Abstract: The conversion of Norway spruce stands into mixed-species forests is currently one of the most pressing challenges to ensure the stability of forest ecosystems in Central Europe. Recently, direct seeding as a method of artificial regeneration and species (re-)introduction has received increased attention in forestry. Considering that environmental conditions have a strong influence on the growth performance of direct-seeded plants, we investigated how differences in soil and environmental conditions affect the growth performance of silver fir (Abies alba Mill.) and pedunculate oak (Quercus robur L.) seedlings. Our study focused on closed-canopy and open-canopy (canopy removal) Norway spruce stands in a low mountain forest in central Germany. Our data indicates that the growth performance of A. alba and Q. robur seedlings is mainly influenced by the availability of photosynthetically active radiation (PAR). The growth of A. alba increased with a higher PAR-ratio, whereas the photosynthetic efficiency, as measured by F_v/F_m (chlorophyll fluorescence), showed sensitivity to it. Conversely, the growth performance of Q. robur showed a linear increase with light availability. Nutrient availability was the second most important factor, while soil pH alone showed no significant effect. The volumetric water content showed no direct effect, though drought appeared to reduce growth. The results stress that A. alba is sensitive to abrupt changes in the light regime at this early stage of development, highlighting the key role of canopy longevity in facilitating growth. Q. robur, on the other hand, appears to be well suited to sites at high risk of canopy loss due to disturbance or where the canopy has previously been removed.

Key words: Microsite, Soil nutrients, Leaf nutrients, Forest conversion, Canopy cover, Chlorophyll f luorescence