Consequences of traditional management in the production and quality of copal resin (Bursera bipinnata (Moc. & Sessé ex DC.) Engl.) in Mexico

Background Copal is a resin of ritual uses in Mexico, extracted from several species of trees of the genus Bursera. The effect of traditional management on phenotypical traits of copal trees has not been suciently studied. This research analyzed the traditional management and human selection on populations of Bursera bipinnata, and their inuence on the quantity and quality of the resin produced by wild and managed trees. Method Management of copal were documented through semi-structured interviews and workshops. Samples of 60 trees from six wild and managed populations were selected to quantify the production of resin during two consecutive years. Fresh resin was collected to identify organic volatile compounds through gas chromatography and Principal Components Analysis (PCA); individuals were classied according to the amount and type of organic compounds produced. Results We identied management strategies from simple harvesting to seeds planting. The criteria for selecting managed trees and seeds are based on the quantity and quality of the resin produced and on higher quantity of resin yield per tree, which were much higher in managed than in wild trees: 190.17 ± 329.04 g vs 29.55 ± 25.50 g, and 175.88 ± 179.29 g vs 63.05 ± 53.25 g for the production seasons of 2017 and 2018, respectively. Twenty organic volatile compounds were identied and the PCA showed that managed trees produce higher percentages of compounds associated with scent.

In the community studied live 298 people, whose main productive activities are agriculture of maize, squashes, and beans, and some small-scale commercial crops like sorghum, watermelon, melon, and jicama are also produced [32]. Copal extraction is widely performed in the region for more than 100 years [26] and is currently widely practiced as a key seasonal livelihood.

Documenting management strategies
Copal trees are tapped ("picado de copal") by making parallel gashes in the branches (occasionally also in the trunk with a machete). This activity is strongly associated with gender since only men do it, fathers being responsible for transmitting to their sons knowledge about extraction and care techniques of copal trees. We documented copal management through 30 semi-structured interviews and participatory workshops [33,34] with copaleros (people dedicated to extract copal). The interviews allowed us to explore what copal varieties are identi ed by copaleros, as well as their selection criteria, knowledge, management practices, organization and associated cultural aspects. The copaleros interviewed were on average 47 years old ± 16, with, on average, 22 ± 19 years of experience extracting copal. Besides working on copal extraction, they also work on agriculture and cattle herding activities. At the same time, a workshop was performed with the participation of 30 copal managers, including some personally interviewed. The workshop focused on documenting the classi cation and selection criteria, the management practices, and the distribution of copal trees in the communal territory.
Quanti cation of resin yield in wild and managed trees Six sampling units (SU) 50 × 20 m 2 were selected, three in wild populations and three in managed populations. Ten trees were sampled in each SU, 60 trees in total (30 trees from wild populations and 30 from managed SUs). Wild populations SUs were de ned as those found in natural vegetation that has not been opened to agriculture, and whose trees have not been used for extracting copal or tapped. Managed populations were those sites where copal trees have been selected for their attributes and dispersed intentionally on the margins and inside plots of crop elds (Fig. 2).
For each sampling unit we randomly selected B. bipinnata trees with diameter at breast height (DBH) between 10 and 20 cm (these are the trees typically tapped), and their height (m), canopy cover (m 2 ) and DBH (cm) recorded. To evaluate possible signi cant differences between these variables in relation to their wild and managed condition, a t-student test was performed using R [35]. Following traditional tapping techniques [26], for each tree an incision was performed and resin was collected, a new incision was made again only when the previous one ended to dripping resin. This routine was repeated as many times as needed during three-month period that the copal tapping season lasts (from August to October), and repeated for two consecutive years (Fig. 3). The studied trees were monitored every three days, collecting resin and changing the collection receptacles following the traditional way of obtaining copal [28]. The total amount of resin produced weekly per tree was weighed on a digital scale (Tanita Professional Mini), to obtain the average amount of resin produced in the three SUs per population type (wild and managed), during two production seasons.
To test signi cant differences in resin production between wild and managed populations and to rule out the potential in uence of environmental variables in each site, a covariance analysis was performed for each collection season, with average temperature and relative humidity as covariables. For measuring the covariables, six environmental monitoring micro-stations were installed, one in each wild and managed population. The stations were programmed to record weekly measurements (10 measurements per condition), during the two tapping seasons. The data were analyzed using the R statistical program [35].
Identi cation of organic compounds in copal resin from wild and managed trees To compare the resin chemical pro le from wild (N = 24) and managed trees (N = 24), a small sample of fresh resin was collected (~ 0.280 g). This resin was immediately stored in an amber vial, with 3 mL of reactive grade hexane (Baker). The samples were transported in ice and kept refrigerated at -10 °C until the analysis [36].
From each sample, an aliquot was mixed in a vortex with 500 µL of a tetradecane solution (0.5 mg mL-1). The mix was concentrated to a volume of 250 µL with gaseous nitrogen. Afterwards, 20 µL of N,O-Bis(trimethylsilyl)tri uoroacetamide (BSTFA) were added and heated at 30 °C for 10 min [37].
From each sample, 2 µL were analyzed with an Agilent 6890 Gas Chromatography equipment with an HP-5MS (5% Phenyl 95% dimethylpolysiloxane) capillary column (30 m x 0.25 mm with 0.25 mm lm thickness), coupled to an Agilent 5973N selective mass detector. Helium was used as a carrier gas at 7.67 psi with a 1.0 mL min − 1 constant ow. The front inlet was maintained at 280 °C in a split ratio of 20:1. Initial oven temperature was set at 50 °C for 5 minutes and increased to 200 °C at a rate of 5 °C min − 1 , and to 290 °C at a rate of 25 °C min − 1 for 13 min. The mass spectrometer was operated in electrical ionization mode (EI), with a ow of 1 mL min − 1 , 70 eV ionization voltage, the interface temperature at 300 °C, and a scan range of 40-500 m/z [38]. The compounds were identi ed by comparing the mass spectra of each constituent with those stored in the NIST2011.L database and with mass spectra from the literature [39,40]. The RI values were compared to those reported in the literature [36,39,41,42]. The concentration of volatile and semi-volatile compounds was calculated based on an internal standard that consisted of a tetradecane solution (0.25 mg mL − 1 ) [42,43]. Due to a lack of uniformity in the weight of the hexane-dissolved samples, percentages of the compounds of the samples were used. We used a limit of detection (LOD) criteria, only including peaks with signal to noise (S/N) ratio equal or higher than 2. Therefore, missing data (7%) were imputed using the Random Forest Algorithm for each wild and managed population. Afterwards, concentration values were transformed using the Box-Cox method [44].
A Principal Components Analysis (PCA) was performed to explore the different concentration patterns of the organic compounds between wild and managed tree populations [35].

Results
Local criteria for classifying copal trees Copal trees managers recognize, name, and classify copal trees based on different criteria. First, they classify them according to their morphological attributes, for which they recognize four species: chino (B. bipinnata), ancho (B. copallifera), ticumaca (Bursera bicolor (Willd. Ex Schltdl.) Engl.) and linaloe (Bursera linanoe (La Llave) Rzed., Calderón & Medina)). From the three locally recognized copal species, only the rst two are tapped. Copal chino is the most valued, mainly for its high resin yield, scent, and consistency, followed by copal ancho. A second criterion is based on the origin of trees: there can be " eld copal" (copal de las parcelas), and "wild" or "forest copal" (copal cimarrón or de monte); the latter are not used for resin extraction because there is a perception that these trees produce very low amount of resin. A third criterion is the consistency of the resin, which is classi ed in two types: "aguada" (liquid) and "sólida" (solid). This classi cation criterion is applied to B. bipinnata as well as B. copallifera, and in general there is a higher occurrence of liquid copal resin in trees from the forest or wild (Table 1). A fourth criterion is scent, people recognizing two scents, the "normal" copal (of soft scent), and lemon-scented copal (of intense and fragrant scent), which is highly valued although it is scarce. A fth criterion is color, and there can be three of them: white, yellow, and greenish blue. One last criterion is the form of the resin, which can be in penca (which has the bar form given by the agave leaf or penca, and is the most valued because of its economic and cultural value), lágrima (tear), goma (gum), mirra (myrrh) or cáscara (bark) ( Table 2).

Small and glossy leaves
Less resin yield Solid (it solidi es when exposed to the air; this is a process that takes time, by the third day after exposition it starts looking crystalline) Liquid (this resin does not solidify and is not extracted). Is more common in the trees named "cimarrón" or "de monte" "Cimarrón" or "de monte" (wild)

Large and ashy leaves
Greater resin yields Solid (it solidi es when exposed to the air; this is a process that takes time, in the third day after exposition it starts looking crystalline) Liquid (this resin does not solidify and is not extracted). Is more common in the trees named "cimarrón" or "de monte" "Cimarrón" or "de monte" (wild)

Smaller trees
Dark stem Copal that comes in the leaf (penca) or "planchita". This copal is collected in an agave leaf and solidi ed into a bar.
"Goma" (gum). This is created by a larvae or worm (as copaleros call it) in the bark of the tree.
Myrrh or bark. These are the resin leftovers that stick to the bark (this resin is seldom collected because it increases the tree´s susceptibility to diseases).

Management strategies and practices
Management practices for B. bipinnata involve a wide range of decisions, from the collective to the family and individual levels. Los Sauces is an ejido, a communal land tenure regime that emerged from the post-1910 revolution agrarian reform. Noteworthy, the constitutional amendments of 1992 enabled the formal recognition of individual tenure of parcels. Information from Los Sauces suggests that these changes resulted in complex formal an informal institutional arrangements where often individual decisions are taken at the plot level, but still communal agreements are taken at the ejido level including commercial forest management planning. Forest management plans (and equivalent instruments for commercial NTFP harvest), which include the authorized annual copal extraction quotas, are approved by the ejido's Assembly and sanctioned and veri ed by the Ministry of Environment (SEMARNAT). In the past, parcels, spots or trees were designated individually for management. The person who was interested in extracting copal would ask for permission from the communitý s General Assembly. With the constitutional changes of 1992, each ejidatario (member of an ejido) is considered the owner of the trees in his/her plots, thus owners do not need to ask for permission to use copal trees found in their parcels. However, it frequently happens that, if the owner of a parcel does not have enough copal trees, he may ask owners of other parcels to let him tap trees in their parcels, so to extract a higher quantity of resin. Copal managers who have low density of trees in their parcels must work more, because they must transplant or plant trees in their parcels and wait at least eight years to extract from those trees.
Decisions at the family level can involve rotation of extraction areas annually or biannually for those cases where copal managers have various parcels to tap resin from. In contrast, if they have one or a few parcels, they may choose to let individual trees rest untapped for the season. In some cases when tapping activities cannot be conducted by a family member, arrangements with other skilled copaleros are sometimes made. The copalero is paid with half of the value of the copal sales from that parcel. Decisions at the individual level imply the use of speci c tools and implements to extract resin, as well as the transmission of knowledge and extraction techniques to their children.
Both in situ and ex situ coal management practices were documented. In situ practices are performed within wild vegetation as well as inside croplands (agroforestry management). These practices include: a) collection (harvesting from trees found in wild vegetation, including resin naturally exuded, and that obtained through tapping); b) tolerance (croplands are cleared before the sowing season, copal trees are left standing on their margins; c) transplant (moving small trees to the margins of croplands, to prevent them from being damaged or eliminated during agricultural activities ; d) promotion (seedlings and trees are taken to pasturelands or other wild vegetation sites with low copal tree densities); and e) protection (activities directed to accelerate plant growth, including eliminating surrounding plants, opening up space in the canopy and getting rid of epiphytes ).
The purpose of all these practices are to increase the density of trees with favorable attributes in managed areas, to prevent erosion, establish resting sites and shade for cattle, limit parcels and protect seedlings and young copal individuals.
Ex situ practices include transplanting of vegetative parts -preferably through stakes-, planting complete individuals, and seeds sowing (Fig. 4). The aim is to achieve the greatest tree survival rate, to have material stock for the long run to reforest degraded areas with low copal densities ( Fig. 5; Table 3). Selection criteria of copal trees Copaleros have several criteria to select the trees that will be tapped (Table 4): a) appearance, that is, sturdy and healthy looking with no visible plagues or diseases; b) size, trees with a diameter less than 10 cm are not tapped, as they produce little resin and tapping them may affect their ability to produce resin in the future, or make them vulnerable to death; c) age, trees are selected after 8 to 10 years old; d) color and consistency of the bark, that is, the bark must be soft and shiny grey. Grey bark is associated to managed trees with higher resin production, compared to wild trees which have a blackish bark, and which produce little resin and are hard to cut; e) scent, in order to characterize this attribute copal resin managers crush some leaves in their hand to perceive scent and, in this way, decide whether to tap or not a tree; those with the strongest scents are considered trees of higher resin quality, and are even selected to produce stakes and their seeds collected to disseminate through community greenhouses; and f) quantity of resin yielded, is one of the de nite selection criteria. A rst sign of good production potential is that a tree exudes resin spontaneously. To further diagnose a tree's potential, copal managers will probe the tree, making incisions in speci c parts of the branches. If after three days no resin is exuded, it means that the tree is not apt for tapping and could eventually be eliminated if the parcel is used for agriculture. In contrast, if large quantities of resin are produced (1 kg per season) it will be considered an ideal tree to work the following years. Therefore, this will call for different management strategies, and therefore, diverse management techniques are practiced (Table 3). Cultural aspects of copal extraction Some copaleros consider important to ask permission to the trees to be tapped. Prayers are also commonly practiced before tapping trees, so as to protect themselves from the risks implied in the tapping activity, for instance bites and stings from animals (e.g. snakes, scorpions and wasps), from falling from the trees while tapping them, and to have a good harvest. Some of them take their tools to be blessed, tools like the "quichala" or "quixala" (a type of sharp chisel, which is struck with a wooden hammer to make incisions in the bark), sledgehammer and machete (Fig. 6). In addition, at the end of the tapping season, copaleros give thanks for the harvest taking candles and owers to church. The cultural factor is a strong incentive for conservation of the copal trees and consequently, of their forests, to maintain and continue with their traditions and at the same time, so that their children may continue extracting copal (Fig. 7).
Structural variables and copal resin production in wild and managed populations In terms of dasometric variables, managed trees were found to be taller, with greater cover and trunk diameter (DBH), compared with wild trees. These differences were statistically signi cant (  (Table 6).  The tapping intensity will also depend on how fast resin exudes, which implies great knowledge and technical skill to identify the resiniferous conduits. According to the copaleros, this process is known as "calentar" or warming the tree and implies identifying the level of rami cations needed to make incisions. Resin extraction of B. bipinnata is carried on only during the rainy season, given the marked seasonality of the TDF, conditioning the tapping season to a determined period, after which, trees enter latency and must be left to rest.

Characterization of resin quality
Twenty organic volatile compounds were found, 16 of them being responsible of the resin´s scent and four for its thickness. The greatest concentrations correspond to α -phellandrene (scent), β-amyrin and betulin (thickness). Wild trees possess greater percentages of α -phellandrene, in contrast, managed trees have greater percentages of β-amyrin and betulin (thickness). Table 7 shows the signi cant differences for the percentage of compounds concentration between managed and wild trees. It can be observed that managed trees possess greater percentages of δ-cadinene, δ-cadinol, β-myrcene (scent) and β-amyrin (thickness). At the same time, wild trees had greater percentages of β-pinene, sabinyl acetate, caryophyllene (scent) and lupeol acetate (thickness). On the other hand, the PCA explains 70.1% of the variation in the rst two components, if ordered per management type, according to the actual organic volatile compounds' percentage. Therefore, the second principal component explains the distinction between managed and wild trees. In Fig. 8, managed trees are ordered at the top of the plot and the wild ones in the lower part. The variables that support the separation of wild individuals from managed ones are δ-cadinol, calemene, δ-cadinene, sabinyl acetate, α-pinene and βamyrin. This rst ve compounds give copal resin its scent and the last one its thickness. Therefore, managed copal trees have a greater percentage of these compounds compared to wild trees ( Fig. 8; Table 8).

Discussion
Copal management strategies and their rationale B. bipinnata receives various types of management practices, along an intensity gradient. In situ practices in forest and agroforestry systems have the main purpose of increasing the quantity of individuals with desirable attributes. This constitutes a frequent practice in Mesoamerica [45] and has mainly been registered for some edible tree species [5,46,47]. Our study is an effort to register strategies in species with ritual purposes. In Northeast Africa, diverse species of the genus Boswellia and Commiphora are harvested for resin in arid landscapes, where they are promoted and protected, among other in situ management strategies [48,49]. In Indonesia, several species of the genus Styrax L., whose resin is tapped, are currently managed in large plantations, although it has been recognized that these production systems have a previous history of silvicultural management [50].
Ex situ management of copal often implies transplanting of seedlings, young plants, and vegetative parts, and even seeds sowing. The same has been registered for Boswellia papyrifera (Caill. Ex Delile) Hochst. [51] and for Senegalia senegal (L.) Britton [52], where reproduction using stakes has the purpose of accelerating growth, shortening time, and assuring the propagation of individuals with high resin yields [14]. Moreover, those who use this propagation technique are aware that this method does not guarantee that offspring will possess the same attributes selected in the parent trees.
At the same time, planting of seeds of both species constitutes a strategy to assure resin production through management intensi cation in anthropic landscapes. Mena [28] reports for the study area, that B. bipinnata has higher densities in agroecosystems and systems transformed by humans, compared to the low densities in wild vegetation [53]. This can express people´s worrisome interest for having high densities of desired species [50], and that, in order to achieve this, they must transform natural spaces, adapting them to have a higher capacity to produce elements valued by people. This situation is similar to that reported in Ethiopia with incense and myrrh-producing species like S. senegal, Vachellia seyal (Delile) P.J.H.Hurter, B. papyrifera, Boswellia neglecta S.moore, Boswellia rivae Engl., Commiphora myrrha (T.nees) Engl. and Commiphora guidotti Chiov. ex Guid. [49]. But this contrasts with reports for species that produce latex, such as Castilla elastica Cerv., where no management practices are reported because, according to the perception of people, these trees germinate on their own and are very abundant [54]. This reinforces the perception that as soon as resources seem to be at risk, the practices, and the intensity of management of valued species increase [8,18].
Also, in B. bipinnata, management practices are also done at the landscape level, where copal is a central part of an agroforestry system where selection and propagation processes are performed with high intensity (Fig. 5).
Management of resiniferous species in agroforestry systems is strongly promoted by several international agencies and constitute notable efforts from public policy in many countries [55]. Such is the case in Asia and Africa for S. senegal, Faidherbia albida (Delile) A.Chev., Boswellia serrata Roxb., Canarium strictum Roxb., Commiphora wightii (Arn.) Bhandari, Cyamopsis tetragonoloba (L.) Taub., Garcinia kola Heckel, and Ocimum gratissimum L., as a strategy to stop deforestation, land degradation due to agriculture and cattle, and also to offer economic alternatives that root people to their territories [52,[56][57][58].
In some Latin American countries, several species that produce resin, gum or latex, have been promoted through agroforestry systems, some, for hundreds of years, as it is the case of B. copallifera [59,60], B. linanoe [61,62], Manilkara zapota (L.)P.Royen [63,64], Hevea brasiliensis (Willd. ex A.juss.) Müll.Arg. [65], and Protium copal (Schltdl.& Cham.) Engl. [66][67][68][69]. These anthropogenic landscapes have high biological diversity because of long selection and manipulation processes carried out consciously or unconsciously by humans in in situ environments throughout generations [4,70]. This is therefore a con rmation that copal management strategies and practices are intimately linked to the initial worry to increase the spatial and temporal availability of plant resources of cultural and economic importance [18].

Arti cial selection criteria
In B. bipinnata human selection of the quantity of resin produced per individual is the main criterion for favoring trees in the wild, or to tolerate, promote, protect or plant them in agroecosystems or silvicultural management. This has been reported for some species of the Burseraceae family, like P. copal, Bursera submoniliformis Engl. and B. linanoe [67,71,72]. In this study, we documented that for B. bipinnata human selection is directed to diverse utilitarian attributes, such as yield, scent, color, and consistency of resin (Table 4). Copal managers identify trees with high resin production according to the strength and size of the stem, as reported for P. copal, Clusia Plum. ex L. sp., B. submoniliformis and H. brasiliensis [65,67,71,73]. For S. senegal, Ladipo [52] reports that selection is done based on the growth rate, resistance to drought, high yield and resistance to plagues and diseases.
For B. bipinnata, we also registered in greater depth that selective pressures include the identi cation of individuals with desirable utilitarian attributes. This can result in more individuals with adequate attributes being kept in wild vegetation or in agroforestry systems. On the contrary, if they don´t possess desired attributes, they are eventually eliminated. This has been documented for various edible and medicinal species in Mesoamerica [4], and particularly in long-lived management species like Crescentia cujete L. in the Yucatán Peninsula in Mexico [74,75].

Association between management and resin production
According to the results, for B. bipinnata we found a linear relationship between the size of trees (expressed as height, cover and DBH) and resin yield. That is: trees with larger canopy cover and trunk diameter yield more resin. This is a tendency registered for many species but is particularly clear in B. papyrifera [76][77][78] and in P. copal [79]. We registered a yield between 31 and 190 g of resin per tree in both types of management. The latter matches with that reported by Cruz et al. [26], who estimate that B. bipinnata produces on average 174 g per tree. In contrast, Cruz-Cruz et al. [80] estimate a slightly higher average yield of 313 g of resin. B. copallifera and B. glabrifolia (Kunth) Engl. produce a slightly higher yield, of 260 and 280 g respectively [80].
We also found that managed B. bipinnata trees produced from three to six more times the quantity of resin than wild trees (Table 6). This is perhaps one of the most important ndings, for it con rms the hypothesis that in situ management promotes the presence of individuals with higher resin yields, due to a long history of selection through time.
However, the resin quantity produced by B. bipinnata is exceptionally low compared to other Burseraceae species. For example, B. papyrifera registers a production between 840 and 3,000 g of resin per tree [48,78,81]; P. copal from 16 to 308 g [79]; and Styrax sp. from 200 to 1,000 g [50]. These differences are probably related to several factors, mainly the size of each of the species. B. bipinnata reaches relatively small heights, between 3 and 6 m. In contrast, B. papyrifera and P. copal can reach heights from 6 to 12 m and from 20 to 30 m, respectively.
Other factors that in uence resin yields are the season, and the duration and intensity of harvest. For B. bipinnata, traditional management establishes that trees can only be harvested once the rainy season has started, after owering and for a period of three months (July to October). This contrasts with B. papyrifera, which can be harvested for a period of more than six months and is harvested during the dry season [51,78], letting it rest during the rains because it is thought that resin can wash off, affecting its quality [48]. P. copal is harvested in the dry season, during a period from 4 to 8 months, depending on the region and culture of those who manage it [79,82]. C. wightii is harvested during the dry season, surely expressing other physiological consequences [83]. This is probably related to the capacity to accumulate secondary metabolites, which occurs in the rainy season, right before tapping, as observed for B. papyrifera [81]. An increase in the quantity of latex in H. brasiliensis is also reported, because they are grown in plantations, a condition that allows them to absorb more CO 2 , compared to trees that grow in places with more shade [84].
In species like M. zapota, S. senegal, and Prosopis spp., the production of resin and latex is often related to environmental variables, mainly to temperature and relative humidity [58,85]. However, according to the environmental data of the six management units studied, the differences in copal resin production for B. bipinnata are due to management and not to environmental variables (Table 6), therefore it is possible to assert that human management is responsible for such differences in resin yields. Thus, B. bipinnata trees produce greater resin quantities do so as a result of intensive and non-random selection processes [78], where yield turns out to be a key factor, over all because this is a NTFP whose commercialization is based on the kilograms of resin extracted [86].
Nussinovitch [23] observes that many species whose exudates and resins are extracted often do not produce enough quantities to extract, regardless of being healthy and growing in favorable environments (Climate and soil). An explanation has been that under mechanic stress conditions, production can increase, especially when damage has been done to the bark [87]. This has been documented for B. papyrifera where large resin quantities are yielded during the rst years of harvest [88]. Ballal et al. [58] also report that trees of S. senegal produce greater resin quantities when harvest is intensi ed. This can be associated to the formation of new conduits as a response to increased tapping rates [89,23].
Nevertheless, when the rates of harvest are increased and if tapping of the tree continues after reaching maximum yield, yield starts to decrease and can even lead to death [81]. Therefore, it is important to consider the harvest method and the post-harvest treatment. For B. bipinnata, copaleros have it clear that making more incisions than can be tolerated by the tree may compromise next year´s yields or the tree itself.
Purata [24] mentions that a greater harvest rate can produce more resin yields; however, it can also hamper growth, as well as the production of owers and fruits [65], as observed in several Prosopis species where the gum exudate increases after the fruits have matured [90]. In future studies it would be relevant to assess the implications of extractive practices are on the reproductive biology of the copal tree, particularly the trade-off between resources allocation for plant protection vs. reproduction, contributing to a more precise evaluation of the use of this resin and its long-term sustainability in diverse regions of Mexico. Our research offers evidence that management can also lead to differences in some physiological parameters, such as the quantity of resin yielded and long life cycles in species with ritual uses.
Organic compounds in copal resin and their potential association with management Composition of organic compounds in B. bipinnata ( It is also similar to compounds identi ed in Protium spp., but shows important differences with the compounds reported for the genera Boswellia, Commiphora and Aucoumea Pierre [93].
Contrary to our results, Case et al. [94] found that the majority of organic compounds in B. bipinnata are germacrene, α-copaene, β-caryophyllene and β-bourbonene. Similarly, Lucero et al. [37] identify nine organic compounds, of which only three coincide with our ndings: α-amyrin, β-amyrin and lupeol. Similar results presented by Gigliarelli et al. [95] note that although B. bipinnata is a species chemically variable, α-pinene can be identi ed as one of its main components. In contrast, our research found that α-pinene ranked 12th among the 20 compounds identi ed.
These differences can be due to various reasons, like the taxonomical identi cation, but mainly to the copal samples condition. As observed by Gigliarelli et al. [95], resin that has just been collected is different in terms of presence of chemical composition, when compared with resin that has been harvested in the past months or has been stored for years. Some reports of identi cation of organic compounds for B. bipinnata have been done with samples of resin that had been bought in markets and stored for many years [94,95] and even obtained from archeological sites hundred years old [37]. These differences may also be due to a confusion with the taxonomic identity of copal species. It is very common, for example, to mistake B. bipinnata with Bursera stenophylla Sprague & L.Riley [96], therefore the botanical distinction may not be clear [95]. These differences can also happen because often different species have the same common name, as in the case of "copal blanco", a generic name used for at least two copal species, like B. copallifera and B. bipinnata [94].
Furthermore, although both B. bipinnata populations (wild and managed) presented the same compounds, these were different in proportion and concentrations. In managed trees, in addition to the three compounds mentioned above, there exists a very important proportion of α-amyrin. In contrast, in wild trees caryophyllene has an outstanding place. According to Table 8, compounds that allow to order copal trees according to the type of management (Fig. 8) are ve, all related to substances that confer scent (δ-cadinol, calemene, δcadinene, sabinyl acetate, α-pinene), as well as one that gives it its consistency (α-amyrin). This suggests that managed trees possess higher percentages of these ve compounds (scent) when compared to wild trees. The latter may mean that management is modifying the abundance of organic compounds that give copal its scent.
These processes could be a result of the selection of attributes that are desirable in this resource, as suggested by Carrillo-Galván et al. [10] and Bautista et al. [7], who found that human selection may be generating changes in the chemical pro le of secondary metabolites.
Our research concurs with others made on aromatic plants that found differences in the chemical composition of managed individuals compared to wild individuals, based on their utilitarian attributes [10,97], which can augment the desired phenotypes and even eliminate non-desired phenotypes [5].

Traditional management and domestication of Bursera bipinnata
Our results suggest that driven by its prolonged cultural importance and use, B. bipinnata is in a domestication process [24][25][26][27]. One key motivation to manage and eventually domesticate these plants is to ensure the availability of the resource and eventually improve its quality [4]. The selection of trees with scented resin and abundant yields re ect copaleros´ concerns, whose strategies seek to increase the frequency of trees with these desirable phenotypes.
We consider the traditional management of B. bipinnata as part of a domestication process that has transited through at least three of four phases of co-evolutionary plant-human interaction according to Wiersum [98,99].
These are: the collection of products from their natural vegetation, b) conscious management of individuals with useful attributes, promoting their production capacity through concrete practices and strategies, and c) the planting of wild trees carefully selected [49]. All these phases can be observed in B. bipinnata and other resiniferous species around the world [50,78,89,94], where some of them are grown in plantations, with intensive genetic improvement efforts as part of the process [100]. In this way, silvicultural management of B. bipinnata and its promotion in agroforestry systems should be considered part of a complex domestication process that satis es production needs and ecological concerns [101]. This argument contradicts that which establishes that traditional harvest practices affect the viability of trees whose resin is extracted [102]. In B. bipinnata, copaleros have promoted and encouraged the productive restoration of the TDF, increasing its population density and with this, enhancing ecosystem services, especially provision services, as documented for other resiniferous species like B. papyrifera [100]. Therefore, we believe that the domestication of B. bipinnata strengthens ecosystem resiliency by reducing its degradation, strengthening its ecological integrity, conserving key elements and ful lling human needs.

Conclusions
Copal management in Mexico´s Central-Southern region has implied intensive selective processes through hundreds of years, associated to silvicultural and agro-silvicultural management practices. This has determined an increase in the frequency of trees that produce greater resin yields, with stronger scent and colors that are demanded by markets and consumers in the copal-producing regions. The management practices involve knowledge and dynamic techniques that are still relevant. This has resulted in a differentiation of wild and managed individuals, as well as in domestication processes in a ritual-purpose species.
Our research suggests new inquiry lines aimed at understanding the agroforestry system in which copal is immersed and the landscape matrix in which it is found. Trees with favorable phenotypes that are managed in this agroforestry system maintain direct connectivity to surrounding forests, with important consequences at the landscape level, for the conservation of the TDF, its elements, and the environmental contributions it provides. Therefore, shedding light on, and documenting traditional management techniques of resiniferous species in Mexico, can contribute to maintaining people´s livelihoods and conserving the forests. Abbreviations