Introduction

Wood is usually the main source of energy for heating and cooking in poor communities in developing countries, both urban and rural (^{Bailis, Drigo, Ghilardi, & Masera, 2015}; ^{Ghilardi, Guerrero, & Masera, 2007}). With population growth, pressure on forest resources has increased, causing the degradation of forests, as well as the impoverishment of millions of people linked to forestry (^{Lacuna-Richman, 2012}).

The use of firewood, worldwide, is usually done outside the legal channels of forest exploitation. One of the great challenges to understand the importance of firewood on the dynamics of forest ecosystems is the generation of formal information that offers real data about the volumes extracted. The data on firewood consumption are based on estimates, mainly because most of the production and consumption of firewood takes place outside the commercial channels, so the information is not recorded. However, it is estimated that the traditional use of firewood represents about 55 % of the world's wood harvest and comprises approximately 9 % of the primary energy used. ^{Bonjour et al. (2013)} estimated that 2 700 million people, mainly in developing countries, use firewood traditionally as the main energy input for domestic activities. Of the tropic regions, Latin America has the lowest firewood consumption (^{Boucher et al., 2011}). In Mexico, approximately 19 million people in rural and marginalized areas used firewood as exclusive source for domestic energy supply (^{Masera & Fuentes, 2006}), and 9 million use the resource together with liquefied petroleum gas (LP) for the same activities (^{Best et al., 2006}). The main firewood consuming states in the country are: Chiapas, Guanajuato, Guerrero, Hidalgo, Michoacán, Oaxaca, Puebla, Quintana Roo, Tabasco, Veracruz and Yucatan (^{Ávila-Bello, 2002}).

From the social point of view, the consumption of firewood has severe consequences on health, particularly in the development of lung diseases due to the incomplete combustion process that releases harmful pollutants (^{Lim et al., 2012}). From the environmental point of view, the impacts on deforestation, degradation of ecosystems and global warming are still uncertain; however, it has been shown that the extraction of firewood has serious implications at local scale in the dynamics of tropical forests (^{Rudel, 2013}) and that the burning of biomass contributes to global warming, through the release of methane and dioxide of carbon to the atmosphere (^{Bond et al., 2013}; ^{Ghilardi et al., 2007}; ^{Ramanathan & Carmichael, 2008}).

Southern Mexico is a region where the interrelation between nature and culture has been built through the perspective of cultural landscapes, which are a representation of the links between local rural communities and their natural environment (^{Fischer, Hartel, & Kuemmerle, 2012}; ^{Rössler, 2006}); that is, the human-nature relationship is accepted, through ecological, biological, cultural or scenic values ​​and conservation is emphasized as a result of this interaction (^{Phillips, 2002}). In this sense, the United Nations Educational, Scientific and Cultural Organization (^{UNESCO, 2014}) named the Calakmul Biosphere Reserve as a World Heritage, due to the ecological knowledge of its inhabitants for the maintenance of ecosystem services. In southern Mexico, a variety of traditional practices is carried out, such as agroforestry systems; slash-and-burn agriculture; the milpa system; family gardens (^{Gliessman, 2002}; ^{Toledo, 2008}), to preserve food production; soil fertility; vegetation cover; water uptaket; and firewood production as an energy component (^{Gómez & Méndez, 2007}).

During colonial period in Mexico, cultural landscapes and traditional practices changed drastically; in the case of the Yucatan Peninsula, large areas of tropical forests were deforested for the extraction of precious woods and non-timber forest products (^{Sluyter, 2004}). Consequently, the cultural and ecological elements were modified (^{Fischer et al., 2012}), as well as the perception of ecosystem services by the inhabitants (^{Martín-López et al., 2012}). The traditional knowledge of the rural communities is basic to elaborate strategies in the consumption of firewood. It has been observed that the lack of this resource is related to negative implications on family welfare, so it is relevant to explore the practices involved in the use (^{Brouwer, Hoorweg, & Van Liere, 1997}), including the value it represents for the families in their uses and customs (^{Food and Agriculture Organization [FAO], 2016}). In this context, the present study characterized the value of wood consumption and the factors that allow its use in the rural area, in the landscape of the region of Calakmul, Campeche, Mexico.

Materials and methods

Results and discussion

The surveys recorded 60 species used as firewood. According to Figure 1, in vegetation sampling, the species with the highest IVI in each age class were: Bursera simaruba (L.) Sarg. (7 to 10 years), Croton sp. (11 to 20 years) and Manilkara zapota (L.) P. Royen (>20 years). The first 10 species with the highest IVI represented 69, 57 and 60 % in each age class. It was also observed that the species richness increased with the age of vegetation (7 to 10 years: 59 species, 11 to 20 years: 64 species; >20 years: 68 species), which agrees with reported studies (^{Báez-Vargas et al., 2017}; ^{Chowdhury, 2007}) for the forests of the region. On the contrary, the number of individuals decreased with the age of sites (7 to 10 years: 2 559, 11 to 20 years: 1 220; > 20 years: 921), which was also observed by ^{Díaz-Gallegos, García-Gil, Castillo-Acosta, and March-Mifsut (2001)}; ^{Martínez and Galindo-Leal (2002)}; and ^{Báez-Vargas et al. (2017)}.

Figure 1 Importance value index (IVI) of the species in three classes of successional age: A) 7 to 10 years, B) 11 to 20 years and C) > 20 years. 

The relationship between the species most commonly used as firewood and the IVI values (Table 1) indicate that, contrary to expectations, the most important species in the region are not the most widely used. For example, the most commonly used species (Haematoxylum campechianum L.) was found only in the youngest age class with an IVI of 0.1, unlike the IVI of B. simaruba (31.2), the most important species in class one. This differs from that found in the studies of ^{Vázquez-Calvo, Cruz-León, Santos-Cervantes, Pérez-Torres, and Sangerman-Jarquín (2016)} and de ^{Yescas-Albarrán, Cruz-León, Gómez-Uribe, Lara-Bueno, and Maldonado-Torres (2016)}, who observed that Acacia pennatula (Schlecht. & Cham.) Benth. (cubata blanco), Gliricidia sepium (Jacq.) Steud. (matarrata) and Lysiloma acapulcense (Kunth) Benth. (tepehuaje) were the preferred species for fuelwood consumption and also the most abundant in the study regions.

With regard to the extraction of firewood, 95 % of respondents mentioned that the main source are trees, trunks and dead branches of standing or fallen trees; the rest indicated that they felled trees to obtain firewood. Firewood is the main fuel in rural communities in southeastern Mexico, which coincides with that reported by ^{Del Amo (2002)}. People use firewood in different proportions, only 12 % depends entirely on firewood; that is, they do not occupy gas or electricity at home. Firewood is used mainly for cooking and heating water; 76 % of respondents indicated that they use firewood daily, while 7.6 % only occasionally; 4.5 % mentioned that they use firewood when cooking special meals or have special festivities; 71.4 % use stoves of different materials, mainly rustic; 24.6 % have wood stoves and only 4 % have firewood saving stoves (built with mud and sand), wood oven or grills.

Firewood is collected, for the most part, by men (81 %). The people who carry out this activity are between 40 and 60 years old, who represent 40 % of the work force, followed by the segment of 25 to 40 years old (21 %), older than 60 years old (16 %) and people aged between 10 and 25 years old (10 %). The demographic and socioeconomic data of the people surveyed are shown in Table 2.

Most wood is transported using motorized vehicle (55 %), followed by bicycle or tricycle (bicycle adapted with a transport container, 22 %), transport by foot (13.5 %), wheelbarrow (9 %) and animals (5.5 %). Firewood source supply is in own land or ejidos (60 %), friends´s land (20 %) and in plots or courtyards (8 %). The respondents indicated that they invest about two hours in the collection of firewood (40 %), one to two (36 %), half an hour or less (15 %) or even all day (8 %). On average, the distance traveled for the collection of firewood is 5 km (49 %), followed by 1 to 5 km (27 %) and less than 1 km (23.5 %).

The most important factor in fuelwood consumption is the economic one; 68 % of respondents said so. In contrast, 65 % of the respondents indicated that the main disadvantage is the risk to health (Table 3). The incomplete combustion of the biomass releases small particles of other components whose harmfulness to human health at home environment has already been documented (^{Smith, 2006}). In suitable stoves it is possible the clean consumption of firewood and coal, which gives rise mainly to carbon dioxide and water; however, such conditions are difficult to achieve in poor rural areas where small, cheap stoves are used that are fueled with firewood. Firewood that does not burn properly is converted into carbon dioxide, resulting in products of incomplete combustion, basically carbon monoxide, but also benzene, butadiene, formaldehyde, polyaromatic hydrocarbons and many other compounds dangerous to health (^{Bonjour et al., 2013}).

Regarding the monetary parameters associated with firewood, 57 % of respondents pay wages (equivalent to one working day) for the extraction of firewood, with an average of 124.00 Mexican pesos (from 50.00 MXN to 250.00 MXN). Of the total number of respondents, 78 % said that firewood represents savings and only 15 % reported some income; only 3.8 % of people allocate resources for the purchase of firewood on a regular basis. As pointed out by ^{Alayón-Gamboa (2014)}; ^{Neulinger, Alayón-Gamboa, and Reinhard-Vogl (2014)}; and ^{Chí-Quej et al. (2014)}, the socioeconomic environment in the region encourages the use of natural resources as a subsistence strategy based on self-consumption, which maintains the practice of traditional land uses such as family gardens, cornfields, plots or activities silvicultural.

Although the extraction of firewood can be considered of low economic importance in terms of the net income that is perceived with respect to forestry, agriculture, livestock or beekeeping (^{González-Abraham, Schmook, & Calmé, 2007}), extraction strategies can compensate the low income in the communities. This is due to the fact that about half of the people collect firewood at distances close to 5 km, which implies a moderate expenditure in the transport and use of fossil fuels.

The results also suggest that the preference for the selection of firewood is associated to biological attributes of species typical of the late phases of secondary succession. This agrees with the study of ^{Del Amo (2002)} in the same region of study, whose preferences were focused on tinto (H. campechianum), chicozapote (M. zapota), chacteviga (Caesalpinia violacea [Mill.] Standl.), jabin (Piscidia piscipula [L.] Sarg.) and zapote (Pouteria campechiana [Kunth] Baehni). In other assessments in southern Mexico similar trends were found on concentrated use in few species. ^{Ramírez-López, Ramírez-Marcial, Cortina-Villar, and Castillo-Santiago (2012)} found preference of 10 of 138 species in coffee communities of Chiapas; while ^{Quiroz-Carranza and Orellana (2010)} mention five preferred species of 41 in Yucatan. The preference in the use of firewood species is related to attributes and characteristics such as availability, economic and energy implications, in accordance with the study of ^{Escobar-Ocampo, Niños-Cruz, Ramírez-Marcial, and Yépez-Pacheco (2009)} in the central area of Chiapas.

The results indicate that, contrary to what is proposed by different authors such as ^{Dufraisse (2012)} and ^{Piqué (1999)}, there is no direct relationship between the dynamics of plant communities and human preferences over species as a source of firewood, since in the south of the Yucatan Peninsula, a great variety of species of different successional phases are used, indistinctly. However, potential fuel capitals increase when extracting firewood from trunks and branches of dead or sick trees that have diminished their carbon capture capacity, which would decrease the intra- and interspecific competition. The effect of extraction of woody material on carbon sequestration and soil nutrients in fragmented forests has been documented by ^{García-Oliva et al. (2014)}, from the quantification of soil organic carbon, while the high contribution of tropical secondary forests in carbon sequestration has been shown by ^{Poorter et al. (2016)}

Modifications in the structure and composition of the species community cause users to adopt different strategies in the use of the resource. For example, the benefits of necromass extraction have been widely reported in agroforestry systems, as well as in multi-stratum crops, since greater growth in biomass of established species has been observed. ^{Chí-Quej et al. (2014)} also suggest that work in this type of crops is a work alternative that contributes to the equitable distribution of work among family members.

The results coincide with studies conducted through interviews and questionnaires in other regions of the world. ^{Martín-López et al. (2012)} evaluated the perception of society and sociocultural values on ecosystem services and functions in communities of the Iberian Peninsula; ^{Agbenyega, Burgess, Cook, and Morris (2009)} did something similar in forests in England, and ^{Iftekhar and Takama (2008)} analyzed the ecosystem benefits provided by mangroves in Bangladesh. The surveys applied in the present study are a simple and practical application tool that allows the approximation of two aspects in the exploration of cultural landscapes, as shown by ^{Hartel et al. (2014)} in rural communities of Romania, through socio-economic, cultural and biological data.

It is important to highlight that the cultural landscape analyzed in this study is in danger due to different factors. The young population is not actively involved in traditional land use practices; aspirations of the new generations do not find satisfaction in rural environments, as indicated by ^{Oroszi (2004)}. The majority of the owners of the land are people over 40 years old, which puts at risk the permanence of ancestral customs and uses in the medium term. In addition, there is a latent threat to ecosystems, due to the expansion of urban, agricultural or livestock areas, which has already occurred in previous decades in the Yucatan Peninsula. Finally, it is important to point out that, although the extraction of firewood per se is not cause but an effect of deforestation (due to the amount of available waste derived from forest extraction), it has been related as a factor of disturbance or degradation of ecosystems (^{González-Espinosa, Ochoa-Gaona, Ramírez-Marcial, & Quintana-Ascencio, 1997}; ^{Masera, Astier, & López-Ridaura, 2000}).

Conclusions

This research is one of the few studies that establishes the relationship between the preferences in the consumption of firewood in a region and the relative importance of the species in the ecosystem. The species used as a source of firewood do not correspond to those that have the highest IVI in the region. Some attributes of the preferred species are availability in the land, calorific value or the flavor of the food associated with cooking using firewood. The socioeconomic dynamics of the communities in the south of the Yucatan peninsula suggest that collecting firewood and using it as fuel is a subsistence strategy; in addition, it represents savings for the activities that are indispensable at home. The segment of the population dedicated to the collection of firewood is made up of older people, while the young population is not actively linked to such activity. As a result, traditional practices associated with the use of firewood may be at risk in the medium term. The cultural landscape and the preservation of the forest resources of this region of Mexico should be addressed with strategies of promotion and training on the use of firewood as an important energy resource.