Artificial light at night decreases plant diversity and performance in experimental grassland communities

Abstract

Artificial light at night (ALAN) affects many areas of the world and is increasing globally. To date, there has been limited and inconsistent evidence regarding the consequences of ALAN for plant communities, as well as for the fitness of their constituent species. ALAN could be beneficial for plants as they need light as energy source, but they also need darkness for regeneration and growth. We created model communities composed of 16 plant species sown, exposed to a gradient of ALAN ranging from ‘moonlight only’ to conditions like situations typically found directly underneath a streetlamp. We measured plant community composition and its production (biomass), as well as functional traits of three plant species from different functional groups (grasses, herbs, legumes) in two separate harvests. We found that biomass was reduced by 33% in the highest ALAN treatment compared to the control, Shannon diversity decreased by 43% and evenness by 34% in the first harvest. Some species failed to establish in the second harvest. Specific leaf area, leaf dry matter content and leaf hairiness responded to ALAN. These responses suggest that plant communities will be sensitive to increasing ALAN, and they flag a need for plant conservation activities that consider impending ALAN scenarios. This article is part of the theme issue ‘Light pollution in complex ecological systems’.

Artificial light at night (ALAN) causes shifts in soil communities and functions

“…In our experiment, the population of plant-feeding nematodes showed a trend towards being higher under high ALAN, although the statistical significance was only marginal. Since stressed plants can be more susceptible to herbivory [58], the higher density of plant-feeding nematodes at high ALAN lends some support to the suggestion that ALAN stresses plant communities and makes them more vulnerable to belowground herbivory, although there is profound seasonal variability in the plant variables [34]. Although the proportion of plant-feeding nematodes was comparatively small in our study, their impact on productivity can be enormous [59], with the potential to enhance the negative impact of ALAN, especially when plants become more physiologically stressed.…”

“…During phase 2, plant biomass was generally higher compared to phase 3 (for more details, see [34]). Interestingly, high levels of ALAN resulted in a reduction in plant biomass compared to low levels of ALAN (figures 1b,e,h,k).…”

“…The effects were most pronounced during phase 2 when plant biomass was overall highest and where ALAN significantly reduced plant biomass (932.1 ± 128.3 g dry plant biomass in phase 2 versus 331 ± 74.6 g dry plant biomass in phase 3 [34]). By contrast, in phase 3, the effect of ALAN was no longer significant, probably because the duration of plant growth was longer (nine weeks in phase 2 as compared to six weeks in phase 3; electronic supplementary material, figure S1), allowing plants to be exposed to the influence of ALAN for a longer period of time [34]. In phase 2 of the experiment, higher levels of ALAN were associated with significantly lower soil basal respiration, indicating decreased microbial activity.…”

“…The higher plant biomass in phase 2 did not reduce soil water content, likely because plant communities with more biomass can reduce transpiration [52], create a more stable microclimate close to the soil surface [53,54] and be more efficient in water use than under high ALAN. This potential positive effect of plant communities on soil became smaller with time ( phase 3), likely because the turnover of the plant community reduced the negative effects of ALAN on plant biomass [34].…”

“…After a settling period of about five months, the soil density was 0.20 g cm − ³. The plant community consisted of 16 grassland species and was sown and exposed to the experimental light regime as described in [34]. Some plant species of the seed bank were introduced as well.…”