in Germany [1]. Hypothetical forest landscapes were generated by resampling plots of the three management systems so that all compositional combinations were represented in steps of 10% with 1000 replications. We asked how gamma-diversity - single taxa diversity and multidiversity - responds to differently composed forest landscapes.

Species richness of all groups responded non-linearly and convexly to landscape composition. Strongest responses were found for beetles (R² = 0.599), spiders (R² = 0.588) and vascular plants (R² = 0.521) with the biodiversity maximum in pure even-aged and the minimum in pure unmanaged forest landscapes. The response pattern was similar for Shannon diversity and for forest specialists. Birds (R² = 0.454) and saproxylic beetles (R² = 0.275) benefited from a high share of even-aged complemented by unmanaged forests. Different response patterns were observed for deadwood fungi (R² = 0.367) and bacteria (R² = 0.358) with the maximum in unmanaged and uneven-aged forests, respectively. Multidiversity cumulated in pure even-aged landscapes preserving 97.5% of total diversity across all 14 taxa. By increasing the share of uneven-aged forests, multidiversity declined to 87.0%. Its minimum was found in pure unmanaged forest landscapes (86.2%).

We conclude that biodiversity of most taxa is largely preserved in even-aged forests. This indicates that a mosaic of different age-classes better promotes biodiversity than high within-stand heterogeneity and management abandonment for the investigated forest landscape under current conditions. Specific features of uneven-aged and unmanaged forests can provide important habitats for certain species, in particular those depending on high deadwood amount. These habitats may even increase in the future, considering the relatively short time span of management abandonment in the investigated secondary natural forests (20 to 70 y) that functioned as an unmanaged reference. Nevertheless, the even-aged forests should be acknowledged as the backbone of forest biodiversity in a landscape that was shaped by humans for centuries.

1. Schall P, Gossner M M, et al. (2018). The impact of even-aged and uneven-aged forest management on regional biodiversity of multiple taxa in European beech forests. J Appl Ecol 55, 267-278