EFFECTS OF N ADDITION RATES ON THE PRODUCTIVITY OF PICEA-SITCHENSIS, THUJA-PLICATA, AND TSUGA-HETEROPHYLLA SEEDLINGS .2. PHOTOSYNTHESIS, 13C DISCRIMINATION AND N PARTITIONING IN FOLIAGE

The effects of differing, exponentially increasing rates of N addition (0.025, 0.05, 0.07 and 0.09 gN gN(-1)day(-1)) on photosynthesis, disc rimination against C-13 and partitioning of foliar N to chlorophyll an d major photosynthetic proteins were compared in seedlings of the ever green conifers Picea sitchensis, Thuja plicata and Tsuga heterophylla. T. heterophylla had the lowest range of foliar N concentrations (N-1m ). Across species, photosynthetic rates (A) increased linearly with N- 1m to a maximum at 21 mg g(-1) and declined at higher N(1m)s. Species differences in A resulted from differences in N-1m, not from differenc es in photosynthetic N use efficiency. Self-shading may have caused A to decline at a high N-1m in P. sitchensis and T. plicata. Measurement s of gas exchange and delta(13)C suggested that carboxylation capacity increased more than did stomatal conductance as N-1m increased. The r esponses were small and confined to N(1m)s associated with the lesser rates of N addition. Concentrations of total protein, ribulose 1,5-bis phosphate carboxylase (RUBISCO) and the light harvesting chlorophyll a /b protein complex (LHC) increased with N-1m, but the fraction of foli ar N allocated to RUBISCO and LHC increased with N-1m only in P. sitch ensis and only between the 0.025 and 0.05N regimes. The responsiveness of A and concentrations of RUBISCO to Nlm were less than reported for deciduous C-3 species.