Socioeconomic data, knowledge, and use of C. prunifera
The carnaúba leaf (C. prunifera) was the most important part of the plant and had the most indications of uses. Although the leaf (straw) has had several indications of uses, the main category was economic, that is, the pure extraction of straw leaves to remove the powder, and later commercialize it. This data corroborates the study by Vieira and Loiola [25] who indicated the economic importance of carnaúba straw by traditional artisans in the northern state of Piauí. The importance of the extraction powder, production wax and carnaúba leaves was also observed in the study by Sousa et al. [26], also indicating the fruit and stem as important parts of the species. The versatility in relation to the forms of use makes carnaúba a species of great economic and cultural importance in its occurrence region [27]. C. alba (Morong ex Morong and Britton), a co-genus of the carnaúba, also has similar characteristics regarding ethnobotanical aspects, such as the use of leaves for construction, crafts, and in animal feeding [28–31].
Furthermore, other tree species have also stood out economically for many local populations. For example, Sousa Júnior et al. [32] indicated the production of pequi oil (Caryocar coriaceum Wittm) from the fruit as a category with as much importance as food; its commercialization is so relevant because the oil can be stored during the period between harvests. Another species that has great economic relevance is the umbu (Spondias tuberosa Arruda), whose fruit commercialization guarantees income for many rural communities in the semiarid region [33]. In several regions of the world, species of the Moringa genus have different uses, from their leaves and fruits, and are economically important due to their applications in nutrition, industries, and medicine [34, 35].
In several ethnobotanical studies, the plant species studied are presented as having various categories of uses, according to the local people who use and manage them. In addition, depending on the species, the target part of extraction, use, and management may be the fruit (as the examples indicated above), the leaf, seeds, stem, tubers, etc. [7]. It is important to note that in many communities, people use different types of plants, some of which have a greater potential for treating diseases (medicinal use), others for subsistence or alternative food (food use), in the production of charcoal, construction of houses, or as wood for combustion (wood use). In a way, all of these categories mentioned above have implications in the socioeconomics of local populations.
However, it is important to note that some species have a greater economic appeal, either through their direct commercialization or from products extracted from it. Thus, the socioeconomic aspect may be one of the driving forces based on the economy [36] that directs the use and management of plant populations of human interest. On the other hand, the economic importance of a species can lead to overexploitation of the resource, harming its potential. Destructive practices in the fruit collection of Mauritia flexuosa Lf, a species of great cultural and economic importance, is an excellent example of this exploitation [37, 38]. Thus, showing the importance of studies that aim at understanding the socioeconomic factors related to the knowledge, uses, and management of plant species of economic importance, as well as their sustainable management.
Therefore, some studies sought to understand the influence of socioeconomic factors on the knowledge and uses of plants, and some made various types of associations, beginning with general plant use [39–41] to categories of specific uses, such as medicinal [42–44] and food [45]. In general, studies on this focus have varied according to the approach of each researcher. Income and education have been addressed in studies on useful plants in general, which suggest that knowledge decreases with the increase in schooling [46, 47] and increased income [41, 48, 49]. Our data, however, did not show a relationship between socioeconomic variables (length of residence, time in extractive activity, and age) with the extraction of carnaúba straw, with the exception of the income obtained from extraction and the number of leaves extracted, which had a significant relationship, indicating an effort to maximize straw collection, including through group organization, according to local extractors. This analysis reinforced the economic importance of carnaúba leaves as one of the factors that drive the use and management of this species locally. Sousa Júnior et al. [12] found a similar situation for C. coriaceum, which showed a great economic importance from the nutritional use of its fruit, the pequi. The same was observed for S. tuberosa through the study by Lins Neto et al. [50], which demonstrated the commercial importance of the umbu fruit as a factor that influences the management of this species. In the case of carnaúba, what is strongly different is the target of human selection (the leaf) compared to other studies in which the tendency has been for fruits as targets for selection. This can indicate the economic factor as the predominant force that directs the management of native plant species in socio-ecological systems.
In regards to age, according to the results, we did not observe an influence of this variable on the use and management of carnaúba straw. Similarly, Sousa et al. [41] demonstrated that the local importance of species from the region of Floresta do Araripe, in Ceará, was not influenced by socioeconomic factors, with the exception of a smaller number of species which had age as a factor that explained the importance of some culturally salient species. Moreover, according to these authors, younger people were motivated mainly by the commercial potential of some of the native forest resources, giving them greater local importance. Unlike other studies, this present study was carried out with only one species (carnaúba) that suffers from intense extractivism, with a focus on the use and management of its leaves. In addition, this study observed that only men perform extractive activities of carnaúba in the communities studied, which did not allow for gender analysis.
Local management and morphological perception of C. prunifera
Data on the perception of extractors are in line with the main results found for native plant species where local people exercise some type of management. The perception of morphological and organoleptic characteristics is an important resource that allows people to manage species of interest, favoring those characteristics that vary between plant populations [12, 50], and thus constituting the basic principle of the artificial selection process [7, 51, 52]. Thus, artificial selection is a criterion that can be based on the perception of characteristics preferred by people; consequently, reflecting on the management practices that drive the morphological and genetic divergences between natural and managed populations [53, 54]. The color and size of fruits, for example, are target characteristics of human selection, a trend observed in several studies [9, 12, 50, 54, 55].
Another morphological structure of plants that has been analyzed from this perspective is the leaf [56, 57] indicated that the color and size and arrangement of the Agave leaves are an important attribute of the species that help producers of “pulque” (a drink prepared with Agave sap in Mexico) in different stages of production. Although the tuberous root is predominantly the target of selection, the leaves of Manihot esculenta ssp. esculenta (Euphorbiaceae), can be the target [7, 58]. One of the main studies on leaf management of the Arecaecae family was the influence of leaf management on individuals of butiá (Butia capitata Martius) in southern Brazil [59]. According to this authors, traditional management seems to be the most interesting both from the point of view of the response of the palm, and also from the productivity of leaves for exploitation. Moreover, according to that study, the artisans of the butiá recognize three distinct types of leaves: new apex leaves, which must remain on the plant, while the rest may be extracted; new leaves for making artisanal products; and discarded leaves, as they are not suitable for handicrafts (either due to their rigidity, stains, or dryness) [59].
These data reinforce the importance of the perception that local extractors have about the resources they manage in different locations. Similar to the study by [59], in this present study on carnaúba (C. prunifera), the interviews also indicated that extractors perceive different types of leaves, which are locally called “eye of the carnaúba,” two larger (larger new leaves) and one smaller (new leaf that is left on the plant to assist in its regeneration). Our hypothesis that people would perceive morphological diversity in carnaúba, therefore, has been confirmed. Thus the local perception, especially of morphological characteristics of interest, may indicate the important role of intentional human action in the selection criteria, use (and preferences), and management of plant species of economic importance, especially by having a more conscious (intentional) selection [12, 60]. A logical consequence of this understanding is that the management of species of economic importance in socio-ecological systems (integration of socioeconomic and biophysical processes and components) [61], is the driving force behind targeting characteristics of human interest [12].
Some studies already carried out on the carnaúba itself indicate the great importance that this species presents, thus justifying the intense management exercised over it in a large part of northeastern Brazil. Ferreira et al. [21] evaluated the effect of different leaf cutting management strategies on a natural population of carnaúba and showed that leaf age has greater relevance on the exploitation, and that the best leaf cutting management strategy is a single annual cut in the month of December. Despite not focusing on the human aspect, through an ethnobiological perspective, this data on management found by Ferreira et al. [21], in which a single cut is the best strategy, can be an indication of possible strategies for the sustainable extraction of C. prunifera straw. In addition, our interviews indicated another mechanism that can also be evaluated as another management strategy: not removing any leaves from some individuals of C. prunifera during one year so that they have more leaf straws with powder the following year. This mechanism is related to the extraction product of the leaves, which is the wax powder. The differences in prices observed from the interviews indicate that the handling of the leaves also aims at a better quality powder. According to the interviews, the powder from the “eye” of the carnaúba straw (closed leaves and new ones existing in the canopy of the carnaúba) has higher quality and therefore higher market value than the powder from the rest of the straws (open leaves that form the carnaúba canopy). According to Sousa et al. [26], the price of powder extracted from the "eye" of carnaúba was estimated at R$ 7.00 (thereabout US$ 1.68 based on the month of November 2019), while the powder removed from ripe straws cost around R$ 5.00 (thereabout US$ 1.20 based on the month of November 2019). Similar data were observed in our study.
Another interesting fact about the carnaúba was the positive correlation between the amount of straw collected and the income obtained, which proved to be strong and positive. That is, the more you collect, the more you earn in that activity. The correlation analysis corroborates the interviewees' own view, recognizing that, despite the morphological preferences for larger leaves and powder extracted from new leaves (the carnaúba eye), the important thing is to have a large number of leaves collected over the period harvest. A possible consequence of this is the variation in prices of leaves and powder caused by the increased supply of those products, resulting from intensive collection activities. Thus, extractivists may have their income impaired due to the fall in product prices at the beginning of the production chain [62]. Another possible negative impact is the ecological effects on the resource [63]. One of the implications of this is the importance of seeking strategies for the sustainable use of biodiversity resources of economic importance, such as the carnaúba leaf, in order to match the conservation of natural resources with income for human populations that use such resources [64, 65].
Therefore, it is important to emphasize that correlation analysis further reinforces the economic importance of carnaúba leaves as one of the factors that guides the use and management of this species. Sousa Júnior et al. [12] found a similar situation for C. coriaceum, which had a great economic importance from the nutritional use of its fruit, the pequi. For carnaúba, what is strongly different is the target of human selection, which in this case is the leaf, compared to other studies in which the tendency has been for fruits as targets of selection. This can indicate the economic factor as the predominant force that directs the management of native plant species in socio-ecological systems. Nevertheless, it is necessary to develop studies that seek to ascertain this hypothesis, as well as the production chain associated with other socio-cultural factors that may be related to the use and management of carnaúba.