Site fertility and the morphological and photosynthetic acclimation of Pinus sylvestris needles to light

Morphological and photosynthetic acclimation of current-year needles to canopy gradients in light availability (seasonal mean integrated quantum flux density, Q(int)) was studied in the temperate conifer, Pinus sylvestris L., at two sites of contrasting nutrient availability. The nutrient-rich site supported a monospecific P. sylvestris stand on an old-field. The trees were approximately 30 years old and 19-21 m tall. Mean foliar N and P contents (+/- SD) were 1.53 +/- 0.11 % and 0.196 +/- 0.017%, respectively. The nutrient-poor site was located on a raised bog supporting a sparse stand of 50- to 100-year-old trees, with a height of 1-2 m, and mean needle N and P contents of 0.86 +/- 0.12% and 0.074 +/- 0.010%, respectively. At both sites, needle thickness (T) and width (W) increased with increasing Q(int), and leaf dry mass per unit leaf area (M-A) was also greater at higher irradiance. The light effects on M-A-the product of needle density (D) and volume to total area ratio (V/A(T))-resulted primarily from large increases in V/A(T) with Q(int) rather than from modifications of D, which was relatively insensitive to light. Although needle morphology versus light relationships were qualitatively similar at both sites, needles were shorter, and the slopes of W, T, M-A and V/A(T) versus light relationships were lower, at the nutrient-poor than at the nutrient-rich site, indicating that the plasticity of foliar morphological characteristics was affected by nutrient availability. As a result of lower plasticity, needles at the nutrient-poor site were narrower, thinner, and had lower MA at high irradiance than needles at the nutrient-rich site. The maximum carboxylase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (V-cmax) and the maximum photosynthetic electron transport rate (J(max)) scaled positively with foliar N and P contents. The correlations were generally stronger with P than with N, suggesting that needle photosynthetic capacity was more heavily limited by the availability of P than of N. The J(max)/V-cmax ratio was positively related to the foliar P/N ratio, indicating that J(max) was more strongly suppressed than V-cmax under conditions of low P availability. Phosphorus and N deficiency also limited the plasticity of foliar photosynthetic characteristics. There was a moderate increase in needle photosynthetic capacity of up to 1.6-fold from the bottom to the top of the canopy at the nutrient-rich site, but net assimilation rates were essentially independent of canopy position at the nutrient-poor site. Stomatal constraints on photosynthesis were similar between the sites, indicating that photosynthetic acclimation was curtailed at the biochemical level. We conclude that the foliar capacity for morphological and physiological acclimation to high light significantly decreases with decreasing nutrient availability in P. sylvestris, and that both N and P availability are potentially important determinants of foliar carbon gain capacities.