Additional File 1 illustrates the plant part used, consumption procedure, food use-category and number of informants mentioning each use for the 97 wild edible species reported in the six areas.
All species gathered were authocthonous except Mespilus germanica and Prunus cerasus, which are now feral. Many species, such as Corylus avellana, Borago officinalis, Laurus nobilis, Castanea sativa, Rubus idaeus, Taxus baccata, Ulmus minor, Mespilus germanica, Prunus avium, Prunus insititia, Ribes uva-crispa and Origanum vulgare, can either be collected in the wild or cultivated in gardens.
Many of the reported uses exist only in the collective memory of the elderly. Most wild fruits, bulbs or flowers mentioned were consumed by children or shepherds as snacks or for amusement on the way to school, or when tending livestock. Some people still pick them on walks to relive the flavours of their childhood.
Food is a very conservative aspect of culture but the erosion on the use and knowledge about wild food plants is higher than that of allotment food plants. The decline in wild food gathering appears to be due to negative connotations, i.e., association with times of scarcity, especially during and after the Civil War (1936–1939). Interestingly, a saying in Piloña – "esi comió berros" (S/he ate Rorippa nasturtium-aquaticum) – refers to the starving. Piloña is the only one of the six regions where the above species, one of the most important wild vegetables in Spain and elsewhere , is not consumed.
By contrast, wild berries and herbs are still used to make homemade jams (e.g. Sambucus nigra, Rubus ulmifolius and Vaccinium myrtillus), desserts and spirits (e.g. Prunus spinosa, Sideritis hyssopifolia) for sale as quality local produce.
Species' Cultural Importance
Figure 2 lists, in order of importance, the twenty most culturally important species in the Peninsular northwest according to the MCI, and their CI value in each survey area. The chestnut is the first, second and fifth most important species in Piloña and Sanabria, Caurel and Montesinho, respectively, but is far less significant in Picos, and especially Campoo, where it does not grow spontaneously, being collected the chestnuts from neighbouring areas.
The ten most significant species include fruits (Castanea sativa, Rubus ulmifolius, Fragaria vesca, Prunus avium), seasonings (Origanum vulgare, Laurus nobilis, Foeniculum vulgare), herbal teas (Chamaemelum nobile), liqueurs (Prunus spinosa) and vegetables (Rumex acetosa). For the six sites as a whole, the species used as fruits are very important, with 4 species in the top 10. Two species used for condiments and one for herbal tea also rank highly. Vegetables are clearly much less important, Rumex acetosa ranking eighth and Rorippa nasturtium-aquaticum fifteenth.
The least culturally important were those used for their subterranean organs and flowers, the first in each category being Conopodium sp. pl. and Cytinus hypocistis (40th and 44th in the ranking, respectively).
Table 2 shows the number and percentages of species and of UR among each food-category at each survey site, which indicate that fruits are clearly the most important category in all areas except Montesinho, where it is plants used for seasoning followed by vegetables.
Differences in CI values for species among the different areas
Figure 2 also indicates appreciable differences among the CI values obtained in the different surveys. Among the ten species with the highest mCI, only Rumex acetosa was not cited in all six ethnobotanical surveys. Moreover, most are important in every region. A common cultural background may explain these similarities.
The next ten species include some used only at two or three study sites, e.g. Calamintha nepeta, Tilia spp., Sideritis hyssopifolia and Thymus mastichina.
Some species grow in most areas, but only have a high CI value in one of them. For example, the tenth species in the ranking of mCI (see Figure 2), Foeniculum vulgare, is the most important species in Montesinho although has a much lower CI in the other areas. Pterospartum tridentatum, the tenth species in the CI ranking in Montesinho, is also used only in Piloña. Finally, the vine Bryonia dioica is consumed only in Montesinho (Portugal), where it occupies seventh position in the CI ranking despite being quite common at all the survey sites. Also in Montesinho, a larger number of plants used as vegetables and for seasoning occur among those with a high CI.
As mentioned in Methods, mean value was calculated considering only the areas where the species grows since a null value may be due to species not growing there or growing but not being consumed. This mean value therefore takes into consideration species selection or rejection and availability; hence, it is lower for species that grow in the area but are rejected or not considered edible. For instance, as the mCI for Vaccinium myrtillus, whose use was mentioned in all five areas where it grows, is obtained by dividing by five, its mCI does not diminish as a result of not growing in one of the areas. Subsequently, the mCIs for Rorippa nasturtium-aquaticum or Crataegus monogyna, which grow in all six areas but are only consumed in five, were obtained by dividing by 6. The fact that they are not consumed despite being available reduces their mCIs.
Interestingly, Figure 2 also indicates that CI values for edible species in Montesinho are generally higher than for species in the other areas. We can hypothesize that local knowledge of wild edible plants and plant gathering are more widespread in that remote Portuguese region. To analyse this supposition in detail, we can calculate the mean of the CI values for all the species in each area (mCIa) as a measure of botanical knowledge. The mCIa value for Montesinho (0.14) was more than double that of the following values: 0.06 for Campoo, 0.05 for Picos, 0.04 for Caurel, 0.03 for Sanabria and Piloña. This is an exceptional example of great differences in knowledge of wild edible plants among different human groups. Although Sanabria and Montesinho are neighbouring territories sharing a similar environment, the difference is significant, and might be partly explained by a greater loss of knowledge in the former. Moreover, Montesinho has been dependent on natural resources for decades as until the 1990s it was isolated due to the very poor national road network. However, some activities, such as smuggling, periodic migratory farm labouring and selling agricultural produce, have maintained and promoted knowledge of plants.
Comparison with other Spanish regions
Nearly all the species with a high CI value are also widely consumed throughout Spain and the Mediterranean area. One exception is Rumex acetosa, which although the most cited vegetable in Piloña and Campoo, is not as commonly gathered in the rest of Spain . On the other hand, some edible species commonly consumed throughout the Iberian Peninsula, such as Silene vulgaris or Taraxacum officinale, are seldom collected in some of the study areas despite occurring in all of them.
Some food species eaten in the study areas have scarcely been documented as food plants in the ethnobotanical literature, especially plants whose roots or flowers (e.g., Crocus nudiflorus, Pedicularis schizocalyx, Fritillaria pyrenaica and Lamium purpureum) were used as sweets and Halimium lasianthum, whose flowering buds or immature fruits were chewed as a snack. Despite the importance of edible flowers [e.g. ], they are often overlooked by researchers, being rejected as merely children's food.
Unusual species occur among plants used for seasoning. In Montesinho, Physospermum cornubiense is used for liqueurs and to flavour sweet foods, and leaves and flowers of Salvia sclarea for seasoning soups. Also, the flowers and young buds of Pterospartum tridentatum are still used to make a local liquid rice dish known as "arroz de carqueja".
Cultural importance of the families
Regarding the diversity of species gathered, Rosaceae was the most important family, with 17 species. Consumption mainly involves eating ripe berries or making liqueurs (see Additional File 1). Other important families are Lamiaceae, with 13 species, used as condiments and digestive infusions and Asteraceae, with six species being consumed as green vegetables or in infusions. Five species of Polygonaceae were mainly consumed as vegetables and five species of Apiaceae occurred in many use categories. If we compare these figures with those for Spain as a whole , the most diverse families of gathered food plants are Asteraceae (92 species), followed by Lamiaceae (53), Rosaceae (34), Apiaceae (25) and Fabaceae (22). These differences are explained by the great importance of wild fruits in the Northwest, most being members of the Rosaceae. In fact, this family is the most diverse in Spain in terms solely of wild fruits. Although almost one-third of the vegetables in Spain belong to the Asteraceae, this category and this family are not so important in the area. The daisy family is also the most diverse family in certain regions of Italy [22,25,52] and the second most diverse in two Turkish areas [13,53].
As we explained in Material and Methods, adding the CI of the species of each family is a good way to measure the cultural importance of the families (CIf). Table 3 shows the most important families in descending order of mCIf. Although a family's cultural importance correlates highly (r = 0.95) with the number of species in each family (see Figure 3) a regression analysis is needed to confirm statistically which families have higher values than expected for the number of species .
Figure 3 indicates that the plant families with more than 5 species and greater cultural importance as wild food in the northwest of Iberian Peninsula are Rosaceae, Lamiaceae, Fagaceae, Asteraceae, Apiaceae and Polygonaceae. However, only the Rosaceae attains a significantly higher figure for cultural importance (P 2). On the contrary, Lamiaceae, Asteraceae, Apiaceae and Polygonaceae present mCIf values significantly lower than expected.
Finally, a regional particularity that is worthy to comment it is that Lamiaceae is the most important family in Montesinho, according to its CIf (2.35, see Table 3). This is due to the higher relevance of condiments in this area, both in relative number of species and UR, as shown in Table 2.
Species selection and availability
The relation between species availability and edible use provides interesting insights into food selection criteria in the six areas. As stated in Methods, we established an index called SI to analyse the relationship between species availability and edible use. Table 4 shows the regional selection index for each food-category and the total value for each region. Significant differences appear in the total values of the RSI. In Caurel and Sanabria under 40% of available species were consumed, whereas in Campoo and Montesinho the figure was over 70%.
There are many possible explanations for such differences. Caurel, for example, is a very small isolated region. Although remote areas are commonly thought to yield a greater traditional ecological knowledge, isolation is also associated with lack of information sharing with other regions. Similar conclusions were reached by Milliken and Albert  who hypothesised that a high degree of human dispersion as a result of semi-nomadic migration could be responsible for vast knowledge of medicinal plants. Piloña also shows a high percentage of rejection (RSI = 0.43). Its mild climate due to proximity to the sea means that cultivating vegetables and fruit in allotments is more productive, and, consequently, fewer wild plants are needed .
On the contrary, the high RSI for Montesinho, Campoo and Picos indicate that a remarkable knowledge of wild edible plants is still employed or at least harboured there. The explanation may lie in cultural reasons such as appreciation of seasoning, vegetables or herbal teas. In the case of Picos, poor communications with other areas, but strong links among local communities, have forged a marked identity, which is also evident in the shared plant uses. The lack of economic development and the taste for certain flavours may explain local appreciation of wild edible plants. Finally, the richness of Campoo could be due to its being a transition area. Its rich flora includes Mediterranean and Atlantic taxa, the easiest route from the plateau of Castile to Santander being across that region. It has therefore received influences from other Cantabrian and Castilian peoples.
The RSI for each food-category further helps to understand the observed differences. If the mean of the RSI for each of the categories is obtained, it is clear that species used for their flowers are much less likely to be selected as edible than fruits or liqueurs (Table 4). That point can be interpreted as cultural pressure or interest to consume wild fruits or liqueurs and as a lack of interest in edible flowers. As mentioned above, the latter plant use is often overlooked by researchers, who do not regard it as proper food, although relevant species may have played a role in human nutrition, especially children's diets.
Wild fruits are widely appreciated in the survey sites, with only Caurel having a RSI below 0.4. The percentage of UR over 40% in Caurel and Campoo and especially in Picos (55%), shown in Table 2, corroborate this fact. Low productivity of cultivated fruit trees, lack of money, bad communications, and limited fruit supply in the markets, especially in winter, meant people could not buy commercial fruits. They depended on countryside fruits such as Mespilus germanica, Sorbus aria, Malus sylvestris or Prunus spinosa. Although not very productive, these plants are well adapted to local weather conditions. Significantly, only a few species of wild fruits are rejected in four or five areas; they include Amelanchier ovalis, rare in the study areas, and the bitter and unpleasant acorns of Quercus robur and Quercus petraea. Although once an essential part of the diet [55-57], the latter two are now regarded as animal food.
Unsurprisingly, a number of wild vegetables appear among the unselected species since in most of the six survey areas diets are rich in beans, cabbage and potato, but quite poor in vegetables, with only a few condiments being used [38,40,41].
Montesinho, however, presents remarkable features, with wild condiments (32% of UR) and vegetables (24% of UR) playing a very important role (see Table 2). Condiments such as Foeniculum vulgare, Pterospartum tridentatum, Calamintha nepeta, Lavandula stoechas or Thymus mastichina were used for apparent variation, mainly in soups and purees. Wild vegetables were used to garnish meat or fish dishes. Sometimes, during periods of scarcity, edible greens were eaten with potatoes as a substitute for meat and fish. Campoo also presents a higher percentage of vegetable selection, probably due to its closer relationship with Central Spain, where more vegetables are consumed .