Leaf structure and photosynthetic properties to the intra canopy light gradient in the reserve “Siro Negri” (Italy): comparison among tree species

The heterogeneous light environment within a tree crown due both to self-shading and to shading by neighboring trees determines leaf trait variations. However, the range of variation is species-specific reflecting the optimization of whole plant resource investment strategy. Most studies of plant response to light have been aimed at underlining ecological implications of the tolerance to the extreme (i.e. tolerance to sun and shade) but noticeably less effort has been invested in exploration of trends in the plastic response to light. The main objective of this research was to analyze leaf morphological and physiological trait variations among the species growing in the Natural Integral Reserve “Siro Negri” (Italy) in term of phenotypic plasticity sensu Valladares (2000). The considered forest which in the past largely covered the fluvial valleys along the Ticino river remained untouched for over two decades. The vertical light profile of the forest showed a significant decrease of the photosynthetic photon flux density from the upper tree crown (PPFD > 1500 µmol m-2 s-1) to the lower tree crown (PPFD < 600 µmol m-2 s-1). All the species had thicker sun leaves compared to shade ones (i.e. 73% higher leaf mass area, LMA, mean value). Leaf thickness could be used as a measure of energy investment per unit of leaf area in full sun conditions, while the larger and thinner shade leaves were more advantageous for light capture under low light. Sun leaves allocated more nitrogen than shade ones as attested by the photosynthetic nitrogen use efficiency (more than 100% higher in sun than in shade leaves) reflecting an increase in carboxylating enzymes (RUBISCO) and proteins, responsible for photosynthetic electron transport in full sun and justifying the significantly higher photosynthetic rates. The ratio respiration /photosynthesis was higher in shade than in sun leaves. The plasticity index (ranging from 0.34 to 0.55) shows significant differences in the response of the considered species to light variations underlining the larger plasticity of Q. robur which might be advantageous in conditions of environmental change.