Fig. 1

Principal component analysis (PCA) plot of Calluna vulgaris samples. Two replicates are shown for each corresponding country and season. The length of the arrow represents the size of the loading, which in turn indicates the explanatory variable’s (i.e. polyphenol subgroup) contribution to the determination of the principal components (PCs). Each polyphenol subgroup is represented by the first four letters of its name. Gall, Galloyl derivatives; HHDP, hexahydroxydiphenoyl derivatives, Kaem, Kaempferol derivatives; Myri, myricetin derivatives; Proc, procyanidin subunits; Prod, prodelphinidin subunits; Quer, quercetin derivatives; Quin, quinic acid derivatives