## 2 Experimental Section  
To assess spatial variability, fourteen _B. robertiana_ plants were collected from different populations in the Basilicata region (Southern Italy) from the coast to the inner mountain area (Figure 1) within altitude ranges from 10 m a.s.l. to 727 m a.s.l. Plants were collected in 2017 and cultivated into pots at the campus of the University of Basilicata (Potenza, Italy). In the following year (2018), infloresances with fully opened flowers were incapsulated in a of 6.5 glass bell (Figure 2). Sampling was performed under light conditions, in an air-conditioned room (21 \(\pm\) 1 \({}^{\circ}\)C) to guarantee a stable temperature. VOC sampling followed the protocol we used in a recent study on the floral volatiles of the genus _Gymnospermium_[26]. Analysis of VOCs was performed using HS-SPME with a DVB/CARB/PDMS fiber. A preliminary set of analysis was conducted to optimize the sampling time: these analyses were performed at three different adsorption times of the fiber (5-24-72 h). The highest number of identified compounds (20) was detected when the SPME fiber was exposed for 24 h (Appendix A, Table A1); therefore, we used this interval of fiber exposition in this study. The fiber was exposed to the headspace and then withdrawn into the needle and transferred to a GC/MS system. A 50/30 \(\upmu\)m DVB/CAR/PDMS module (57328-U, Supelco, Milan, Italy) was employed to determine VOCs. Analyses were accomplished with an HP 6890 Plus gas chromatograph (Agilent) equipped with a Phenomenex Zebron ZB-5 MS capillary column (30 m \(\times\) 0.25 mm i.d. (inner diameter) \(\times\) 0.25 \(\upmu\)m FT) (Agilent, Milan, Italy). An HP 5973 mass selective detector (Agilent) was utilized with helium at 0.8 mL/min as the carrier gas. A splitless injector was maintained at 250 \({}^{\circ}\)C and the detector at 230 \({}^{\circ}\)C. The oven was maintained at 40 \({}^{\circ}\)C for 2 min, then gradually warmed, 8 \({}^{\circ}\)C/min, up to 250 \({}^{\circ}\)C and held for 10 min (Figure 3). Tentatively identification of aroma components was based on mass spectra and NIST 11 library comparison. A single VOC peak was considered as identified when its experimental spectrum matched with a score over 90% with ones present in the library and if the retention time was in agreement with the reported retention index. Retention indices were calculated using standard _n_-alkane solution (49452-U, C7-C40 saturated alkanes standard, Sigma-Aldrich, Milan, Italy). Figure 1: Study area and sampling point locations in the Basilicata region (southern Italy). Labels of the samples correspond with the municipality names where plants were collected. A multivariate analysis of VOC compositions was performed through non-metric multidimensional scaling (NMDS) as an ordination technique and hierarchical cluster analysis (HCA). Bray-Curtis dissimilarity was used for both NMDS and HCA using the relative abundance of each compounds as sample variable. For HCA, we used the unweighted pair group method with arithmetic mean (UPGMA) technique as an agglomerative method. Variables with [PERSON]'s rho correlation coefficient > 0.65 with ordination axes were superimposed in the scatter diagram. Concerning spatial analyses, we tested whether geographic distance and environmental variables influence the VOC compositions of samples. Site-specific soil data were not available; therefore, we approximated differences in the environmental niche using slope, aspect, altitude, and bioclimatic data. For each sample point, the most relevant phytoclimatic indices [27, 28] were extracted from the high-resolution raster dataset developed to realize the bioclimate map of Italy [29].