DOI: 10.5061/dryad.hhmgqnknt
title: Artificial light at night (ALAN) decreases plant diversity and performance in experimental grassland communities – Data on species biomass and traits

description: 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 on plant communities as well
 as the fitness of their constituent species. ALAN could be beneficial for
 plants as they need light as an 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.
description: The EcoUnits were filled with 1.23 m<sup>3</sup> of
 unsterilised, well-mixed soil from the vicinity of the EcoTron, as we also
 monitored soil communities in the same experimental setup (see Cesarz et
 al.). Plant communities comprising 16 plant species were sown into soil on
 19 February 2020 (see Table S1). Because the soil was not sterilised, some
 of the local seed bank was also transferred into our experiment. Plant
 communities were harvested by clipping aboveground plant biomass (2 cm
 above topsoil) on June 11, July 3, and August 28 (establishment period),
 as well as on October 27 and December 8 (measurement period). This harvest
 regime mimics typical intensive grassland management in central Europe,
 with short growth phases in between harvest events. 
 For this study, we analysed the last two harvests in detail to
 address temporal variations and accumulated effects of the ALAN treatment
 (see Table S1; hereafter referred to harvest 1 and 2, respectively). The
 harvests differed in length: harvest 1 encompassed a time for regrowth of
 9 weeks, whereas harvest 2 only encompassed 6 weeks, as this was embedded
 in a bigger experimental setup. The biomass of one-eighth (0.19
 m<sup>2</sup>) of each EcoUnit (subplot) was separated into
 species (both sown and not sown as well as ‘unknown’) and then dried to
 constant weight at 60°C for three days. Plant identification was sometimes
 not possible when the plants were not fully mature. These species were all
 clustered as ‘unknown species’, whereas for others only the genus could be
 determined. Dead biomass was also recorded. Plant functional trait data
 was collected for one species each per functional group of grasses
 (<em>B. hordeaceus</em>), non-legume forbs (<em>P.
 lanceolata</em>) and legumes (<em>T. repens</em>) just
 before the harvests in October and December. The species were selected
 based on their frequent occurrence in the EcoUnits. However, not all plant
 traits were measured on all species and in all EcoUnits. <em>P.
 lanceolata</em> was originally not sown into the communities but had
 become one of the dominant species in the EcoUnits by October and was thus
 selected for our experiment. It was not very abundant by the end of the
 experiment as it did not regenerate well after the harvest in October. All
 traits were collected and measured just before the harvest unless stated
 otherwise. Stretched plant height of three representative individuals per
 species and EcoUnit was measured using a ruler. Then, ten healthy leaves
 from at least three manually randomly selected individuals per species and
 EcoUnit were harvested and transported to the laboratory, where SLA, LDMC,
 toughness, hairiness and wettability were measured. All
 ten leaves were scanned on an Epson Expression 11000 XL scanner and the
 resulting images were analysed using ImageJ to determine the leaf area. In
 the case of <em>T. repens</em>, only the lamina was scanned.
 Leaves were weighed and subsequently dried at 70°C for at least 48h, and
 dry weight was recorded to calculate SLA (leaf area of fresh leaf/ dry
 weight) and LDMC (dry weight/ fresh weight). All weights were measured
 using a precision scale (QUINTIX315_1S, Sartorius Lab Instruments GmbH
 & Co. KG, Goettingen, Germany). A few days afterwards, the
 chlorophyll fluorescence measurements and the SPAD values were determined
 on living plants in the EcoUnits, just before the harvests. The hairiness,
 or rather the density of trichomes, of the leaves was analysed by counting
 the hairs from an image taken at 400-fold magnification using a light
 microscope and focussing on the middle part of the leaf (Ocular 10x/22,
 Di-Li-2009, Distelkamp-Electronic, Kaiserslautern, Germany) in ImageJ. For
 that, four of the leaves used in SLA measurements were chosen at random.
 Hairs were counted on the upper and lower leaf side and then added to make
 a total for both leaf sites. <em>T. repens</em> did not show
 any hair on its lamina. The samples of this species were excluded from the
 subsequent analysis. The leaf thickness was measured with a digital
 caliper (WEZU Messwerkzeuge Remscheid GmbH, Remscheid, Germany) at the
 same spot as leaf toughness. For leaf toughness, the puncture resistance
 was measured using a surgical blade at a speed of 129 mm min<sup>-1
 </sup>on an electric test stand (Sauter GmbH, Wutöschingen, Germany)
 and the force of the cut was measured with a power meter (FH 50, Sauter
 GmbH). The leaf toughness was than calculated as the quotient between the
 puncture resistance and the thickness. The leaf wettability was
 investigated via measuring the contact angle (CA) of a water droplet and
 the leaf, where high CA means low wettability. For that, a droplet of 5 µl
 distilled water was placed on a flat leaf surface for 90 seconds and then
 photographed (Nikon D5300 with a Sigma DC Objective, Chiyoda, Tokyo,
 Japan). The CA was then measured using ImageJ. Chlorophyll fluorescence
 was measured using a PocketPEA device (Hansatech, King’s Lynn, Norfolk,
 UK). We measured the parameters PI<sub>abs</sub> as well as
 plant stress via F<sub>v</sub>/F<sub>m</sub> after
 30 min of dark adaption to ensure a full reduction of the photosystems on
 three replicate individuals for each EcoUnit and species. These
 measurements were not performed on <em>P. lanceolata</em>, as
 not many individuals were abundant after harvesting the leaves for the
 previous analysis. The SPAD value was measured using a SPAD 502 (Minolta
 Camera Co., Osaka, Japan) on the same individual. For each individual,
 three replicate measurements were performed as the values varied within
 individuals.
description: # Artificial light at night (ALAN) decreases plant diversity and
 performance in experimental grassland communities – Data on species
 biomass and traits
 [https://doi.org/10.5061/dryad.hhmgqnknt](https://doi.org/10.5061/dryad.hhmgqnknt) In ALAN\_specis\_biomass.csv, Biomass per species in the proportion sorted is given. In ALAN\_biomass.csv, the biomass of the entire Ecotron is given. In ALAN\_traits.csv, traits of the species are given. ## Description of the data and file structure ALAN\_specis\_biomass.csv: The species are abbreviated by 3 letters genus and species. unit describes the name of the EcoUnit (numbered consecutively), light refers to the light regime (given in lux), mass indicates the biomass (in g dry matter) and harvest denotes whether it was the first or 2nd harvest studied (categorical information). ALAN\_biomass.csv: biomass refers to biomass (g dry matter), unit describes the name of the EcoUnit (numbered consecutively), and harvest denotes whether it was the first or 2nd harvest studied. Treatment refers to the light regime (in lux). ALAN\_traits.csv: Species and EcoUnit are given as above. Treatment refers to the light regime (in lux).. Hair density is hairiness (hairs mm-2), besides that toughness (N mm<sup>-1</sup>), contact angle (CA\_Mean, °), fresh weight (g), dry weight (g), leaf area (cm<sup>2</sup>), specific leaf area (SLA; cm<sup>2</sup> g<sup>-1</sup>), leaf dry matter content (mg g<sup>-1</sup>), SPAD, plant height (cm) and the chlorophyll parameters FvFm and PIabs are given as described in Methods.