How can oak regeneration in the Leipzig Floodplain Forest be effectively supported by femel plantations? Application of a demographic forest model

Like many temperate floodplain forests in Europe, the Leipzig Floodplain Forest (LFF) has been impacted by anthropogenic disturbances, such as historical forestry practices and hydrological alterations. As a result, the forest's tree composition has changed, as evidenced by the declining abundance of typical floodplain species—like pedunculate oak (Quercus robur)—which play an important role in maintaining biodiversity. Notably, oak is not naturally regenerating in the mature forest, situating a need for management intervention to assist oak regeneration. One such management intervention is to plant oak in femels, i.e. small-scale clearings, or canopy gaps. However, it remains unclear whether small femels (<0.5 ha) can ensure effective oak regeneration. Moreover, city foresters aim at reaching 40 % average oak cover in the forest overstory by planting oaks in femels, but uncertainty remains regarding how much of the area would have to be planted with oaks per year to reach this goal.

To explore these questions, we collected forest inventory data from oak femels ranging from 0.1 to 0.6 ha in size and compared oak density, size, growth, and mortality across femel sizes. Furthermore, we used the demographic Perfect Plasticity Approximation (PPA) model to simulate oak overstory cover over time under a femel management regime to assess the percentage of the forest area that should be planted with oaks per year accounting for different groundwater conditions. To parameterise the model, we used forest inventory data to quantify growth rates of oaks in the femels and growth, mortality, and recruitment rates of all common tree species in the Leipzig Floodplain Forest that may regenerate under the oak canopy over time.

Oak density, size, growth, and mortality were similar across femel sizes, indicating that smaller femels are as effective as larger ones in promoting oak regeneration, while minimising the impact on the forest structure. To reach the foresters’ goal of 40 % average oak overstory cover, it is recommended that approximately 0.3 % of the hardwood floodplain forest area is planted with oaks each year, independent of the groundwater conditions. This study illustrates how ecological modeling can assist conservation planning in the context of forest management decision-making and these results may also be relevant for other European floodplain forests where oak regeneration requires support through active management to conserve long-standing biodiversity.