Biomass accumulation in forests . . . BIOMASS ACCUMULATION IN FORESTS WITH HIGH PRESSURE OF FUELWOOD EXTRACTION IN CHIAPAS , MEXICO

1 Received on 03.05.2018 accepted for publication on 31.07.2018. 2 CONACYT-Universidad Autónoma de Chiapas, Facultad de Ciencias Agronómicas, Chiapas México. E-mail: <drajar@conacyt.mx>, and <polo_tex1@hotmail.com>. 3 Universidad Autónoma de Chiapas, Facultad de Ciencias Agronómicas, Chiapas México. E-mail: <ruizc.roldan@gmail.com>, <hgomezc1@yahoo.com>, <francisco.gouvara@unach.mx>, <pinto_ruiz@yahoo.com.mx>, <alejandro.ley@unach.mx> and <moraless@outlook.com>. 4 Biodiversidad, Medio Ambiente,Suelo y Agua , Chiapas México. E-mail: <alonsoloc75@gmail.com> and <cavs58@gmail.com>. 5 Universidade Federal de São Carlos, Programa de Pós-Graduação em Planejamento e Uso de Recursos Renováveis, Sorocaba, SP Brasil. E-mail: <iessaiec@gmail.com>. *Corresponding author.


1.INTRODUCTION
Nearly 2.7 billion people in the planet depend on traditional fuels like firewood, charcoal and animal dung for domestic energy (IEA, 2011).But the demand and sources of energy vary depending on regions.In many countries, these resources account for over 90% of household energy consumption (IEA, 2013).In Mexico, total energy consumption from the residential sector in 2015 was of 755.5 PJ, of which 33.5% was obtained from fuelwood (SENER, 2018).About one fourth of Mexican households (27.2 million people) depend, either exclusively (18.7 million people) or in combination with liquefied petroleum gas (LP gas) (8.5 million people), on fuelwood for cooking (Masera et al., 2005).High fuelwood consumption is concentrated especially within rural and peri-urban households in Mexico (Masera et al., 2005).In the state of Chiapas, nearly 50% of the families depend on firewood as their main source of energy for cooking and heating their homes.The average per capita fuelwood consumption in the rural communities of El Ocote, Chiapas, ranged from 1.3 to 3.3 kg day -1 (Marquez-Reynoso et al., 2017) to 9.5 kg day -1 person -1 in two rural communities of Villaflores, Chiapas (López-Cruz, 2016).
The high demand for this resource contributes significantly to deforestation, forest degradation and loss of biodiversity (Marquez-Reynoso et al., 2017).The total forest biomass removal for fuelwood in Mexico increased from 0.47 to 0.63 million m 3 between 1990  and 2011 (FRA, 2015).Since the total forested area in Chiapas has greatly decreased in the past decades due to the expansion of pasturelands, fuelwood pressure on the remaining forest patches has been increasing.Despite the high demand for fuelwood by the families living around or adjacent to frontier forests, there are few studies that evaluate changes in the biomass storage of remnant forests as a consequence of fuelwood extraction (García-Oliva et al., 2014).The purpose of this research was to evaluate the biomass stocks of the pine and oak forests between four rural communities with different fuelwood extraction pressure in Chiapas, Mexico.The study also addressed possible technical and ecological alternatives for fuel saving and forest conservation in the region where fuelwood is the main source of household energy.

2.MATERIALS AND METHODS
To evaluate the changes in forest cover in Chiapas, we compiled published reports presenting total forest area at different times and analyzed the INEGI vegetation and land use maps (III, IV and V series).Four sites within the municipality of Villaflores, Chiapas, were selected to evaluate forest biomass considering the gradient of fuelwood extraction pressure.Niquidambar and Belen are the high fuelwood pressure sites and Nambiyugua and Jerusalen are considered the low fuelwood pressure sites.Easy access to the community and the demand for firewood were the criteria adopted to define fuelwood pressure.Apiculture and agroforestry practices are common livelihood activities in low fuelwood pressure sites, while extensive animal farming and maize monoculture are predominant in high fuelwood pressure sites.Pine forest is the main vegetation type in Niquidambar and Nambiyugua, while the oak forest predominates in Belen and Jerusalen.Villaflores is located in the southern limits of the central depression and the Sierra Madre de Chiapas, of predominantly mountainous terrain.Its geographical coordinates are 16°14.0'N and 93°16.0'W. The predominant climate in the region is tropical sub-humid with annual precipitation ranging from 1200 mm to 1600 mm.The average monthly temperatures range from 19°C to 29°C.Leptosols and Cambisols are the dominant soil types at the study sites.
To evaluate land use changes over time within the selected sites, we used a participatory historical mapping approach and field visits with geo-referencing.Land use changes were analyzed using the ArcGis software.Forest Biomass stocks of pine and oak forests were quantified by measuring dasometric variables within 16 forest monitoring plots of 400 m 2 (four plots by community).Stand structural parameters like tree diameter Biomass accumulation in forests... at breast height (DBH), total height of the tree, canopy diameter and branch free trunk height were also measured.Published allometric equations (Eq. 1, 2 and 3) were used to calculate aboveground biomass (Cairns et al., 2003;Vargas-Loreta et al., 2017).Equations 1 and 2 (Vargas-Loreta et al., 2017) use the sum of different biomass components calculated separately.For equation 3 (Cairns et al., 2003), wood density corrections were applied because the original equation did not include the wood density parameter.Where, AGB is aboveground living tree biomass (Mg), DBH is the diameter at breast height (cm), TH is the total height of the tree (m), P is the wood density of each tree species (g cm -3 ), and Pm is the mean wood density of the tree species used to develop the equation (0.72 g cm -3 ).
Basal areas were calculated as the sum of the crosssectional areas of the tree trunks at the height of 1.3 m from ground level.All trees within each plot were measured and extrapolated per hectare.. Linear regression analysis was performed to evaluate the temporal trend of forest cover changes in Chiapas.One-way ANOVA (p = 0.05) was used to compare the statistical differences in forest biomass and stand structural properties between the sites with high and low fuelwood pressure.

Changes in forest cover
In Chiapas, forest areas have decreased sharply over time.Nearly 40% of the total forest lands have been converted to other land uses during the past 60 years (Figure 1).
Land use pattern changes between two communities of Chiapas, one with high fuelwood pressure and the other with low fuelwood pressure, varied strongly over time.The loss of forest cover from 1995 to 2015 was of 70% at the high fuelwood pressure site, compared to 15% at the low fuelwood pressure site (Figure 2).

Changes in forest structure and biomass storage
Fuelwood pressure greatly affected the stand structural properties of both the pine and oak forests as many of them varied between two sites.Average tree DBH, tree height, basal area and canopy diameter were significantly higher (p<0.05) at the low fuelwood pressure site, compared to the high fuelwood pressure  site (Table 1).However, tree density per hectare was higher at the high fuelwood pressure site.There were no significant differences in average wood density and branch free trunk height between sites.The lower basal area and higher tree density at the high fuelwood pressure site were explained by the presence of shorter and smaller trees, which is one of the common symptoms of forest degradation.Such differences in forest structural properties are normally linked to the variation in their biomass storage capacity.
The aboveground biomass (AGB) stocks varied significantly between the sites with different level of fuelwood pressure (ANOVA, F = 7.95, p = 0.018).Pine and oak forests with low fuelwood pressure stored, respectively, 213.374 and 189.51 Mg of AGB ha -1 .The AGB stock was 35% and 51% lower for pine and oak forests, respectively, at the high fuelwood pressure sites (Figure 3).

4.DISCUSSION
Deforestation for the expansion of pasture and agricultural lands was very high between the 70s and the 90s because of governmental incentives in Mexico.
In recent years, it has been reported that the intense deforestation in southern and southeastern Mexico has declined, and that secondary vegetation is growing.However, degradation of the remaining forests due to the extraction of fuelwood and other forest products is still ongoing in rural areas of Mexico because of household energy demand (Rüger et al., 2008;Barody, 2013;De Jong, 2013).Land use changes at community scale can be attributed to the differences of main livelihood activities between the two communities.Apiculture was the main livelihood activity at the low fuelwood pressure site, whereas crop and livestock production were the main livelihood activities at the high fuelwood pressure site.
Easy access from the rural community increased fuelwood extraction, thereby reducing total AGB storage.Livelihood activities like apiculture require conservation of natural flora and this explains the highest level of forest conservation in one of the communities analyzed in this study.Rüger et al. (2008)    Understanding tree growth, recruitment and mortality at different stages of forest succession can be a useful tool in the adequate harvesting of the correct species without deviating the successional trajectories, species composition and carbon dynamics (Aryal et al., 2014).The average per capita fuelwood consumption in these communities was of 9.5 kg person -1 day -1 (López-Cruz, 2016).Marquez-Reynoso et al. ( 2017) reported that fuelwood consumption in the communities around the El Ocote biosphere reserve of Chiapas ranged from 1.3 to 3.3 kg person -1 day -1 .Alternative strategies like improved cooking stoves can save up to 26% of fuelwood consumption and related greenhouse gas emissions (García-Frapoli et al., 2010;Vázquez-Calvo et al., 2016).From ecological perspectives, the establishment of dendroenergy banks, the management of agroforestry systems and secondary forests for family fuelwood consumption would be the sustainable alternatives in rural areas with high fuelwood demand (McCrary et al., 2005;Sharma et al., 2016).

5.CONCLUSION
Fuelwood demand in rural areas significantly reduces forest area and biomass stocks of the adjacent forest ecosystems.Ecofriendly livelihood activities like apiculture were more efficient in forest conservation compared to extensive agriculture and livestock production activities.Continued extraction of some species due to their preferred fuelwood properties can change the stand structure and community composition of the forest ecosystems.Technological and ecological alternatives like improved cooking stoves, the establishment of dendroenergy banks, and the management of agroforestry systems are recommended for a sustainable fuelwood supply to people living in rural areas around the frontier forests.

6.ACKNOWLEDGEMENTS
The authors are thankful to Consejo Nacional de Ciencia y Tecnología (CONACYT) for financing the project "Estrategias de mitigación de impactos ambientales del sector agropecuario en Chiapas (381-2015)" and to Alianza MéxicoREDD+ for supporting field work.Thanks to Noé Alejandro Ruiz Orantes for his support in laboratory works.We appreciate the comments from two anonymous reviewers on the earlier version of the document.

Figure 2 -
Figure 2 -Differences in land use pattern changes between two communities of Chiapas.Figura 2 -Diferenças entre duas comunidades de Chiapas no seu padrão de mudança no uso da terra.
) b first branching (m) Different superscript letters next to the values (mean ± 0.95 confidence interval) indicate the significant differences between two sites (one way ANOVA, p<0.05).
also reported similar changes in forest structure and tree community composition due to fuelwood harvesting in tropical montane cloud forests of Veracruz, Mexico.González-

Table 1 -
Structural variation of pine and oak forest between sites with high and low fuelwood pressure in Chiapas, Mexico.