A field study on solar-induced chlorophyll fluorescence and pigment parameters along a vertical canopy gradient of four tree species in an urban environment
Introduction
In addition to light reflected by vegetation, plants emit a weak far-red radiation better known as chlorophyll (Chl) fluorescence. Typically, Chl fluorescence competes with photosynthesis processes and heat for the use of a leaf's absorbed light. Therefore, Chl fluorescence has the advantage of being highly vegetation specific since it is only emitted by the fluorophores (e.g. Chl a). Reflectance, in contrast, is influenced by any compound that absorbs in the wavelength range observed (e.g. Chl a and b as well as green paint). Whereas reflectance based vegetation indices in remote sensing are measured since the '70s, the technology and methodology for the airborne detection of Chl fluorescence are available only quite recently (Damm et al., 2010, Guanter et al., 2013, Lichtenthaler, 1988, Mcfarlane et al., 1980, Rascher et al., 2009, Stoll et al., 1999). Chl fluorescence is emitted in the spectral range of 650–850 nm and is characterized by two peaks. One peak occurs in the red around 687 nm and the other in the far-red around 741 nm. So far, the red/far-red peak ratio of Chl fluorescence is a parameter often used for the detection of leaf and plant stress since it reveals changes in chlorophyll content due to a partial overlap of the red fluorescence peak with the absorption bands of chlorophyll (D'Ambrosio et al., 1992, Hak et al., 1990, Lichtenthaler and Rinderle, 1988). Moreover, by measuring steady-state fluorescence, changes in physiological stress can be identified before any damage and/or a detectable loss of chlorophyll occurs (Meroni et al., 2009, Van Wittenberghe et al., 2013). Despite this large potential for stress monitoring the fluorescence peak ratio is also known to become quite insensitive to Chl content above a Chl concentration of 300 mg m− 2 (Gitelson et al., 1998). However, Agati et al. (1995) pointed out that the utility of the Chl fluorescence peak ratio reaches beyond the simple detection of changes in chlorophyll concentration. Since the peak ratio has been linked to the underlying mechanisms of photosynthesis, it can serve as an instant monitor of CO2 uptake by plants (Damm et al., 2010, Freedman et al., 2002, Rascher et al., 2009).
The use of Chl fluorescence as a tool of plant stress is, therefore, extremely appealing since it can be used in remote sensing applications (Agati et al., 2000, Stoll et al., 1999). Nevertheless, a better understanding of the spectral shape and variability of steady-state fluorescence emission (calculated as fluorescence yield) within whole canopies is required. Especially since different conditions of light or stressors along a tree canopy gradient may influence the photosynthetic apparatus and, therefore, the fluorescence emission, along the vertical gradient. Downward fluorescence, which is seldom taken into account, is the fluorescence that is emitted downward from the lower leaf surface when the incoming light arrives at the upper leaf surface. Due to the longer light path through the leaf, the downward fluorescence signal is shown to be a more sensitive indicator of the leaf's Chl content compared to the upward signal, and is, therefore, a better stress indicator (Van Wittenberghe et al., 2013).
The research presented here had as a main objective to investigate both the importance of within-canopy and between-canopy variation for several fluorescence yield (FY) and pigment parameters. To do so, both the effects of sampling height and tree location were investigated for four tree species. Individuals of each species were selected to represent different traffic exposures in an urban environment. Urban motorized traffic releases large amounts of particulate matter (PM), a complex mix of both liquid and solid particles of organic and inorganic substances suspended in the air. PM deposition on urban vegetation is shown to decrease by height and distance from the source, illustrated by the biomagnetic value of leaves (Hofman et al., in press, Kardel et al., 2012). Therefore, a vertical gradient in traffic pollution exposure may well exist quite regularly. Due to the previous results showing the potential of sun-induced FY parameters of lower canopy leaves as stress indicators of urban traffic (Van Wittenberghe et al., 2013), the following hypotheses were formulated:
- (i)
Leaf saturation isothermal remanent magnetization (SIRM) values vary along the canopy and show a decreasing dust deposition from bottom to top;
- (ii)
The effect of tree location might be more important than sampling height on FY indices when variation in pollution is larger among trees than within trees, and;
- (iii)
Within species, FY indices are rather determined by the photosynthetic pigment system than by leaf structural parameters such as specific leaf area, nitrogen content and leaf thickness.
Section snippets
Field site
The field study was conducted in the city of Valencia (39°28′ N, 0°22′ W, 15 m a.s.l., with 809,267 inhabitants in 2010). Three dicotyledonous deciduous trees Celtis australis L. (European nettle tree), Morus alba L. (White mulberry), Platanus × acerifolia (Aiton) Willd. (London plane), and one monocotyledonous evergreen tree, Phoenix canariensis Chabaud (Canary Island date palm) as well, were selected based on occurrence, morphological leaf characteristics, and absence of diseases. Due to their
Location and sampling height impact on SIRM
The highest SIRM values were obtained at the bottom canopy layer of the C. australis, P. canariensis and M. alba at traffic square (Fig. 2). The palm tree (P. canariensis) had the highest absolute SIRM values of 191 × 10− 6 A, while P. × acerifolia showed comparatively low values, without distinct extremes between the three different sampling heights. To obtain an indication of the pollution exposure for each tree, we compared the SIRM values with threshold values based on previous research in the
Conclusions
The hypothesized decrease of magnetic particle deposition from bottom to top was only visible at the traffic location. Deposition of magnetic particulate matter was for all species more influenced by tree location than by sampling height, illustrating a more location dominated pollution level. We also observed that several FY indices showed a higher variation among tree individuals (significant location effect) than within tree individuals (non-significant height effect) for M. alba and P.
Acknowledgments
The research presented in this paper is funded by the Belgian Science Policy Office (BELSPO) in the frame of the STEREO II program-project BIOHYPE (SR/00/131) and partially supported by the Spanish Ministry for Science and Innovation under the project AYA 2010-21432-C02-01. The authors wish to thank Adrian Del Amo and Violeta Ortiz for their field assistance and laboratory analyses. We would also like to acknowledge Claus Buschmann for his helpful literature suggestions.
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