Introduction

Global Climate Change (GCC) is the result of the increment in the temperature of the planet's surface and it is one of the most serious environmental problems faced by humanity since over a decade ago (^{IPCC, 2001}).

Forests act as a valuable deposit of carbon since they are greenhouse effect gases (GEG) mitigation or reduction sources, particularly carbon dioxide (CO₂). However, when they are disturbed or destroyed, the reserves in the aboveground biomass are partially or totally removed from their original source, and later, transformed into wooden products or released into the atmosphere by combustion or in a longer time, by microbial decomposition (^{Ordóñez et al., 2008}).

Within the forest vegetation, tree and herbaceous strata put together the highest biomass, and, therefore, carbon; storage capacity and capture depend on plant community, weather, moisture, species, site quality and density, for example (^{Ordóñez, 1999}). The main components of the aboveground biomass of the tree layer are the stem, the branches and the foliage.

The temperate forests of central and southern Mexico currently experience an accelerated process of deforestation and degradation, with rates comparable to that of the country's forests change rates. ^{Bocco et al. (2001)} mention that deforestation in Michoacán ranges from 1.0 to 1.8 % per year in temperate and tropical forests, respectively, which are among the highest in Mexico.

In Mexico there is no thorough information on carbon stocks by ecosystem type and land use, nor of its net flows, derived from the patterns of land use change on a regional scale (^{Ordóñez, 1999}).

This study aims to make a contribution to the knowledge of the content of carbon in temperate forests through a detailed analysis that comprises the estimation of the potential carbon content and capture in the aboveground biomass of the ejido of San Pedro Jacuaro, Michoacán, plus making stands and to estimate the tree areas of the place, making a forest inventory in order to quantify the cubic meters of wood volume and calculate the carbon content by total area and biomass per hectare.

The ejido was selected to carry out the project that includes a natural forest in good condition that is subject to forest management that keeps it that way.

Materials and Methods

The study area is located between the 2'191 000 north latitude and west longitude and 2'180 000 north latitude with west longitude in the Eje Neovolcánico physiographic province and the Mil Cumbres subprovince at 1 500 to 3 000 m (Figure 1) over 1 842 has.

Figure 1 Location of the study area, San Pedro Jacuaro , Ciudad de Hidalgo municipality, Michoacán . 

Eleven types of sites or stands present at the selected lands were determined, in which seven sampling plots were established by each stand, which summed up 77, which, based upon the sampling intensity, is representative of the total area, according to the following formula of sample size:

Where:

In order to distinguish the type of sites, Bocco's method (^{Bocco et al., 1991}) was applied, as it uses a geographic information system (GIS) to characterize the place and define landscape units. Each sampling plot was made up by three 1 000 m² circles (^{Velasco et al., 2002}), inside of which was included a smaller one of 500 m² (Figure 2).

Source: ^{Ordóñez et al., 2008}; ^{Ordóñez, 2012}.

Figure 2 Sampling plots design. 

For the estimation of carbon content of the forest aboveground biomass, the procedure recommended by ^{Brown and Roussopoulous (1974)}, ^{SARH (1995)} and ^{Brown and Delaney (2000)} was followed. This method measures ND (normal diameter or at breast height) at 1.30 m in all the trees with sizes above this number.

Afterwards, from a stand according to the existing types of vegetation (pine and oak), four generic equations (^{Ayala, 2001}) were selected to estimate the total volume based upon ND and height.

Where:

This equations favor a more precise estimation when height is considered, compared to those that only include ND, which lead to others, lower or more conservative, and, therefore, do not fall into overestimations.

To determine biomass, volume was multiplied by wood density (0.45 g cm^-3 for pine and 0.60 g cm^-3 for oak) (^{INECC, 2013}); next, by the number of individuals at the site and the calculation was transferred to the hectare and the total area of the study. Finally, to determine carbon content, the value of biomass was multiplied by the 0.5 carbon concentration (^{IPCC, 1996} and ²⁰⁰³; ^{Ordóñez, 2008}).

In addition, in order to know the evolution of carbon contents in the forests, it was decided to use the Current Annual Increment (ICA, for its abbreviation in Spanish) through the Loetsch method (^{Ordóñez and Masera, 2001}). ICA is the volume increase of a tree, which is influenced by the species, age, soil type, moisture and competence, among other factors, that as a whole conform the quality of station and when dimensional analysis is made, values are transformed from m³ ha^-1 yr^-1 into Mg C ha^-1 yr^-1 (^{Ordóñez, 2008}).

Results

The carbon content in the vegetation of the study area was estimated with a high and a low estimate; the first found a total of 129.1 Mg C ha^-1, with values between 71 and 198 Mg C ha^-1, while the second gave a total of 103.1 Mg C ha^-1, with values between 67 and 177 Mg C ha^-1. Figure 3 shows significant differences between using only the diameter (high) and replaces the value in the allometric equation (one for pines and three for oaks) and the other (lower), which considers the diameter and height and adjusts the values of the allometric equation (two for pines and four for oaks).

Figure 3 Carbon content (t C ha^-1 or Mg C ha^-1) in aboveground biomass of the sites of San Pedro Jacuaro , Michoacán . 

In the high estimate, the greatest concentration of carbon stored was recorded in the 11^th site with 195 Mg C ha^-1, even where more than 2 000 trees per hectare were counted, it should be noted that the major diameters were 12 and 13 cm (when replacing values in equation one and three); in the low estimate to replace the values (in equations two and four which include diameter and height), a value of 95 Mg C ha^-1 was obtained, that is, a difference of 51 % from the estimated value is confirmed.

Sites 3 and 10 showed percentage differences in carbon stocks of 3 and 8 % respectively, as site 3 has 529 trees ha^-1 on average with dominant diameters of 70 to 75 cm; site 8 has 839 trees ha^-1 with dominant diameters of 39 and 40 cm, suggesting that the selected allometric equations are not designed for trees with a diameter below 35 cm.

From ICA parameters, potential scenarios were developed to capture carbon after 5, 10, 15, 20, 25, 40 and 50 years. It was taken as a premise that management did not exist and that the evaluated forests develop naturally. Results indicate that there is good potential for carbon sequestration with an average of 1.54 Mg C ha^-1 yr^-1 (the national average growth of natural forest is 1.0 Mg C ha^-1 yr^-1 (^{INECC, 2013}).

Expressed in tons of equivalent carbon dioxide it is possible to predict an annual capture of 5.65 Mg CO₂ e ha^-1 and for the 1 842 has of the property, there is a potential carbon capture of 10 401.16 Mg CO₂ e.

Discussion

The forests of conifers and hardwoods in Mexico are distributed around the Transmexican Axis and they exist virtually in all mountain ranges (^{Rzedowski, 1994}), demonstrating their value as carbon sinks.

To estimate the carbon content in a forest it is required to know, essentially, its biomass which allows determining the storage of carbon and other chemical elements in each of its components in a given area (^{Brown et al., 1996}; ^{Ordóñez and Masera, 2001}; ^{Ordóñez et al., 2008}); to weigh the potential amount of carbon that can be sequestered, growth should be measured from the stem volume (^{Ordóñez, 2008}).

In this study the biomass of standing trees was calculated by allometric and volumetric equations from the wood density of the genres of interest, coupled with the use of a biomass expansion factor for shafts, suggested by the ^{IPCC (1996}; ²⁰⁰³⁾ and ^{INECC (2013)} for the corresponding to the entire tree. Compared with other locations of temperate forests in which the process have been carried out, the minimum and maximum values of this research surpass the intervals recorded in previous experiences (^{Ordóñez et al., 2008}; ^{Ordóñez, 2012}; ^{Orozco and Mireles, 2014}), resulting from the use of generic equations.

To carry out carbon assessment projects on forest ecosystems demands reliable measurement methods that meet the requirements established by the ^{IPCC (1996} and ²⁰⁰³⁾, that are efficient to measure the carbon content and potential capture (^{Ordóñez and Masera, 2001}; ^{Ordóñez et al., 2008}). Therefore, when making a low estimate of carbon stocks, the premise not to overestimate the content of the element in question is fulfilled, which is recommended by the IPCC for quantification projects thereof, in order to reduce uncertainties. The most important thing is to make in situ cubication and to estimate, with local data, the following variables: actual stocks, density of trees per hectare, genders present, volume per tree, tree biomass and current annual increment, complemented by wood density by genus or species, and, finally, to try to justify the use of expansion factors if they should be employed.

Conclusions

It was possible to make an estimation the carbon content in the aboveground biomass of the forests of the ejido of San Pedro Jacuaro in the state of Michoacán by means of the allometric equations, as the use of generic equations leads to significant differences of more than 51 % of the estimated carbon replacement values, as they do not take into account diameters under 35 cm. The accurate estimation is 237 618 Mg C, while the conservative estimate would suggest a total of 220 642 Mg C in the 1 842 has comprising the study area.

The carbon sequestration potential is 1.54 Mg C ha^-1, which in tons of equivalent carbon dioxide is 5.65 Mg CO₂e ha^-1 and for the studied land, 10 401.16 Mg CO₂e per year. To place bonds for carbon sequestration it is necessary to make a balance between what is cut and what grows, in order to have a clear appreciation of how carbon sequestration occurs as an environmental service.

Conflict of interests

The authors declare no conflict of interests.

Contribution by author

José Antonio Benjamín Ordóñez Díaz: selection of study units and sampling sites in the field, structuring of the document, data base, review and data analysis and funding negotiations; Ricardo Rivera Vázquez: data analysis and processing and review of the document; María Erika Tapia Medina: definition of the research topic, selection of study units and sampling sites in the field, data base, structuring and review of the document; Luis Raúl Ahedo Hernández: selection of study units and sampling sites in the field, structuring and analyzing data base.