Seeing the light: germination, growth and flowering responses in wildflowers under artificial nighttime lighting

Abstract

The introduction of artificial nighttime lighting due to human settlements and transport networks is increasingly altering the timing, intensity and spectra of natural light regimes worldwide. Artificial light at night (ALAN) is predicted to constitute a significant anthropogenic pressure on natural biological systems, which are organized foremost by light, and particularly by daily and seasonal cycles of light and dark. Much of the research on the impacts of nighttime light pollution on organisms has focussed on animal species. However, little is known about the impacts of daylength extension due to outdoor lighting technologies on wild plant communities, despite the fact that plant growth and development are under photoperiodic control. The present study evaluated the effects of photoperiod extension that comes from cool white light-emitting diode (LED) street lighting at ground-level illuminance on seed germination, individual plant growth and flowering in wildflower species. Seed germination in Cichorium intybus was accelerated by this lighting treatment. Low-irradiance cool white LED lighting also increased the plant height of individuals of six wildflower species that were exposed to ALAN over the course of a year and under field conditions. In winter, the white LED treatment promoted both individual flowering and the development of an erectophile habit in long-day plants of Lapsana communis, Leontodon hispidus, and Ranunculus acris. Flowering in Oenothera biennis was also hastened by artificial nighttime lighting, the changes being observed in summer. The impacts of ALAN on grassland communities were also examined through long-term field experiments. 13 grassland species growing in mixed communities (‘mesocosms’) were exposed to light treatments simulating low-pressure sodium (LPS) and cool white LED street lighting at ground-level illuminance. Most of these plant species did not exhibit changes in biomass accumulation after five years of exposure to ALAN. However, the white LED treatment decreased biomass production in the herbaceous species Lotus pedunculatus. Low-irradiance cool white LED lighting also increased the inflorescence density of the grass species Agrostis tenuis and Holcus lanatus in a semi-natural grassland. Similarly, an increase in the density of inflorescences of H. lanatus was recorded under the light treatment simulating LPS lighting. Long-term exposure to ALAN at low light levels may account for shifts in the composition and/or structure of plant communities, as suggested by the growth and flowering responses to photoperiod extension found in the present research. Knowledge of the effects of current street lighting, such as cool white LED lighting at low illuminances, on the composition and diversity of plant communities could be further improved by conducting studies on the impacts of these sources of ALAN on the flowering of short-day plants and also on interspecific interactions including plant competition under different nutrient supplies, pollinator visitation and herbivory.