Forest productivity is closely related to how effectively woody plants utilize the most important environmental factors—light and moisture. Assessment of the ecological plasticity of structural and functional traits in woody plants is necessary to predict the dynamics of forest communities in the changing natural environment and climate. In this study, needle xylem anatomical and hydraulic traits and their relationships with leaf CO2/H2O-gas exchange parameters were investigated in Scots pine (Pinus sylvestris L.) trees during natural reforestation after clear-cutting of boreal pine forest in Eastern Fennoscandia. We analyzed the effect of shading on needle structural and functional traits in Scots pine trees of the same age in a clear-cut site and under a bilberry-type pine forest canopy in the middle taiga. The highest values of tracheid lumen diameter (D95), number of tracheids per needle (Tnum) and xylem area per needle (Ax), theoretical needle hydraulic conductivity (Kth_n), and theoretical leaf-specific hydraulic conductivity (Ks_leaf), stomatal conductance (gs), rates of photosynthesis (A) and transpiration (E), number of stomata per unit needle area (Nst) and, on the contrary, the lowest values of photosynthetic water use efficiency (WUEi, WUE) and plasticity index (PI) of all structural and functional traits were noted in Scots pine trees growing in the clear-cut and getting sufficient amounts of light. At the same time, the values of theoretical needle xylem-specific hydraulic conductivity (Ks_xylem) were similar in habitats with high (clear-cut site), medium (shading in the clear-cut), and low (forest canopy) light levels. The features of the relationship between the hydraulic structure, photosynthetic capacity, and water use efficiency in Scots pine trees under different habitat conditions are discussed.

Clear-cut logging currently is a key factor transforming forest communities in many boreal regions. The dynamics of biogeochemical processes taking place in clear-cuts makes them a good model for studying the response of plants to changes in environmental conditions. This study aimed to assess the response of nutrient, carbon and water exchange parameters in coniferous and deciduous plants after clear-cut forest harvesting. The effects of environmental factors on the functional traits of regrowing trees in Scots pine (Pinus sylvestris L.), silver birch (Betula pendula Roth), aspen (Populus tremula L.) and grey alder (Alnus incana (L.) Moench) were analysed during four growing seasons (2016–2019) in a clear-cut site and under the canopy of an undisturbed bilberry-type pine forest in the middle taiga of Karelia, Northwest Russia. Unidirectional changes were observed in the specific nutrient content, specific leaf area, biological macronutrient absorption capacity, stomatal conductance, and rates of photosynthesis and transpiration in the different tree species along a gradient of environmental factors. In most cases, the nutrient concentrations and N:P:K ratio were consistent in all species as environmental conditions changed. Species-specific responses were observed for the N:P:K and K:Ca:Mg ratios, photosynthetic water use and nitrogen use efficiency. Contrasting environmental conditions in the clear-cut and under forest canopy most significantly affect plant capacity to absorb nitrogen. Variation in functional traits in the coniferous and deciduous species reflects their specific resource-use strategies in a heterogeneous environment and, hence, indicates the different adaptation potentials for various plant species.