WEWeb EcologyWEWeb Ecol.1399-1183Copernicus GmbHGöttingen, Germany10.5194/we-15-39-2015Do tree-species richness, stand structure and ecological factors affect the
photosynthetic efficiency in European forests?BussottiF.filippo.bussotti@unifi.itPollastriniM.martina.pollastrini@unifi.itUniversity of Florence, Department of Agrifood Production and Environmental Science (DISPAA), Piazzale delle Cascine 28, 50144 Florence, ItalyF. Bussotti (filippo.bussotti@unifi.it) and M. Pollastrini (martina.pollastrini@unifi.it)13November2015151394116September201522October201526October2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://we.copernicus.org/articles/15/39/2015/we-15-39-2015.htmlThe full text article is available as a PDF file from https://we.copernicus.org/articles/15/39/2015/we-15-39-2015.pdf
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.
Pearson correlation (p values) of the OJIP-test parameters of
spruce in relation to tree diversity in exploratory and experimental sites
of the FunDivEUROPE project. Arrows indicate the direction of the
correlation (↓= negative; ↑= positive).
Significances of correlations are presented for p < 0.05.
Discriminant analysis applied on the exploratory sites of Romania
and Finland. Homogeneous groups are defined in based on the species richness. We observe
that few species plots are roughly distinct from the majority of species plots.
Fluorescence rise OJIP curves were induced by 1 s pulses of red light (650 nm, 3500 µmol m-2 s-1). 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 fluorescence OJIP transients
were analyzed with the OJIP test (Strasser et al., 2004; Tsimilli-Michael and
Strasser, 2008). For this study, the following ChlF parameters were
considered: (i) the capacity of the photosystem II (PSII) reaction centres
to trap an absorbed photon (FV/FM, quantum yield efficiency); (ii)
the density of the reaction centres per unit of absorbing pigment (RC/ABS);
(iii) the probability of a trapped electron to move into the electron
transport chain (ΨEo,); (iv) the efficiency of a trapped electron
to move into the electron transport chain, from QA- to the PSI end
electron acceptors (ΨRo); (v) the performance indices for energy
conservation of photons absorbed by PSII, through the electron transport
chain (PIABS) and (vi) for energy conservation until the reduction of
the final acceptors beyond the PSI (PITOT).
The general pattern of the ChlF parameters, evidenced by means of principal
component analysis (PCA), displays that the above parameters are grouped in
two clusters: the main cluster (factor 1, 35.3 % variability explained)
included FV/FM, ΨEo, RC/ABS and PIABS and was
directly related to the latitude, LAI (leaf area index) and basal area. The second cluster
(factor 2, 17.8 % variability explained), including ΨRo and
PITOT, was directly related to solar radiation, temperature, drought
condition, and inversely to latitude.
Different responses of spruce in relation to tree-species richness were
assessed among the sites of the experimental and exploratory platforms
(Table 1).
In the experimental platform (Satakunta, Finland and Kaltenborn, Germany)
tree diversity induced changes in the quantum yield capacity
(FV/FM), with an opposite patterns between the two sites.
Differences between the two sites were connected to the different levels of
development of the stand; Satakunta's spruces grow under the
protection of the tallest birches (Betula pendula Roth.) in mixed plots, as to avoid photoinhibition (Pollastrini et al., 2014). Among the exploratory sites, no
effects of diversity on ChlF parameters were detected at Hainich (Germany)
and Bialowieza (Poland), whereas opposite trends were observed at
Râşca (Romania) and North Karelia (Finland). In the first site, the photosynthetic
efficiency was enhanced by tree diversity, whereas in the second it was
depressed. Such patterns were connected to the behaviour of ΨEo
(capacity to reduce primary acceptors of electrons). That may indicate
different strategies in the competition for soil resources. In particular,
the detrimental effect of diversity in Finland may be attributed to the
competition for limited resources in extreme ecological conditions.
A multivariate statistic (discriminant analysis) based on ChlF parameters
was applied to evaluate the consistency of the groups attributed (site and
species richness). The results show that the same species at different sites
(exploratory and experimental) forms distinct clusters, suggesting that the
ChlF properties were influenced by the ecology of the site and by the degree
of development of the stand. As far as the diversity is concerned, the
capacity of ChlF to identify different levels of tree-species richness
is less evident. However, in Romania and Finland, few species plots were
roughly distinct from the majority of species plots (Fig. 1).
In conclusion, the most relevant findings can be summarized as follows. (1) Tree diversity affects the tree photosynthetic efficiency. The effects,
detected with ChlF analysis are variable depending on the relative position
of the tree crowns in the canopy layer as well as the competition at the above-ground
(competition for light) and below-ground (competition for water and
nutrients) levels. (2) Data on young experimental plantation and mature
forest cannot be compared because of the different degree of evolution of
the forest stand and soil. (3) The multivariate statistical analyses of ChlF
parameters allow for the identification of trees of the same species growing in
different ecological condition and (although to a lesser extent) the effects
of tree-species richness.
Acknowledgements
The research leading to these results received funding from the European
Union Seventh Framework Programme (FP7/2007-2013) under grant 265171. We
thank all the FunDivEUROPE community, especially the “leaf
team”, the site managers, technicians and tree climbers.
Edited by: D. Montesinos
Reviewed by: H. Kalaji and V. Goltsev
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