Modification of light-acclimation of Pinus sylvestris shoot architecture by site fertility

Adjustment of shoot architectural characteristics—needle angle with respect to shoot axis (α), needle mass per unit shoot silhouette area (MS), needle number per unit shoot axis length (KL), silhouette to total needle area ratio (SS)—to seasonal average daily integrated quantum flux density (Qint) was investigated in conifer Pinus sylvestris L. in an old field (fertile site) and in a raised bog (infertile site). For both sites, KL increased with increasing Qint, leading to a greater foliar area and dry mass per unit shoot length, as well as to larger MS and foliar nitrogen content per unit silhouette area at higher irradiance. The five-fold increase in MS with Qint via architectural modifications allowed biomass concentration in high light environment where the photosynthetic returns were the highest. However, the negative correlations between SS and Qint indicated that enhanced needle production also led to a lower sunlit needle area fraction and greater self-shading within the shoot at higher irradiance. Simulations using canopy gap fractions across the sky hemisphere incident to the shoots in their natural position further demonstrated a decrease in light interception efficiency relative to a flat surface (ϑ) with increasing Qint. A shoot architectural model based on a turbid medium analogy suggested that the decreases in the efficiency primarily resulted from increased clumping and larger needle area density at higher irradiances. The relationships were qualitatively similar at both sites, but the needles were shorter and the packing of needles was higher leading to a lower gap fraction within the shoot volume and to greater self-shading within the shoot at the nutrient-limited site, especially under low irradiance. We conclude that both needle and shoot level modifications contribute to plastic alterations in shoot light harvesting efficiency in P. sylvestris, but also that site fertility may significantly constrain the light-acclimation in shoot architecture.