Assessing the impact of street lighting on Platanus x acerifolia phenology

https://doi.org/10.1016/j.ufug.2018.05.015Get rights and content

Highlights

  • Artificial light affects leaf fall phenology of Platanus x acerifolia.

  • Trees exposed to higher level of light maintained green leaves longer in winter.

  • Lighting and greening should be jointly considered in the design of public spaces.

  • Presence-absence protocol for phenology can be useful in real context monitoring.

Abstract

Autumn phenology is an important part of the tree growing season that is still poorly understood. In addition to the environmental factors that might affect its timing, there are artificial effects introduced by modern society that could interfere with it, such as the increasing use of artificial light to illuminate urban nights. This study investigates the relationship between outdoor public lighting and leaf senescence of Platanus x acerifolia that constitutes with more than 4000 individuals, and 6% of public greening in Florence, Italy. The difference in autumn phenology under two lighting conditions was assessed by analysing data collected in a real context, using a presence-absence protocol of green leaves on 283 trees during leaf fall season from 2014 to 2017. Trees were classified in two groups of different light exposure. In 2016–2017, data were also collected at Cascine park, the main green area within the city and darker than the monitored sites. According to the analysis, the percentage of trees with green leaves under luminaires was significantly higher than trees far from the luminaires, for all sites from mid-December to the end of January, and this effect was enhanced during 2016–2017 which was characterised by a colder winter. In the same year, the period of absence of green leaves at Cascine started at least 20 days earlier than the other sites. These findings should be taken into consideration by scientists because artificial light could affect autumn phenology and therefore the length of the vegetative season, and by urban greening and light managers during the design and management of public green spaces. Moreover, the presence-absence protocol proved to be suitable for collecting observations because it was easy to perform in a real context.

Introduction

The use of artificial light at night (ALAN) for facilitating human activities has increased at rates of 3–6 % in the last decades (Hölker et al., 2010), and this increase will likely continue because the availability of more energy-efficient lamps could encourage over-illumination of public places (Kyba et al., 2014). ALAN can alter night-time natural light directly by irradiance and glare and indirectly by the scattering of light caused by atmospheric particles (sky glow), which affects illumination levels of larger areas (Kyba and Hölker, 2013). This spreading also causes the alteration of diurnal and seasonal cycles of light in natural ecosystems far from urban settlements, as it may be the major source of light (Bennie et al., 2015; Gaston et al., 2015a). Hence, the natural pattern of day and night that regulates natural rhythms of biological life can be disrupted by ALAN (Gaston et al., 2015b). Several studies have assessed the negative impacts of ALAN on the behaviour of living beings, especially nocturnal species, both at the individual level and within populations and ecosystems. For a complete and systematic review of the ecological effects of ALAN and the future perspectives in this study sector see Gaston et al. (2013); Gaston et al. (2015b), and Bennie et al. (2016).

Regarding flora, it is well known that photoperiod and air temperature play a central role in plant development and particularly in plant phenology, the study of the periodic life cycle events that are influenced by seasonal climatic variations. These studies are very important because environmental modification induced by human activities like climate change can cause different shifts and, as a consequence, a mismatch in phenology of mutually dependent species (e.g. pollinator and plant) with potentially dangerous effects on the survival of one of the species (Donnelly et al., 2011). Indeed, the effect of air temperature on phenology has been documented by a significant number of studies worldwide, both in natural environments (Parmesan and Yohe, 2002; Menzel et al., 2006; Penuelas et al., 2009) and in an urban context (Jochner et al., 2012; Massetti et al., 2015), where the temperature increase is higher due to the UHI effect (Oke, 1976). While the effect of air temperature on spring phenology has been broadly investigated, there are less studies that focus on autumn phenology because monitoring is more challenging and the effects of temperature are not clearly in one direction (Estrella and Menzel, 2006; Yu et al., 2016). However previous investigations suggest that photoperiod is the leading factor influencing autumn phenology (Perry, 1971; Basler and Korner, 2012; Yu et al., 2016), and warm temperatures in autumn may delay the timing of leaf fall (Menzel, 2003; Garonna et al., 2014; Estiarte and Peñuelas, 2015). Even though autumn phenology is less studied it is as important as spring phenology, because it affects the length of the growing season and the reproductive cycle of trees (Richardson et al., 2010).

Although ALAN can alter the photoperiod, most investigations have been performed in laboratory experiments and have focused on the effects of quantity, spectral quality and duration of light on basic mechanisms regulating plant growth (Briggs, 2006). Indeed, only few studies were conducted in natural or quasi natural environment. A study reported that Abelia grandifolia V. and Robinia pseudoacacia L. individuals under artificial light continued to produce new shoots later in the season and as a consequence are more exposed to first winter frost damage than those far away from artificial light (Kramer, 1937). Another study conducted at the Agricultural Research Center in Beltsville, Md. (USA) found that the growth response of 7 out of 22 ornamental species was affected by mercury vapor and metal halide lamps, and 16 out of 22 by high pressure sodium lamps (HPS) (Cathey and Campbell, 1975). Other studies on wild plants are documented in Bennie et al. (2016). Recently, the use of citizen science data to assess the role of ALAN on the phenological shift of four deciduous species was investigated (ffrench-Constant et al., 2016). While isolated observations and commentary have been previously reported, this phenomenon has yet not been systematically studied or analysed in a real-world context (Bennie et al., 2016). Therefore, there are no studies that investigate ALAN effects in an urban environment, even though vegetation is extensively used in cities to provide multiple ecosystem services (Nowak and Crane, 2002; McPherson et al., 2011), such the moderation of urban microclimate due to shading (Napoli et al., 2016). Within this context, the main objective of this study is to investigate the effect of public outdoor lighting on Platanus x acerifolia winter phenology under different lighting conditions in a real urban context. This objective is achieved by monitoring the presence-absence of green leaves in individual trees in three main streets of Florence, Italy for three years and at Cascine park (the city's largest green area) for one year. Trees were divided in two groups according to the distance from light poles, and based on the number of trees with green leaves, the statistical differences between the two group were assessed.

Section snippets

Material and methods

Florence is an historically prominent city lying on a plain to the southwest of the Appennine mountains in the central part of Italy (Lat: 43.77 N; Long: 11.26E; elevation 50 m a.s.l.). It is characterized by a sub-Mediterranean climate with a hot and dry summer and wet winters, with rainfall distributed among the other seasons. Monthly temperatures during the study period (November through January, in 2014–2017) were compared to the 1971–2000 climatological record (CLIM). Meteorological data

Results

The sky brightness of the city of Florence measured at M was (16.52 ± 1.13 mar/arcsec2. Images of Platanus x acerifolia during the study period are shown in Fig. 4.

Data and statistics on ALC and NALC trees were reported daily for each site (Table 2) and weekly for all the sites (Fig. 5). In the first year, the percentage of ALC trees with GL was significantly higher than NALC trees between nday 19 and 58 at site A. B and C were also monitored twice and a significant difference between ALC and

Discussion and conclusion

According to the present study, the percentage of ALC trees with green leaves was significantly higher than NALC trees, both for each site and for all sites together for each year. These results suggest that a different level of light exposure can affect the persistence of green leaves on Platanus x acerifolia during winter. It must be pointed out that the level of exposure (ALC and NALC) was not measured in this study, except for some single measurements), but it was classified as a bimodal

Acknowledgements

The author wish to thank Dr. Andrea Giacomelli for sharing ideas that helped to improve this research and Prof. David Pearlmutter of Ben Gurion University of the Negev,for suggestions and revisions of the text.

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