Physiological responses of black spruce layers and planted seedlings to nutrient addition

We investigated effects of nutrient addition on several physiological chara cteristics of 60-cm-tall black spruce (Picea mariana Mill. B.S.P.) layers ( i.e., rooted branches of overstory trees) and 20-cm-tall planted seedlings on a clear-cut, N-limited boreal site. After two growing seasons, current-y ear and one-year-old needles of fertilized trees (layers and seedlings comb ined) had higher net photosynthetic rates (A(n)) and maximum capacity of Ru bisco for CO2 fixation (V-max) than unfertilized trees. One-year-old needle s of fertilized trees had higher stomatal conductance (g(s)), higher water- use efficiency, and lower intercellular to ambient CO2 ratio than unfertili zed trees. Additionally, fertilized trees had higher predawn and midday sho ot water potentials than unfertilized trees. Stomatal conductance of 1-year-old needles was 23% higher in seedlings than in layers, but there were no significant differences in g(s) of current-ye ar needles between the regeneration types. For both needle age-classes, A(n ) and V-max of layers were 25 and 40% higher, respectively, than the corres ponding values for seedlings. The higher values of A(n), V-max and foliar N concentration of layers compared with seedlings after two growing seasons may be associated with the larger root systems of the layers compared with the transplanted seedlings.