The role of photosynthesis and light conditions for competition dynamics among Quercus petraea, Fagus sylvatica, Carpinus betulus, and Rubus subg. Rubus in the regeneration phase

Naturally regenerated seedlings of Quercus petraea (Mattuschka) Liebl. are often outcompeted by tree species such as Fagus sylvatica L. and Carpinus betulus L., and understorey species like Rubus subg. Rubus . Since plant growth is fundamentally driven by photosynthetic capacity and efficiency, the competitive dynamics between species are influenced by their ability to adapt to varying light conditions through morphological and physiological plasticity. To explore these adaptations, we measured a number of variables indicating growth performance or potential of 60 12-year-old seedlings of Q. petraea , F. sylvatica , and C. betulus as well as individuals of R. subg. Rubus along a gradient of canopy openness and thus radiation. These variables included: a) key leaf traits, including specific leaf area (SLA) and leaf nitrogen (N) content, b) different photosynthesis measurements under constant and fluctuating light, and c) annual shoot length, total height and root collar diameter. Solar radiation was quantified as total site factor (TSF). In all four species, an increase in leaf N content was observed with increasing TSF, which was accompanied by an increase in maximum photosynthetic rate (A max ) and growth. However, while this increase was continuous in C. betulus and R. subg. Rubus , a significant increase in A max and growth in Q. petraea and F. sylvatica occurred only in the radiation ranges between 1 % and 20 % and 50–70 % TSF. Measurements of photosynthesis in relation to simulated lightflecks suggest that leaves of Q. petraea are better adapted to prolonged high photosynthetic photon flux density (PPFD) exposure than to fluctuating light. Under these light conditions, especially at TSF levels > 60 %, Q. petraea showed a higher photosynthetic performance than F. sylvatica and C. betulus , in addition to comparable diameter and height growth. To promote Q. petraea regeneration against F. sylvatica and C. betulus competition and reduce necessary vegetation control interventions, we recommend radiation levels > 60 % TSF after the initial establishment phase, when oak seedlings have reached a height of about 0.8 m. • Leaf traits and growth in C. betulus and R. subg. Rubus acclimated almost continuously to increasing levels of radiation. • Leaf traits and growth in Q. petraea and F. sylvatica responded to increasing radiation only between 1 % and 20 % and > 50 % TSF. • F. sylvatica and C. betulus responded significantly stronger to short-term lightflecks, than Q. petraea . • Leaves of Q. petraea are better adapted to conditions of prolonged and high PPFD exposure • At TSF levels > 60 %, Q. petraea showed a higher photosynthetic performance than F. sylvatica and C. betulus .