Chemical analyses
We performed gas chromatography to quantify the sampled VOCs. We sampled the volatile compounds of the two plant species by passive trapping with polydimethylsiloxane (PDMS) tube cuttings following a modified protocol of that used by (Kallenbach et al. 2015) and (Klimm et al.2020). Using the PDMS tubes, we sampled volatile compounds from ca. half of the saplings. First, we attached the tubes by copper wire to the experimental sapling. We then tightly enclosed as many leaves as possible and the PDMS tube in polyamide oven bags. We sampled VOCs from the headspace for 24 hours. We removed the tubes, placed them to sterile glassy vials and brought them to lab for further analyses. The PDMS cuttings were analysed by a thermal desorption-gas chromatograph-mass spectrometer (TD-GC-MS) consisting of a thermodesorption unit (MARKES, Unity 2, Llantrisant, United Kingdom) equipped with an autosampler (MARKES, Ultra 50/50). PDMS cuttings were transferred to empty stainless steel tubes (MARKES) and desorbed with helium as carrier gas and a flow path temperature of 160°C using the following conditions: dry purge 5 min at 20 ml/min, pre purge 1 min at 10 ml/min, desorption 8 min at 200°C with 60 ml/min, trap temperature 0°C, pre trap fire purge 1 min at 60 ml/min, split flow 20ml/min, trap heated to 230°C and hold for 4 min. The VOCs were separated on a gas chromatograph (Bruker, GC-456, Bremen, Germany) connected to a triple-quad mass spectrometer (Bruker, SCION) equipped with DB-WAX column: (30m x 0.25mm inner diameter x 0.25um film thickness, Restek). The temperature program was the following: 60°C (hold 1 min), 30°C/min to 150 °C, 10°C/min to 200°C and 30°C/min to 230°C (hold 1 min). Helium was used as carrier gas at a constant flow rate of 1 ml/mi. MS conditions were set at a 40°C manifold, 240°C transfer line and 220°C for the ion source. The scan-range was 33 –500 m/z for a full scan and scan-time was 250 ms. We selected the most prominent peaks in the chromatograms (signal to noise ratio > 10). Peaks that were also present in air blanks were regarded as systemic contamination and were excluded from further analysis. VOCs were tentatively identified by comparison to the NIST database and comparison to retention indices from the literature. The peak areas of these compounds were calculated using the Bruker Workstation software (v8.0.1).