Materials and Methods

The timber harvest of the main conifer species used in the Mexican forest industry, obtained from the natural forests of the GMZs, was determined. The GMZs defined by ^{Conafor (2016)} and distributed across the entire forest area of the country were selected; they were chosen because they have a level of regionalization that allows them to be used as areas for the management of forest species at the national level (^{Flores, 2019}).

Two of the most harvested genera, which contribute 73.4 % of the national timber forest production in 2017, were analyzed (^{Semarnat, 2020}): Pinus and Abies. These taxa are part of the cold temperate forests of the country's mountain ranges: Western Sierra Madre, Eastern Sierra Madre, Transversal Neovolcanic Axis, Southern Sierra Madre, Central American Sierra Madre and Chiapas Highlands. They are found in pure areas or mixed with other conifer species or with oaks (^{Rzedowski, 1979}).

For each genus, the annual and average timber harvest was estimated from the databases of the species included in the state reports of the Ministry of Environment and Natural Resources (Semarnat, 2013 to 2017). The percentage contribution of each taxon to the total national volume was also calculated. This information was used because it is a record of the forest production by species, utilized to prepare the Statistical Yearbooks of Forest Production from 2013 to 2017.

On the other hand, the annual and average volumes of timber harvested from 2013 to 2017 in each GMZ were determined, using the ^{Flores method (2019)}. This method consists in making the proportional allocation of the average timber forest production (m3r) of each genus based on the distribution of the species, which was determined based on the geographic data (latitude and longitude) of the plots of the National Forest and Soil Inventory 2004-2007 (^{Conafor, 2018}) depicted with the QGIS software (http://qgis.osgeo.org) (^{QGIS Development Team, 2015}). In addition, the average timber harvest was calculated for the species with the highest contribution in the most productive GMZs.

Finally, the volumetric increments were measured at a compound interest rate for the Pinus and Abies genera with their respective species, and for the GMZs, using the average annual growth rate (AAGR), which is used in medium- and long-term periods (^{Moctezuma and Flores, 2020}). In the case of the present study, the five-year period was medium-term. Its mathematical function is as follows (^{Cuevas- Reyes, 2017}):

Where:

Fv = Final value for the period

Iv = Initial value for the period

n = Number of years of the period

In those instances in which certain species and GMZs exhibited series from which records for one or more years were absent, their AAGR was not considered because they were not comparable.

Data on timber volumes harvested by state are registered and available for consultation in the Zenodo repository (https://zenodo.org/), a European open access online repository of research results data. Reference must be made to the citation of this article in order to access this database.

Results

The timber harvest of 20 conifer species in the GMZs of Mexico was determined using state records for two genera. Specifically, Pinus had the highest contribution to timber volume with 19 species and Abies the lowest with one taxon (Table 1). As for the domestic timber harvest, the volumes of P. patula Schiede ex Schltdl. & Cham., P. douglasiana Martínez, P. maximinoi H. E. Moore, P. pseudostrobus Lindl., P. teocote Schiede ex Schltdl. et Cham., P. oocarpa Schiede ex Schltdl. et Cham., P. montezumae Lamb., and P. devoniana Lindl. timber made up 81.39 % of the total for the genus in the period considered; while in the case of sacred fir, A. religiosa, the percentage was 100 % because it was the only Abies species recorded in the consulted sources.

Considering the total timber harvest as the sum of the average forest production of Pinus species plus that of Abies, the contribution of this genus was 84.92 %, which shows that these are the dominant genera in the production of m3 of timber in Mexico. As for the AAGRs of both genera during the study period (2013 to 2017), Pinus increased its harvested volume by 8.25 %, while Abies registered a decrease of -2.14 %. The pine taxa with the best AAGR were P. devoniana, with 86.94 %; P. douglasiana, with 77.94 %, and P. herrerae Martínez, with 74.98 %.

In regard to the annual and average volumes of timber harvested by genus in each GMZ, the Pinus species were distributed among 20 GMZs and accounted for 84.55 % of the total timber harvest; the Abies species was located in five GMZs, and its timber harvest amounted to 15.45 % (Table 2). Therefore, these areas have an important potential for timber harvesting at the national level.

In particular, eight GMZs (X.3, V.3, XII.5, XII.3, X.1, XII.2, III.1, III.2) excelled in the average Pinus timber harvest, as they contributed 84.14 % (Figure 1); however, only GMZ X.3 contributed a significant amount of Abies timber, i.e., 72.16 %; this was due to the presence of a higher density of trees in that zone (Figure 2).

Figure 1 Pinus timber volume percentage harvested in the germplasm movement zones. 

Figure 2 Abies timber volume percentage harvested in the germplasm movement zones. 

The AAGR of the GMZs with Pinus was 16.95 %; while in those with the Abies genus, it decreased by -2.14 %; for the zones as a whole, on a weighted basis, it amounted to 13.78 %. The negative rate of Abies shows that the production of the species decreased during the analyzed period, which led to a decrease in its AAGR. The first year of analysis was 67 242.09 m3, and the final year was 60 327.57 m3; despite the fact that in the intermediate years the values were above the initial value.

In relation to the average timber harvest, the species with the highest contribution were: P. patula, P. douglasiana, P. maximinoi, P. pseudostrobus, P. teocote, P. oocarpa, P. montezumae, and P. devoniana. A. religiosa also contributed significantly (Table 3).Revista Mexicana de Ciencias Forestales Vol. 12 (66) Julio - Agosto (2021)

Table 3 shows that the best zones for the Pinus genus, in order of importance, were X.3, V.3 and XII.5; while for Abies it was the X.3 zone. In general, the best GMZ was X3, which may be related to the density of trees in it and to the growth and increase in the number of forest stands in the zone (^{Flores, 2019}).

Discussion

This study estimated the volume of timber harvested from the Pinus and Abies genera in the GMZs, which allowed to identify that eight pine species and one fir species had the greatest contribution to the total volume recorded in the germplasm movement zones. The results of the study showed that the amount of m3 of timber in the GMZs was variable due to the presence/absence of different taxa in each zone.

According to the information obtained in the annual state reports of the General Directorate of Forest and Soil Management of Semarnat: P. patula, P. douglasiana, P. maximinoi, P. pseudostrobus, P. teocote, P. oocarpa, P. montezumae, and P. devoniana had the highest participation in the GMZs. Nevertheless, during the data analysis there were some states that were not included because they lacked records on the volume harvested by species, and only had them by genus; for example, Durango. The timber harvest of each taxon in the GMZs presented in this paper is an approximation of what is harvested.

In general, Pinus had an upward trend as measured by its AAGR for the 2013 to 2017 harvest, and so did Abies as well; the latter genus, however, exhibited a significant decrease in 2017. This is due to the fact that in Mexico there is a greater distribution of pine species than of firs, and consequently, the contribution of Pinus to the national timber output and GDP is higher (^{Moctezuma and Flores, 2020}). Still, there may be increases in the volume harvested, as indicated by ^{Torres-Rojo et al. (2016)} who point out that the national forest harvest is variable and does not meet the annual demand for products, having a deficit of $6 165.60 million USD (^{Conafor, 2019}).

As for the timber harvest by GMZ, those GMZs with significant Pinus participation had an average annual output of over 20 000 m3 per year; while that of Abies was >50 000 m3. In this regard, zones X.3, V.3, XII.5, XII.3 and X.1 are considered very relevant; since, according to ^{Flores (2018}, ²⁰¹⁹), a GMZ is taken as highly productive when the timber volume harvested in it is >40 000 m3. This characteristic is related to tree density in natural forests (^{Ruelas and Dávalos, 1999}). In both genera, the greatest volume harvested was concentrated in the zones located at the center of the country, which shows the need to implement sustainable management in that region, since the harvesting of the species studied is higher there than in the rest of the zones analyzed.

The eight GMZs that contributed most to timber harvesting require management in order to keep a sustainable production by species, for example, by legally authorizing forest harvesting and regulating the harvesting of forests subject to illegal logging (^{Caballero, 2005}). As for the rest of the GMZs, strategies must be implemented to augment the logging volumes and the forest zones through actions that will increase the survival rate of the plants used in reforestation programs, and commercial forest plantations must be established.

The species with the greatest participation in the timber harvest in the GMZs have great relevance for the forestry sector and the forest industry of the country. Thus, P. patula and P. oocarpa are used in plantations because of their rapid growth and of the timber volume that they produce (^{Dvorak, 2012}; ^{Gwaze et al., 2000}); P. pseudostrobus, P. oocarpa, and P. devoniana have the potential to restore areas with a low or medium degradation (^{Flores et al., 2021}); P. douglasiana, P. montezumae, and P. maximinoi are used in the sawmilling industry for the production of square timber (Zavala and Hernández, 2000); P. devoniana is used in the paper industry (^{Escoto et al., 2017}); P. teocote is important for resin production (^{Wadsworth, 2000}); P. ayacahuite has an ornamental use, as a Christmas tree (^{Álvarez et al., 2009}), and A. religiosa is used to manufacture pulp for paper, packing boxes, poles, beams, pilings, firewood and charcoal (^{Arriola et al., 2015}).

The results obtained on the annual timber harvest volume of the species under study show that regionalization by GMZs is an option for tree stand management, since it is based on the number of species and their distribution. These elements determined the variability of the volume of timber in the zones, as well as the differences in the amount of wood from the conifer species analyzed.

Conclusions

Pinus and Abies are the main conifer genera that contribute to the timber harvest in the GMZs; and of these, the greater contribution comes from pine taxa, in particular: Pinus patula, P. douglasiana, P. maximinoi, P. pseudostrobus, P. teocote, P. oocarpa, P. montezumae, and P. devoniana; while in the case of fir, it comes from A. religiosa. This confirms the proposed hypothesis that timber harvesting in the GMZs depends on these species.

In regard to the impact of the results obtained based on the research questions formulated, the information generated on timber production in the GMZs was particularly helpful.

On the other hand, this study is a basis for decision makers, as it will allow them to prioritize activities in the preparation of forest development plans and programs to increase the management efforts in the zones with the highest timber harvest in order to guarantee the sustainability and conservation of the species they harbor.

In terms of growth, pine timber shows a positive trend over the five years of the study, which highlights its importance in production. The opposite is true of fir, which exhibits a negative behavior. This calls for the intervention of managers and officials from various government agencies and non-governmental organizations, as well as from national and international research centers, and public and private academic centers related to forest activities that contribute to the orientation towards forest research, management and transfer of the monetary, material and talent resources allocated to them.