2.1.1 Plant-plant interaction experiment
We conducted the plant-plant interaction experiment in a greenhouse
located at the Botanical Garden Leipzig, Germany, in May 2017. We
recorded an average temperature of 22.6°C and an average relative
humidity of 51.6% for the time of the experiment in the greenhouse. We
used 2 L microcosms (rose pot 2.0 L, Hermann Meyer KG, Rellingen,
Germany) filled with autoclaved (twice at 134°C for 20 min) 50:50
sand-peat (Floradur B Pot Clay Medium, Floragard, Oldenburg, Germany)
mixture. We flushed each filled microcosms with water twice to remove
pulsed nutrients and toxins prior to transplanting seedlings (Alphei &
Scheu, 1993; Trevors, 1996). To allow for similar soil conditions
between the plant-plant interaction experiment and the plant-soil
interaction experiment (see below), we chose to use a commercial
sand-peat mixture as it was not possible to retrieve enough soil from
the field site in Jena, Germany. We established the following plant
diversity levels and communities: (1) monocultures of each species, (2)
the three possible two-species mixtures, and (3) the three-species
mixture (Appendix Table A1 ). We transplanted twelve similarly
developed seedlings in each microcosm, and each plant community was
replicated ten times (total number of microcosms: 70). The relative
proportion among species was equal, i.e., six seedlings per
species in the two-species mixture and four seedlings per species in the
three-species mixture. In the two-species mixture, we transplanted the
species in an alternating pattern, while we randomized the position of
each seedling in the three-species mixture. All microcosms were randomly
placed on tables in the greenhouse and covered with net cages to prevent
unwanted herbivory. We watered all microcosms three times per week and
randomized the position on the tables every 7 days. We fertilized all
microcosms with 250 mL Hoagland solution after 5 weeks to counteract any
loss of nutrients and ensure optimal growth.
After 7 weeks of growth, we harvested five microcosms per plant
diversity level (see below). The next day, we infested two randomly
selected plants per species and microcosm of the remaining microcosms
with three 2nd instar Spodoptera exigua larvae
each. We covered and closed each plant just above the soil with an
organza net to ensure that the larvae could not escape. To ensure
similar development of the larvae (eggs purchased from Entocare
Biologische Gewasbescherming, Wageningen, the Netherlands), we
maintained a laboratory colony on artificial diet in a growth chamber
(25°C, 12 h light, 45% relative humidity). After 7 days of herbivory,
we harvested the remaining microcosms (see below).