Sustainable regeneration in uneven-aged mixed deciduous forests managed by selection silviculture: the role of demographic structure

Selection silviculture aims to create and maintain uneven-aged forests with a diameter at breast height (DBH) structure that is balanced at small spatial scales such that the stem number in each DBH class is high enough to replace the harvested and outgrowing trees over time by ingrowth from the next lower class. In these forests, natural regeneration of shade-tolerant species is at an advantage over shade-intolerant species. This is particularly pronounced in continuous-cover stands dominated by European beech (Fagus sylvatica L.), which develops its crown laterally as a reaction to release events. The conditions necessary to sustain a mixture with less shade-tolerant species, e.g. sycamore (Acer pseudoplatanus L.), have been little studied. Therefore, we explored growth patterns and stand structures in mixed deciduous forests with light availability quantified using vegetation height models. Harvesting and growth patterns were derived from inventory data of beech-dominated selection forests in Thuringia, Germany and long-term forest monitoring plot data from four stands in Switzerland. Based on these data, models of stem number distributions confirmed that stand basal area in deciduous forests dominated by beech should not exceed 21-25 m2 ha-1 to maintain a sustainable structure. In these forests, a total of ∼90-120 stems per ha are needed in the ingrowth DBH class (DBH 8-11.9 cm) to ensure demographic sustainability. At canopy light transmittance <10 per cent, total stem number required in the thicket stage (DBH < 8 cm) is 800-1700 ha-1. Under such shady conditions, only a small proportion of sycamore was observed (<25 per cent) in the thicket stage, as the species is likely to require canopy gaps >400 m2 (gap diameter > 22.5 m) to recruit successfully. Selection silviculture with shade-intolerant species therefore requires much lower stocking volume and larger canopy openings created by group selection cutting than what is routinely applied in practical forest management using single-tree selection principles.