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Introduction
In Mexico, temperate forest ecosystem represents 17.4 % (34 million hectares) of the country's surface (195 million hectares) and groups together the cloud forest and the coniferous and broadleaf forest (Instituto Nacional de Estadística y Geografía [INEGI], 2015). In these forests, the carbon sequestration rate is estimated to be 3.431 ± 0.870 Mg C·ha-1·yr-1 (Casiano-Domínguez, Paz-Pellat, Rojo-Martínez, Covaleda-Ocon, & Aryal, 2018). Pan et al. (2011) mention that forests, as carbon reservoirs, absorb about 30 % of all CO2 emissions in a year and considerably increase their importance when these are incorporated into silvicultural management.
Carbon sequestration and storage by forest vegetation occurs in two scenarios: natural forests and managed areas. Managed forests fix high amounts of CO2 according to stand age, site quality, species composition, density, climate, edaphic and topographic conditions, and silvicultural treatment (Avendaño, Acosta, Carrillo, & Etchevers, 2009; Pan et al., 2011). Atmospheric carbon sequestration capacity tends to decrease as forest age increases; at early and intermediate ages, carbon sequestration rate is high (Fonseca, Benayas, & Alice, 2011). This is related to the rate of biomass accumulation, such that forests with net growth can capture more CO2 than they emit, through respiration, and the capture rate is directly proportional to such growth (Casiano-Domínguez et al., 2018). In general, it is accepted that the rate of carbon fixation through photosynthetic processes is higher in young stands than in mature forests, but total carbon storage in the system is higher in mature forests.
Forests go through different stages of development during establishment: brinzal, monte bravo, vardascal, latizal, and fustal. Forest management involves the implementation of a program of silvicultural practices, using regeneration methods and application of silvicultural practices (site preparation, clearing, pruning, thinning, protection, and promotion) that are carried out during the forest management period (Monárrez-González, Pérez-Verdín, López-González, Márquez-Linares, & González-Elizondo, 2018). Pacheco-Aquino, Durán-Medina, and Ordóñez-Díaz (2015) show signs that forests under forest management represent important carbon stores, with carbon sequestration capacity and that the sites with timber harvesting recover 15 to 20 % of the carbon removed compared to that originally present in a forest without previous interventions; these values are reached seven years after harvesting is carried out.
The ideal scenario for carbon fixation and storage by forests is where forest stands are kept dynamic through the constant incorporation of organic matter to the soil, coming from adult trees, while the natural regeneration of species is established and other young trees are in full photosynthetic activity (Razo-Zarate, Gordillo-Martínez, Rodríguez-Laguna, Maycotte-Morales, & Acevedo-Sandoval, 2013).
Forest carbon sequestration under the market scheme in socially owned forests is operating and has great potential, especially where timber is harvested based on legal, technical and social planning and supervision; since, in addition to land tenure security, there is a forest management culture (Pacheco-Aquino et al., 2015). Similarly, Álvarez and Rubio (2013) mention that, although currently the main value of the pine-oak forest lies in its use as a source of timber, carbon sequestration could represent an important additional value. The objective of the present study was to estimate biomass and aboveground carbon in the development stages of a Pinus patula Schiede ex Schltdl. & Cham. forest, grown in the ejido Atopixco, municipality of Zacualtipán, Hidalgo, to know the potential of the species to mitigate climate change.
Materials and Methods
Study area
The study area is located in the region known as Sierra Alta of Hidalgo, in areas incorporated to silvicultural management in the ejido Atopixco, municipality of Zacualtipán, Hidalgo, coordinates 20° 37’ 26’’ - 20° 35’ 20’’ N and 98° 35’ 23’’ - 98° 37’ 48’’ W, with an average altitude of 2 062 m. The area is dominated by the C(m)a type climate, temperate sub-humid with summer rainfall, precipitation of 1 780 mm and average temperature of 13.5 °C (Aguirre-Salado et al., 2009; García, 1981), with Feozem haplic soils (Hh), rich in organic matter, and Regosol calcarico (Rc) on the steeper slopes (Santiago-García, De los Santos-Posadas, Ángeles-Pérez, Valdez-Lazalde, & Ramírez-Valverde, 2013). Vegetation corresponds to temperate pine-oak forest, where P. patula predominates over other native species such as Pinus teocote Schiede ex Schltdl. & Cham., Quercus rugosa Née, Q. laurina Humb. & Bonpl., Alnus arguta Schl., Prunus serotina Ehrh., Vaccinium leucanthum Schltdl., Clethra mexicana DC., Crataegus mexicana DC. and Ternstroemia sylvatica Schltdl. & Cham.
Description of forest development stages
The Atopixco ejido has a total area of 1 188.9 ha, of which 789.1 ha are covered by forest and only 658.7 ha have been incorporated into the sustainable forest management scheme. Silvicultural treatments are aimed primarily for P. patula, which is the main species of commercial timber interest. Thus, based on the proposals of Aguilar-Luna (2018), Aguirre, Díaz, Muñoz, and Muñoz (2019) and derived from field experiences, the present study proposes the classification of the development stages of the P. patula forest shown in Figure 1.
Brinzal
This is the initial forest development stage, from seedling emergence to about 1 m of height. In cultivated forests, it is usually established during the first rainy season after regeneration or thinning treatments. In the case of P. patula in the study area, it manifests itself in trees up to two years old (Figure 1). At this stage, no forest products are obtained from the clearing of vegetation carried out to favor the development of the species of interest.
Monte bravo
This is the second stage of development and is identified by the marked competition for light, space and nutrients between trees of the ecosystem, leading to a greater increase in height; trees reach between 1 and 3 m and diameters at the base of less than 5 cm, with abundant dry low branches that intertwine to form an impenetrable mass. The average age of this stage in the study area is four years (Figure 1). As it is a cultivated forest, pre-thinning and pruning of the basal branches is carried out, and the process of obtaining products such as wood energy material in low quantities for domestic use begins, giving rise to the dynamics of biomass and aboveground carbon extracted from the forest.
Vardascal
Condition in which the stand shows high density with thin and flexible trees that have lost their lower branches (beginning of natural pruning) generating abundant dead material on the ground. Most of the trees have an average age of up to 6 years, an average diameter at breast height lower than 10 cm and heights of 3 to 8 m (Figure 1). With the application of non-commercial thinning, products such as firewood, props and poles are obtained for self-consumption, which represents the beginning of the extraction of biomass and aboveground carbon from the forest.
Latizal
This is the stage of development in which the trees show the greatest growth in height and natural pruning intensifies; in addition, most of the trees show crown differentiation with average heights of 10 m and average diameter at breast height of 15 cm. In the study area, P. patula reaches this condition at an average of 11 years of age (Figure 1). From time to time, intermediate thinning is applied to obtain props, poles, roundwood for sawmilling and cellulosic material for sale. The extraction of biomass and aboveground carbon from the forest is destined for products with a medium life span in the environment before carbon is released into the atmosphere.
Fustal
This is the final stage of forest development in which trees reach physiological maturity, natural pruning is completed and they produce large quantities of viable seeds for natural regeneration. The height of the trees exceeds 20 m and the diameter at breast height is greater than 20 cm (Figure 1). At this stage the final harvest of the forest takes place, in which the trees reach their maximum dimensions and generate a greater quantity of sawmill products, culminating the growing cycle, corresponding to the commercial shift. Wood and biomass volumes reported are abundant. Extracted wood is used to produce long-lasting products where carbon is stored for many years.
Sampling sites and measurement of variables
With the use of Geographic Information Systems and the QGIS® tool, the polygon incorporated into the ejido's forest management was delimited and the areas that presented the five stages of forest development in similar conditions of exposure and slope of the terrain were located and selected. Subsequently, the stratified systematic sampling was designed, regarding 15 circular sites of 1 000 m2 (Figure 2). Tree measurement characteristics (height and diameter at breast height), the forest development stage and the number of trees per site were measured with the support of terrestrial orientation and navigation equipment (SUUNTO® Mc-2 Compass and GPS Garmin® 60Cx). The basal diameter at brinzal and monte bravo stages was measured with a Steren® digital vernier of 150 mm with an accuracy of 1 mm. For the vardascal, latizal and fustal stages, the reading was taken at a height of 1.30 m (diameter at breast height), using a Haglof Sweden®, model Mantax Blue 800 mm. Total height of each tree was measured with a Truper® tape measure for trees up to 2.0 m and with the support of a Suunto® clinometer for greater heights.
The information gathered in the field was processed in the laboratory to obtain composite variables such as basal area, volume, tree density, biomass and carbon content for each stage of forest development.
Basal area
For the areas at brinzal and monte bravo stages, the basal area (BA) per hectare was found by the direct sum of the basal area of a given tree (abi, m2) calculated from the diameter data at the stem base [
Volumen
The volume (V, m3) at brinzal (Br) and monte bravo (Mb) stages was calculated with the value obtained from AB (m2) and the height (h, m) of the tree using the equation
where,
VTACC = total tree volume with bark (m3)
d = diameter at breast height at a height of 1.3 m (cm)
h = total tree height (m)
a0, a1, a2, b0 = model parameters: 0.0000253, 1.6939421, 1.4175090 and 0.0000680, respectively.
Biomass
Among the most common alternatives for estimating biomass in forests are destructive (Figueroa-Navarro, Ángeles-Pérez, Velázquez-Martínez, & De los Santos-Posadas, 2010; Soriano-Luna, Ángeles-Pérez, Martínez-Trinidad, Plascencia-Escalante, & Razo-Zárate, 2015) and non-destructive methods (Razo-Zarate et al., 2013; Rodríguez-Laguna, Jiménez-Pérez, Aguirre-Calderón, Treviño-Garza, & Razo-Zárate, 2009;). In this study, we chose to employ the latter, for which we used the basic wood density value of 0.46 g·cm-3 reported by Vázquez-Cuecuecha, Zamora-Campos, García-Gallegos, and Ramírez-Flores (2015).
The data collected for volume (actual stock [ER], m3) at each stage of development were multiplied by the corresponding wood basic density (D, kg·m-3) to find the biomass value (B, kg) based on the equation
Aboveground carbon
Aboveground carbon (C, kg) at each forest development stage was estimated with the value determined from the biomass equation (B, kg) multiplied by the carbon coefficient (CC, 0.5) used by different authors (Petersson et al., 2012; Rodríguez-Laguna, Jiménez-Pérez, Meza-Rangel, Aguirre-Calderón, & Razo-Zárate, 2008).
Statistical analysis
With the data collected in the field and using equations, composite variables were derived and submitted to an analysis of variance (P ≤ 0.05) and Tukey's multiple comparison of means tests, to identify differences between mean carbon contents of forest development stages. Data were transformed with natural logarithm (ln) and the Kolmogorov-Smirnov normality test was applied. Statistical analysis was performed using the Minitab® version 18.1 software (Minitab, 2017).
Results and Discussion
Tree measurement characteristics by stage of development
According to Table 1, the area incorporated into the silvicultural management of the Atopixco ejido, which is at different stages of development, had trees with diameters ranging from 0.15 cm at the brinzal stage to 42 cm for adult trees. It should be clarified that the areas regenerated naturally, and seeds often fall in years after the regeneration cut, which is why there are trees with heights less than those mentioned for each stage of development. The values recorded are similar to those described by Aguilar-Luna (2018) and Aguirre et al. (2019) for P. patula, which indicates that the non-destructive method provides reliable estimates of the carbon content of this species. Additionally, there are high densities in the forest stand structure causing the presence of dominated or suppressed trees that are characterized by being thin and of low height.
Table 1 Tree measurement variables by stage of development of Pinus patula in the ejido Atopixco, Zacualtipán, Hidalgo.
| Stage | Density (trees·ha-1) | Diameter (cm) | Height (m) | ||||
|---|---|---|---|---|---|---|---|
| Minimum | Average | Maximum | Minimum | Average | Maximum | ||
| Brinzal | 2 217 | 0.15 | 0.79 ± 0.2 | 3.02 | 0.13 | 0.67 ± 0.2 | 1.7 |
| Monte bravo | 3 447 | 0.4 | 1.72 ± 1.0 | 4.81 | 0.29 | 1.49 ± 0.4 | 3.5 |
| Vardascal | 1 817 | 1.1 | 4.06 ± 2.6 | 9 | 0.8 | 2.87 ± 1.0 | 5.2 |
| Latizal | 2 093 | 2 | 11.35 ± 11.3 | 24 | 2 | 9.93 ± 7.7 | 15 |
| Fustal | 937 | 13 | 26.46 ± 27.9 | 42 | 8 | 23.19 ± 16.8 | 29 |
± population variance
Based on the proposed stages of forest development, Figure 3 shows that the growth dynamics for tree diameter and height was higher for the vardascal to latizal stage, with accelerated growth when moving from latizal to fustal.
Aboveground biomass and carbon content
The analysis of variance showed that aerial biomass and carbon content differed significantly (P ≤ 0.0001) among forest development stages. According to Table 2, Tukey's comparison test grouped each stage as a category. Trees at fustal stage had on average 294.8 kg of biomass. In the transition from the latizal stage to the fustal stage, the greatest difference in biomass of up to 271 kg was recorded in a period of less than 10 years, reflecting the high rate of carbon sequestration by the cultivated trees. For carbon content, the values showed the same trends.
The results reaffirm that managed forests fix high amounts of CO2 according to stand age (Avendaño et al., 2009; Pan et al., 2011) and agree with Razo-Zárate et al. (2013), who mention that the rate of carbon fixation by photosynthetic processes is higher in young stands compared to mature stands.
Table 2 Aboveground biomass and carbon content per tree during the development stages of Pinus patula cultivated forest in the ejido Atopixco, Zacualtipán, Hidalgo.
| Stage | Biomass (kg) | Carbon (kg) |
|---|---|---|
| Fustal | 294.82 ± 87.44 a | 147.41 ± 43.72 a |
| Latizal | 23.72 ± 2.78 b | 11.86 ± 1.39 b |
| Vardascal | 1.10 ± 0.34 c | 0.55 ± 0.17 c |
| Monte bravo | 0.12 ± 0.05 d | 0.06 ± 0.03 d |
| Brinzal | 0.02 ± 0.02 e | 0.01 ± 0.01 e |
Mean values (± standard deviation) with different letter indicate significant differences in biomass and carbon content among developmental stages, according to Tukey's test (P ≤ 0.05).
Aboveground biomass
Aboveground biomass in cultivated forests consist of the trees established after regeneration cuts are applied, where the number of trees is gradually modified by cultivation activities such as pre-thinning cuts and commercial thinning. The highest tree density is present at the juvenile stages, highlighting monte bravo with averages of 3 447 trees∙ha-1. Thus, with the management in the following stages of development, tree density decreases, favoring growth and development of trees that will reach the final harvest (mature forest); that is, those that will generate the best material, environmental and economic benefits.
Figure 4 shows tree density and biomass by stage of development, highlighting the fustal stage with the lowest density (937 trees·ha-1) and highest biomass (162.8 Mg·ha-1). This reflects the effect of thinning as part of silvicultural management, because it reduces competition between trees and favors the increase in volume and biomass of residual trees (Ramírez-Martínez, De los Santos-Posadas, Ángeles-Pérez, González-Guillén, & Santiago-García, 2020), which are efficient for carbon sequestration and storage. In this regard, Fragoso-López et al. (2017) mention that it is necessary to implement sustainable forest management that favors soil productivity to achieve maximum yield in a forest; likewise, Ramírez-Martínez et al. (2020) conclude that the management of tree density is essential to maximize timber yield and that the redistribution of tree spacing will improve total production in volume and take advantage of the potential productivity of P. patula.
Figure 4 Average aboveground biomass and tree density by stage of development in the cultivated forest of Pinus patula in the ejido Atopixco, Zacualtipán, Hidalgo
According to that reported by Chávez-Aguilar et al. (2016), in a forest of the same region and species of about 80 years old (without silvicultural intervention), the total aboveground biomass was on average 208 Mg·ha-1; a value 21 % higher than that estimated in the present study with an approximate age of 23 years (162.8 Mg·ha-1) for the fustal stage. Similar values were found by Monárrez-González et al. (2018), who report that, at the ecosystem level, coniferous forest and broadleaf forests store 179 Mg·ha-1 and 153 Mg·ha-1, respectively.
Other studies by Soriano-Luna et al. (2015) and Figueroa-Navarro et al. (2010), using destructive methods, report a total of 166.6 Mg·ha-1 of biomass for forests in the study area; that is, a difference of only 2.3 % compared to the present estimate at the fustal stage.
Carbon content
The information in Table 3 reveals the direct relationship between the stage of forest development and the amount of carbon stored. During the juvenile stages, competition for light, water and nutrients promotes rapid growth in height; however, through intermediate treatments (thinning) it is possible to redistribute biomass and carbon, favoring fixation in the stem by an increase in the measurement characteristics of residual trees in the intervention areas.
Table 3 Estimates of aboveground biomass and carbon for each stage of development of the Pinus patula cultivated forest in Atopixco, Zacualtipán, Hidalgo.
| Stage | Density (trees·ha-1) | Biomass (Mg·ha-1) | Carbon (Mg·ha-1) |
|---|---|---|---|
| Brinzal | 2 217 | 1.9 | 0.94 |
| Monte bravo | 3 447 | 7.5 | 3.73 |
| Vardascal | 1 817 | 10.1 | 5.05 |
| Latizal | 2 093 | 36.4 | 18.18 |
| Fustal | 937 | 162.8 | 81.4 |
The management of the Atopixco ejido forest, based on the silvicultural development method, favors its productive capacity, the economic activity that timber harvesting represents and the potential of sustainable forest management for climate change mitigation.
Table 4 compares six results of studies of carbon content in other forests in the country, regarding only the adult stage (fustal). Concerning estimates made in similar ecosystems, the results of the present study are close to those reported by Rodríguez-Laguna et al. (2009) and Figueroa-Navarro (2010), taking into account that the estimation models are different; in addition, they confirm the versatility of alternatives for carbon quantification in forest ecosystems by non-destructive methods.
Despite considering an average variation of 22 % in terms of estimating carbon content per unit area, remote sensing techniques, used by Fragoso-López et al. (2017) and Aguirre-Salado et al. (2009), are tools that allow rapid estimates and are more efficient in terms of human, material and financial resources compared to direct measurements.
Table 4 Estimates of carbon stored in temperate forests in Mexico.
| References | Estimation method | Ecosystem | Geographic location | Estimated carbon* (Mg C·ha-1) |
|---|---|---|---|---|
| Aguirre et al. (2009) | SPOT 5 HRG remote Sensing (regression and k-nn) | Pine-oak | Hidalgo | 63.98 |
| (managed) | ||||
| Fragoso-López et al. (2017) | RapidEye-4 remote sensing | Sacred fir | Hidalgo | 105.72 |
| B = 0.0713 D2.5104 | (no managment PNA) | |||
| Hernández-Moreno et al. (2020) | B = 0.1549 DBH2.3572 | Pine-oak | Michoacán | 128.44 |
| CC = 0.07744 DBH2.3572 | (managed) | |||
| Álvarez and Rubio (2013) | Model CO2FIX v3.2 | Pine-oak | Oaxaca | 118.6 |
| (managed) | ||||
| Rodríguez-Laguna et al. (2008) | B = a0 * DBHa1 | Pine-oak | Tamaulipas | 82.9 |
| (no managment PNA) | ||||
| Acosta-Mireles, Carrillo-Anzures, and Díaz-Lavariega (2009) | B = 0.0948 * DBH 2.4079 | Pine-oak | Tlaxcala | 118.3 |
| (managed) | ||||
| Current study (2021) | B=AS * D | Pine-oak | Hidalgo | 81.4 |
| (managed) |
*For the conversion of biomass (B) to carbon (C) the equation 𝐶 = 𝐵 ∗ 𝐶𝐶, where CC = carbon coefficient (0.5) was used. DBH: diameter at breast height, AS: actual stock, D: wood density, PNA: protected natural area.
Conclusions
The use of tree measurement and wood density variables as calculation inputs for rapid estimates of biomass and aboveground carbon content are very useful for the quantification of the ecosystem services provided by the forest. In this case, carbon storage in the Atopixco forest represents 81.40 Mg C·ha-1. The classification by stages of development represents an important tool for marketing strategies of environmental services for carbon sequestration, which has imminent market potential for the mitigation of climate change impacts. The use of non-destructive methods guarantees the practical usefulness to generate quantitative estimates of aboveground carbon in forests and its advantages include low cost and relative speed; however, it does not consider important reservoirs in terms of forest carbon content, such as soil.
