Seasonal dynamics of nitrogen level and gas exchange in different cohorts of Scots pine needles: a conflict between nitrogen mobilization and photosynthesis?

Needles of evergreen conifers are not only photosynthetic organs, but they may also serve as storage sites for carbohydrates and nutrients. Since nitrogen is both a component of photosynthetically active molecules and a nutrient stored in the needles and mobilized for shoot regrowth, we searched for evidence of a trade-off between needle N storage and photosynthetic capacity. Using sequential sampling, we tracked seasonal patterns in needle structure, nitrogen (N_mass) and carbohydrate concentration, and gas exchange in needles of all age classes (current-year, 1- and 2-year-old) present on Pinus sylvestris trees. In both 1- and 2-year-old needles, N_mass increased slightly in the spring, fell subsequently after the onset of shoot growth, followed by replenishment in 1-year-old and further decline until abscission in 2-year-old needles. However, only 2-year-old needles showed a positive correlation between N_mass and photosynthesis, consistent with their overall lower N level that indicated a tighter N budget. The 2-year-old needles had a higher leaf mass per area and lower photosynthesis in comparison with 1-year-old needles. They also had a lower photosynthetic nitrogen use efficiency, which suggests that in addition to N withdrawal, structural change and biochemical modifications might have contributed to photosynthetic decline in the final year of needle life. Thus, whereas seasonal N mobilization observed in 1-year-old needles did not seem to interfere with photosynthetic potential, resorption of N could have contributed to gradual photosynthetic decline in 2-year-old needles.