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

Habitat tree group (HTG) retention is increasingly practiced for biodiversity conservation in production forests, yet its impacts on wood production and other ecosystem functions have hardly been quantified for Central European forests, where HTGs are embedded in continuous-cover forests. Quantifying this impact would facilitate modelling approximate costs and benefits of retention. For this purpose, we assessed the effect of 71 HTGs on the surrounding regeneration (seedlings: <1.3 m, saplings: 1.3–4 m) and regrowth (>4 m, < 20 cm DBH) in European beech and silver fir stands in southwestern Germany. Regeneration and regrowth were measured along transects radiating from the center of HTGs. Generalized linear mixed models were fitted to analyze the effects of distance from HTGs and other factors on regeneration and regrowth structure and composition. Regrowth and regeneration height, basal area and tree species richness were significantly reduced beneath the crown projection area of HTGs, up to 10 m distance from HTG crown edges. Sapling and seedling densities were highest within the perimeter of the HTGs, suggesting HTGs provide favorable conditions for regeneration. Tree volume, density, vitality, and composition of HTGs had no significant effect on regrowth dimensions, indicating that HTGs with different attributes have similar effects on stand production. Our study suggests that habitat tree retention has a limited and well-defined influence on productivity, as the suppressive effect on regeneration and regrowth was limited to the crown projection area of HTGs. This effect can be easily modelled or quantified via remote sensing.