Small-Scale Environmental Heterogeneity Enhances Tree Recruitment through Carbon Recharge and Water Use Diversification

Studying regeneration processes in oak-dominated forests requires a multi-faceted approach that considers local factors, disturbances, management actions, and tree responses. Our aims were to quantify the carbon and water-use responses of saplings and evaluate ecological consequences at the early tree regeneration phase of a pedunculate oak-hornbeam forest. We measured plant eco-physiological parameters using an open-chamber IRGA equipment in large experimental canopy gaps (instantaneous field data) and greenhouse (climate-controlled reference data) conditions. We used the non-parametric Kruskal–Wallis ANOVA test to analyze differences and similarities in the gas-exchange response. Functional fingerprints indicated shared resource use and efficiency functions at species-specific performance levels with temporal variations. Medium-level and seasonally balanced carbon uptake and water-use functions characterized pedunculate oak (Quercus robur L.) and European hornbeam (Carpinus betulus L.). In contrast, the response patterns in wild cherry (Prunus avium (L.) L.) and green ash (Fraxinus pennsylvanica Marshall) were dominated by water use. Goat willow (Salix caprea L.) had consistently elevated gas-exchange levels with the largest seasonal variation among the study species. We found that trees could be ranked on a relative isohydric-to-anisohydric scale regarding their species–environment interaction. According to the carbon-gain response pattern coupling with a non-structural carbohydrate exchange scheme, we were able to classify tree species as having medium- and long-term carbon resource management. In conclusion, spatially heterogeneous and temporally balanced canopy gaps facilitate tree species’ development and mixed-stand regeneration by providing a functionally diversifying recruiting environment.