Archive, bibliographic, and archeological sources
Archival research under the heading Dyeing Plants involved primary sources held in the State Archives of Naples at the Bourbon Royal House and the floristic lists of the Botanical Gardens of Naples and Palermo [12,13,14]. Scientific production that reviewed regional contexts related to rural culture [8, 15,16,17,18,19,20,21,22,23,24] has been also analyzed. In addition, the archeological remains found in archeological sites of Southern Italy, which document fabrics, objects, and paintings colored with plant-derived pigments, have been considered [5, 10, 25, 26].
Flora of dye plants
A flora of dye plants (Supplementary file 1) has been created starting from the analysis of the flora produced by Briganti  and other works of the seventeenth to nineteenth century [13, 14]. Species reported have been updated in the nomenclature using the Catalogue of Life: 2019 Annual Checklist . Information reported on this flora are as follows: species name , family name , habitat and Italian locality where the species can be found or where they have been reported, altitudinal range and chorology , chromosome numbers (if known) [28, 29], pigment color, and pigment sources . Data relating to the pigments, colors, and parts of the plants used have been studied and reformulated to adapt to the current technical-scientific language. Species are listed in alphabetical order to allow an easier search of the items on the list. The frequency of the species on the Italian territories is reported with RR (very rare), R (rare), C (common), and CC (very common) .
Plants, algae, lichens, and fungi catalogs
A floristic catalog of dyeing plants (Supplementary Table 1) has been produced, reporting some information present in the Flora and adding, where found, further recent bibliographic references relating to the species included in the table and their dyeing use [1, 2, 6, 7, 9, 11,12,13,14,15,16,17,18, 21,22,23,24, 30,31,32,33,34,35,36,37,38,39]. Records on the table are ordered according to the criteria of the Angiosperm Phylogeny Group (APG) IV to add an information level, linked to the phylogeny of the species concerned . In the literature, citations concerning mushrooms, lichens, and algae used for dyeing purposes have been founded [12, 22, 24]. This information has been reported in two catalogs, one concerning fungi and lichens (Supplementary Table 2) and the other concerning algae uses as dye sources (Supplementary Table 3). The tables show the names of the species updated according to the current nomenclature , the families to which the species belong , the resulting colors, and the part of the organism used to derive the pigments [12, 22, 24]. Data relating to the pigments, colors, and parts of the organisms used have been studied and reformulated to adapt to the current technical-scientific language. The entries in the two tables are listed according to the alphabetical order of the families to which the species belong.
Floristic investigation of the cited species
During a 2-year period (from 2016 to2018), a floristic survey was carried out on the territory of Southern Italy and of the Major Islands (Sicily and Sardinia) to confirm the presence of dyeing species cited in the literature [1, 2, 6, 7, 9, 11,12,13,14,15,16,17,18, 21,22,23,24, 30,31,32,33,34,35,36,37,38,39]. The geographical information concerning the positions of the species reported in our catalog was found in the flora of Italy  and in the Prodrome of Italian Vegetation produced and updated by the Italian Botanical Society . Species recorded for the regions of Southern Italy (Molise, Campania, Calabria, Puglia, and Basilicata) and for the Major Islands (Sicily and Sardinia) were taken into consideration. All the species actually found in the localities mentioned in the literature [28, 41] have been subsequently recorded. Herbarium vouchers have been taken from Herbarium of Palermo (PAL), Virtual Herbarium of Lake Van Basin (VHLVB), Linnean herbarium—Department of Phanerogamic Botany Swedish Museum of Natural History (S-LINN), Herbarium Luigi Paolucci (H.PAOL), New York Botanical Garden (NYBG), Herbarium Horti Botanici Pisani (PI), and Naturalis Biodiversity Center (NL). All the vouchers have been reported in the flora of the dyeing plants (Supplementary Materials 1).
A guide text has been produced for the realization of semi-structured interviews to be carried out on a sample of individuals. We used the snowball sampling technique to recruit a group of participants in each Southern Italian Region (Molise, Campania, Puglia, Calabria, Basilicata, Sicily, and Sardinia) . The sample is composed of men (37%) and women (73%) (i) older than 70 years of age and (ii) resident from birth in the same villages of Southern Italy. In each region, a different number of interviews were carried out due to the availability of the interviewees and the different size of the population. In Molise, 57 interviews were carried out, in Campania 149 interviews, in Puglia 138 interviews, in Calabria 142 interviews, in Basilicata 58 interviews, in Sicily 134 interviews, and in Sardinia 153 interviews. The total number of interviews collected is 831. The semi-structured interview was preferable to any other form of investigation due to the level of education of the respondents (for 83% only elementary or lower) and for the possibility offered by the method of establishing a more empathic relationship conducive to dialog. The interview was conducted in Italian and anonymously. Respondents were asked if they remembered the following: (i) uses of plants to dye, (ii) which plants were used, and (iii) other people who were interested in plant dyes. Prior informed consent was verbally obtained before starting each interview, and ethical guidelines were rigorously followed . The semi-structured interviews were listened to, and the non-interesting interviews for this work were discarded (when the interviewees claimed not to remember any plant used for dyes). Initial interviews were 831, and around 18% were eliminated, resulting in a total of 680 valid interviews. The resulting information, useful for the purposes of this work, was recorded for analysis.
Once the dye plant catalog has been built, the percentages of the different parts of the plant used to extract the pigments, in relation to families, have been determined. Hence, the various anatomical parts of plant have been grouped into broad categories: leaves, roots, stem, flowers, fruits, and other. In addition, the percentages of the colors obtained from the different families have also been estimated. Graphical representations were produced for both data analyses. The same percentages were also analyzed for algae, fungi, and lichens. The data from the floristic survey of the species cited in the literature have been compared with the specific richness reported in the flora of dyeing plants. In this way, it was possible to verify the percentage of the flora actually represented and present in the Mediterranean territories of Southern Italy, after about two centuries. Data from semi-structured interviews have been recorded and compared with the flora of dyeing plants; informant consensus has been obtained with a simple frequency count of the single species cited in the total of citation emerged in the interviews . In this way, it was possible to obtain an estimate of the percentage of TBK conserved over two centuries in the investigated territories, and on the other hand, the percentage of that knowledge that was lost. From the analysis of the answers of the interviewed, the relations between the species and the regional contexts were investigated.