Shading effects needle xylem traits and leaf gas exchange parameters in Scots pine

Forest productivity is closely related to how effectively woody plants utilize the most important environmental factors - light and moisture. Assessment of 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 bilberry-type pine forest canopy in the middle taiga. The highest values of tracheid lumen diameter (D₉₅), number of tracheids per needle (T_num) and xylem area per needle (A_x), theoretical needle hydraulic conductivity (K_{th_n}) and theoretical leaf-specific hydraulic conductivity(K_{s_leaf}), stomatal conductance (g_s), rates of photosynthesis (A) and transpiration (E), number of stomata per unit needle area (N_st) and, on the contrary, the lowest values of photosynthetic water use efficiency (WUE_i, WUE) and plasticity (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 (K_{s_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.