Plasticity in Pinus pinaster populations of diverse origins: Comparative seedling responses to light and Nitrogen availability

The effects of light and N availability, and population on seedling morphology and biomass allocation were assessed in eleven months seedlings from ten Pinus pinaster populations. We conducted a garden experiment using a light gradient (full sunlight, HL= 337.1 μmol m2 s-1; medium light, ML= 120.6 μmol m2 s-1 and low light, LL= 30 μmol m2 s-1) and two contrasting soil fertility regimes: low-N and high-N availability. Light availability was an important factor controlling phenotypic plasticity. However, differences between productivity environments were observed, especially in full sunlight and medium light conditions, which suggest that soil fertility may be very important at early successional stages at open and medium shaded microsites, but would exert low influence in deep shade conditions. Population-specific responses were observed for many traits, which suggest a genetic control of morphological plasticity. All populations produced more total dry weight with HL and high-N. When light and/or N richness varied, growth depended upon which resource was more limiting. Results indicated ontogenetic drift but when seedlings were compared at the same size, results fitted with optimal partitioning theory. Allocation to needles was higher in HL and ML/high-N conditions, while allocation to root was higher in low-N seedlings in HL and ML conditions. With shade (ML and LL) all populations allocated more dry weight to stem, regardless N availability. A significant increment in stem height was observed with shade increase.