Do tree-species richness, stand structure and ecological factors affect the photosynthetic efficiency in European forests?

Tree diversity improves the stability of forest ecosystems and their capacity to deliver ecosystem services. The functionality of the forests depends largely to the health and physiological performances of singular trees. There is a large body of literature that evidences functional interaction (competition and/or facilitation) between neighboring trees belonging to different species. These interactions may be at hypogeal (use of water and nutrient) or epigeous (use of light) level and can be assessed by mean of foliar analysis. Among all the parameters retractable from leaves, the photosynthetic efficiency (i.e., the conditions and functionality of photosystems, by mean of chlorophyll a fluorescence – ChlF – analysis) demonstrated to be very informative. Within the project FunDivEUROPE (Functional Significance of Forest Diversity in Europe), ChlF parameters (JIP test) were assessed through two European tree diversity platforms, constituted respectively by experimental and explorative forests. The overall survey covers all the main vegetational and climatic conditions, from Mediterranean to boreal regions. The experimental platform was constituted by young artificial plantations, whereas the explorative platform comprised natural mature forests. The patterns of ChlF parameters in relation to tree diversity was strongly dependent from ecological factors of sites and tree species. We refer to the case of Picea abies, the most widely distributed species in the FunDiv network. P. abies trees in the Northernmost explorative site (Finland) decreased their photosynthetic efficiency with increasing diversity, but the opposite trend was observed at the Southern site (Romania). The role of climatic and structural factors (leaf area index, tree density) is analyzed and discussed. P. abies responded differently also in the two experimental site at Satakunta (Finland) and Kaltenborn (Germany). Opposite patterns were attributed to the competition for light at different state of development of the stands. More information on the overall status of trees can be obtained by combining ChlF parameters with other physiological measurements (such as foliar total nitrogen and carbon stable isotopes) and the conditions of crowns. Results from a first survey in Tuscany are reported here. In conclusions, the use of foliar analysis and ChlF parameters is a powerful tool that can be used in forestry to assess the vitality of trees and their responses to changing environmental conditions and the effectiveness of management measures.