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

Forest trees live in a multi-factor environment that includes the abiotic characteristics of the site (climate, soil, bedrock) and the structural features of the forest stand (tree age, density, leaf area index, tree species composition). The analysis of the functional traits (morphological, chemical and physiological, see Bussotti and Pollastrini, 2015) at leaf and tree level allows for the assessment and evaluation of the responses of trees to changing environmental factors. Among the physiological traits, the analysis of the chlorophyll fluorescence (ChlF, namely the prompt fluorescence and OJIP test, Strasser et al., 2004) is an effective tool to assess in vivo plant stress in experimental studies. The application of ChlF on mature trees in large-scale studies is more problematic due to the difficulty to reach tree canopies in forests, although some experiences were carried out at the local scale (Koprowski et al., 2015). ChlF measurements and analyses with the OJIP test allow to collect a great amount of data on the light-use efficiency in photosynthetic processes (one measurement takes 1s and it is possible to make many replications in a short time). Furthermore, the ChlF induction curve, evaluated by means of OJIP test, produces multi-parametric information on the potential photosynthetic efficiency. A large-scale application of OJIP test in forests was carried out within the 7FP project FunDivEUROPE (Functional Significance of Forest Biodiversity in Europe), aimed at assessing the functional significance of forest diversity in Europe (Baeten et al., 2013). The effects of tree diversity on the photosynthetic efficiency of tree species were assessed in the exploratory platform of FunDivEUROPE, that includes six European mature forests (monocultures and mixed up to five species) distributed along a latitudinal gradient (from Mediterranean to boreal). FunDivEUROPE also included an experimental platform, consisting of mixed forest stands planted ad hoc with different levels of tree-species richness. These experimental stands were installed during the implementation of the previous projects when trees were still young. The aims of this contribution are (i) to explore the variability of ChlF parameters along European ecological gradients and (ii) to compare the responses to diversity in young mixed plantations and in mature forests. For the latter purpose, we selected the sites with Picea abies(L.) Karst. (spruce), the most widespread tree species in experimental and exploratory sites. The leaf sampling was carried out in the summers between 2011 and 2013, by means of tree climbers, extension loppers and gun shooters, according to the height of the trees, the stand structure, and the operational conditions in each region. After sampling, branchlets were put in hermetic plastic bags and humidified to avoid leaf dehydration. ChlF measurements were done with a Handy PEA fluorimeter (Plant Efficiency Analyser, Hansatech Instruments Ltd., Petney, Norfolk, UK) after 4–5 h of sample dark adaptation on 16 leaves per plant (in conifers only current year needled were measured). A long dark adaptation period was necessary to reduce the effects of photoinhibition. Fluorescence rise OJIP curves were induced by 1 s pulses of red light (650 nm, 3500 μmol m s). Plotted on a logarithmic time scale, the fluorescence transients show a polyphasic shape. The initial fluorescence level, indicated with “O”, is the beginning of the fluorescence emission. Following the “K” time step (300 μ s), the “J ” (∼ 2 to 3 ms) and “I” time steps (∼30 ms) reflect the intermediate level of the fluorescence emission. The maximum level of the fluorescence emission is “P ” (the peak, at 500–800 ms–1 s). The