All research mentioned in this paper was conducted in six rural and mountainous areas of the northwest of the Iberian Peninsula. In Spain: Campoo in the south of the Cantabria Autonomous Region 38; Picos de Europa, a geographical region that straddles the autonomous administrative regions of Asturias, Cantabria and León province in the Autonomous Region of Castilla y León [39, authors personal observations]; Piloña in central-eastern Asturias 40; Caurel, in the south-east of Lugo province (Autonomous Region of Galicia) 41; and Sanabria in the north-west of Zamora province (Autonomous Region of Castilla y León) 42. In north-eastern Portugal: Montesinho 43 adjoining Zamora province (Figure 1).

All six survey sites are culturally and biologically rich and most lie in protected areas, e.g. Picos de Europa National Park, Sanabria Lake Natural Park and Montesinho Natural Park. Bordering both Mediterranean and Eurosiberian floristic regions, the six sites have climates that vary from oceanic (wet and relatively mild) in Picos de Europa, Piloña, Caurel and the north of Campoo to wet-Mediterranean (drier in summer) in Sanabria, Montesinho and most of Campoo.

Landscapes include a mosaic of meadows, forests, rivers and high mountain vegetation growing on varied geological materials and soils. The predominant vegetation consists of beech (Fagus sylvatica L.) forest, several oak species, e.g. Quercus robur L., Quercus petraea (Matt.) Liebl., Quercus pyrenaica Willd.,Quercus faginea Lam. and Quercus ilex L., chestnut (Castanea sativa Mill.), broom scrubland consisting of Cytisus scoparius (L.) Link, Cytisus multiflorus (L'Hér.) Sweet, Genista florida L., and heath comprising Erica cinerea L., Erica vagans L., Erica australis L., Erica umbellata Loefl. ex L., Calluna vulgaris (L.) Hull. Fagus sylvatica, Quercus robur and Quercus petraea are more common in the wetter northern areas, while Quercus pyrenaica, Quercus faginea and Quercus ilex grow in drier areas.

Until a few decades ago the survey sites' economies were based on agriculture, cattle rearing and several less important activities. Most of the population engaged in traditional stock farming involving few animals. Short-distance vertical transhumance and long-distance southwards transhumance of cattle and sheep were particularly important in the Cantabrian Mountains. In regions such as Campoo, low salaries meant that even people working in the steel, cement and glass industries combined wage labour with livestock farming. The largely subsistence-based household economy was boosted with income from the sale of animals, eggs, butter and handicrafts. Other important economic activities were smuggling and forestry in Montesinho, and door-to-door hawking in Sanabria, chiefly using mules.

Many fields once used to grow cereals (for bread), pulses, turnip and potatoes now provide grazing for cattle. Agriculture plays only a minor role and new economic activities, such as rural tourism, are increasingly important.

Ethnobotanical information was obtained through informed consent semi-structured interviews with key informants over the last twenty years (1989–2004) (Table 1). Informants with a sound traditional knowledge of useful wild plants, mostly elderly long-time residents, were interviewed. Open questions about wild food consumption sought to ascertain knowledge about past and present-use, mode of consumption and preparation, collection time and collection sites for each species 44.

For this study, data were grouped into the following categories of edible plants based on folk perceptions: "vegetables", plants whose leaves, stems or even unripe fruits or seeds were consumed; "wild fruits", plants whose fruits or seeds were consumed when ripe; home-made "liqueurs" or other alcoholic drinks; "herbal teas", used in general as a digestif; plants used for "seasoning"; and finally, "flowers" and "underground organs", eaten for their sweetness.

Every plant species mentioned by an informant within one use-category was counted as one use-report (UR) [see 45]. For instance, the raw fruits of Prunus spinosa in Picos de Europa were reported as consumed by 17 informants and used in liqueurs by 21, totalling 38 UR. However, a total number of 27 informants cited the species as useful since some informants reported use both for liqueurs and for raw consumption of fruits. We have rejected species with only one UR because such data are less reliable and sometimes dubious for drawing comparisons.

Voucher specimens were deposited at the herbaria of the Royal Botanical Garden of Madrid (MA, Real Jardín Botánico), the University of Oviedo (FCO, Universidad de Oviedo) and the School of Agricultural Engineering at Bragança (BRESA, Escola Superior Agrária).

To estimate the cultural significance of each species, we used the Cultural Importance Index (CI), whose definition and use are discussed in another paper [see 46], with the following formula:

C I = ∑ i = 1 i = N U U R i N MathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacH8akY=wiFfYdH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8kuc9pgc9s8qqaq=dirpe0xb9q8qiLsFr0=vr0=vr0dc8meaabaqaciaacaGaaeqabaqabeGadaaakeaacqWGdbWqcqWGjbqscqGH9aqpdaaeWbqaamaalaaabaGaemyvauLaemOuai1aaSbaaSqaaiabdMgaPbqabaaakeaacqWGobGtaaaaleaacqWGPbqAcqGH9aqpcqaIXaqmaeaacqWGPbqAcqGH9aqpcqWGobGtcqWGvbqva0GaeyyeIuoaaaa@3F4E@

The index, which is based on previous indices 4748 was obtained by adding the UR in every use-category (i, varying from only one use to the total number of uses, NU) mentioned for a species, divided by the number of informants in the survey (N).

The CI was calculated for each region. For example, Foeniculum vulgare in Montesinho was reported as used in liqueurs by 10 informants, for seasoning by 32 and for herbal teas by 23. The total number of survey participants was 90.

CI_{Foeniculumvulgare} = 10/90+32/90+23/90 = 0.11+0.36+0.26 = 0.722

This additive index takes into account the spread of use (number of informants) for each species and versatility, i.e. diversity of edible uses. The theoretical maximum value of the index is the total number of different edible use categories.

A mean Cultural Importance Index (mCI) of the species was used to assess wild food plant use in the Peninsular northwest as a whole. It is also useful in evaluating CI differences among the various sites. Since a null value may be due to either the species not growing in the area or growing but not being consumed, the mean value preferably needs to be calculated by considering only regions where the species grows and is available. For example, if the null values of the areas where it does not grow (Sanabria and Montesinho) are rejected, the mean value for Fagus sylvatica is 0.055; however, the figure decreases to 0.037 if all six areas are considered. Thus, the mean value takes into account species selection or rejection and availability.

To measure the cultural importance of families (CIf), we added the CI of the species from each family, following Galeano 49. We preferred using the sum instead of the mean as proposed by Phillips and Gentry 48 so as to highlight more diverse families which would otherwise be underestimated.

When comparing the edible floras of different regions, it is crucial to differentiate between plants growing in the area but not consumed and those which cannot be consumed because they are absent. To quantify this factor, a regional selection index (RSI) was created to assess differences in edible species selection or rejection among regions. It was obtained by dividing the number of species consumed at a site by the number of species growing there. For instance, the RSI for Sanabria is 0.37 (29/78), since 29 out of 78 available species are used. A regional index for each edible category can be further calculated to assess regional differences in selection among categories. For instance, the RSI for vegetables in Sanabria is 0.19 (5 out of 27 available) versus 0.4 (10/25) for fruits.

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.

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.

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 37. 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. 51], 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".

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 37, 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 222552 and the second most diverse in two Turkish areas 1353.

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 48.

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 < 0.05) than that expected for the number of species. This fact remarks once more the high significance of wild fruits category in most of the survey areas, with a large proportion of UR (see Table 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.

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%.