Effects of habitat tree retention on adjacent advance regeneration in continuous-cover forests in Central Europe

This new study examines how habitat tree retention influences regeneration, stand development and tree-species composition in Central European continuous-cover forests, providing practical guidance for integrating biodiversity conservation with timber production. Habitat trees are increasingly retained in managed forests because they provide important structures such as large old trees, cavities, deadwood and other tree-related microhabitats. However, their effects on the surrounding regeneration and on future stand productivity have remained poorly quantified under close-to-nature forestry systems.

The study, prepared by researchers in the TRANSFORMIT project, focused on 71 habitat tree groups in state forests in Baden-Württemberg, southwestern Germany, in stands dominated by European beech and silver fir. The retained habitat trees were at least 120 years old, while the surrounding younger cohort was around 10–20 years old and consisted predominantly of naturally regenerated trees. Regeneration and regrowth were assessed from the centre of the habitat tree groups to their crown edges and at increasing distances into the surrounding young stand.

The results show that habitat tree groups reduce the growth of young trees in their immediate vicinity, but that this effect is spatially limited. Regrowth trees at the centre of habitat tree groups were around 22% shorter than trees growing 30 m from the group edge, while those at the edge were around 14% shorter. Regrowth basal area was reduced by approximately 52% at the group centre and 27% at the edge. In contrast, no significant reductions in either height or basal area were detected at distances of 10, 20 or 30 m from the habitat tree group edge.

This relatively short-ranging effect is particularly relevant for forest management because it suggests that the production costs of habitat tree retention are largely concentrated beneath the crown projection area of the retained trees. Rather than causing a broad reduction in productivity across the surrounding stand, competition from habitat trees appears to be confined mainly to the area occupied by their crowns. This makes the spatial extent of productivity losses comparatively predictable and could help estimate the opportunity costs associated with habitat tree retention, including in compensation or incentive schemes for forest owners.

At the same time, habitat tree groups did not prevent natural regeneration. Seedling densities were relatively high within the groups, reaching approximately 2,900 seedlings per hectare in the group centres. Sapling densities were also high and were greatest near the group edges, at approximately 2,860 saplings per hectare. Considerable variation occurred among habitat tree groups, indicating that local stand conditions remained important, but overall the findings suggest that retained trees can coexist with substantial natural regeneration.

The pattern highlights an important trade-off between regeneration establishment and subsequent growth. Habitat tree groups appear capable of providing favourable conditions for the establishment of seedlings and saplings, while the overstory trees simultaneously suppress height and diameter development through competition. Continued seed input from retained trees, lower stem densities and small canopy openings caused by declining or dead habitat trees may all contribute to regeneration within the groups. As retained trees eventually decline and die, already established regeneration may subsequently be released.

Habitat tree retention also affected the composition of the younger stand. Shade-tolerant species dominated across the study area, but their dominance was strongest within habitat tree groups. Around 98% of regrowth basal area at the group centres consisted of shade-tolerant species, compared with 89% at 30 m distance. The share of intermediate and shade-intolerant species increased from around 2% at the group centre to around 11% at 30 m. Species richness of larger regrowth trees was also generally higher outside the retained groups.

These patterns are relevant where forest management aims to establish mixed stands or promote less shade-tolerant species such as oaks. Habitat tree groups dominated by shade-tolerant species, particularly European beech, can reinforce the regeneration of those species through both shading and seed input. Canopy openings outside the retained groups therefore remain important where management objectives include greater tree-species diversity or the establishment of light-demanding species. The study suggests that habitat tree groups and regeneration gaps can serve complementary functions: retained groups provide old-growth structures and habitat continuity, while nearby openings create opportunities for a broader range of tree species.

The characteristics of the habitat tree groups themselves had comparatively little influence on the growth of the surrounding young stand. Group size, tree density, standing volume, vitality and species composition did not significantly alter regrowth performance. This suggests that the crown area occupied by retained trees is more important for predicting their influence on production than the precise size or spatial arrangement of the group. From a management perspective, this may provide flexibility in how habitat trees are retained and indicates that selecting trees with high ecological value can be more important than adhering to a specific group configuration.

The findings also point to possible advantages of retaining a mixture of tree species and vitality classes. Trees of lower vitality and standing dead trees often have smaller crowns and therefore occupy less productive space while continuing to provide valuable microhabitats. Combining living habitat trees with declining or dead individuals may therefore help maintain structural diversity while limiting competition with the surrounding regrowth.

Overall, the study suggests that habitat tree retention creates a measurable but relatively localized trade-off with stand productivity in continuous-cover forests. Reduced growth was largely restricted to the area beneath retained crowns, while natural regeneration remained abundant within habitat tree groups. The results therefore support management approaches that combine permanent retention structures with complementary canopy openings, allowing biodiversity conservation, natural regeneration, tree-species diversification and timber production to be addressed within the same stand. The findings are most directly applicable to continuous-cover forests similar to those studied in Baden-Württemberg, particularly systems dominated by European beech, silver fir and other relatively shade-tolerant species.