Introduction

Terrestrial ecosystems provide a host of ecosystem services to humankind, including food, fodder, fiber, fuel and timber forest products. The demand for land products and services is degrading the ecosystems. About one third of the world's arable land is affected by degradation, which results in an increase in the number of people living in poverty in developing countries (^{Boer et al., 2017}).

Soil degradation is a serious global problem for many communities and is related to food insecurity, vulnerability to climate change and poverty (^{Barbier and Hochard, 2016}). This degradation comes in various forms, including soil nutrient depletion, salinization, agrochemical contamination, soil erosion, vegetative degradation (e.g., deforestation) as a result of overgrazing, and the clearing of forests for use as farmland (^{Scherr and Yadav, 2001}).

Deforestation is a major problem for developing countries because it causes loss of biodiversity and increases the greenhouse effect (^{Hein et al., 2018}). Most deforested areas occur in the temperate and subtropical zones (^{Angelsen et al., 1999}).

Around the world, there are several causes identified as the main drivers of deforestation: expansion of agricultural land, logging and firewood extraction, overgrazing, fires, mining, urbanization, military conflicts and tourism (^{Chakravarty et al., 2012}). All of these must be addressed by each country in order to reduce their impacts. Deforestation brings with it some problems that globally affect natural resources and the human population (^{Chakravarty et al., 2012}): climate change, loss of soil and water resources, flooding, decline in biodiversity, economic habitat loss, as well as social consequences. There are some essential strategies for reducing deforestation, which vary by region and time (^{Hein et al., 2018}).

In Mexico, temperate forests extend over an area of about 323 305 km² (around 17 % of the country), provide timber and non-timber resources, and are home to species essential to its biodiversity (^{Galicia et al., 2015}). However, these ecosystems have been reduced in almost 45 % of the country, due to increased land degradation (^{Semarnat-Colegio de Postgraduados, 2002}). National projections for deforestation rates have varied from 260 000 to 1 600 000 ha year^-1 over the past three decades, according to the record of academic studies and official reports (^{Couturier et al., 2012}).

The main causes of deforestation in Mexico are land use change for agriculture (82 %), illegal logging (8 %), as well as forest fires and diseases (6 %) (^{Goldstein et al., 2011}). The government's response has been to legislate and establish public policy programs (^{Goldstein et al., 2011}; ^{Cotler et al., 2013}), such as those of the Federal Environmental Protection Agency (Profepa), forest certification programs, afforestation and reforestation efforts, the creation of natural protected areas, and payment for environmental services programs. However, some other programs have favored or encouraged deforestation, including Procampo and Alianza para el Campo, since they promote agricultural activities at the cost of reducing forest areas (^{Schmook and Vance, 2009}).

Mexico's National Forestry Commission (Conafor) established Germplasm Movement Zones (GMZs), equivalent to seed zones, defined as areas with similar ecological and climatic characteristics that host populations with relatively uniform genotypes or phenotypes (^{Flores et al., 2014}), in order to reduce the movement of germplasm out of its natural distribution. Zoning helps to increase the survival rate of established seedlings in the field, which is affected when species are planted outside their local distribution; therefore, they exhibit high mortality rates and poor adaptation to different growing conditions (^{Rehfeldt et al., 2014}). Although these zones have been defined, there is still a movement of germplasm among the GMZs that affects plant growth and diversity.

The reforestation program in Mexico is a permanent strategy to recover and increase forest areas and reduce forest land degradation; for example, in 2020 100 000 ha were reforested (^{FAO, 2020}). However, the main problem is the low survival rate of seedlings (^{Burney et al., 2015}) which is associated with poor quality seedlings (^{Escobar-Alonso and Rodríguez, 2019}). The low survival percentages cause that the goals of reforestation are not fulfilled, which seek to restore and to conserve the forests of the country.

In spite of the efforts to restore forests, none of the current degraded areas have been considered, nor have the level of degradation or the number of seedlings produced in nurseries per ecological zone or GMZ. The first is an area with wide formations of natural vegetation, but relatively homogeneous, similar in physiognomy although not necessarily identical (^{FAO, 2001}). In order to propose a national restoration strategy, it is necessary to evaluate and use this information. Therefore, the objective of this research study was to assess the restoration potential of conifer forests in order to reduce land degradation by GMZs, by comparing priority areas for restoration with the most effective reforestation efforts.

In this regard, the following questions were raised: 1) Does the amount of seedlings vary among conifer species produced in the nurseries?; 2) Is the deterioration of land that is home to conifers dissimilar in different production zones and restoration zones by ZMG?; 3) Does the survival rate of seedlings vary for conifers by GMZ?; and 4) Is the restoration potential of conifers different within each GMZ?

This information is essential for planning reforestation actions to be initiated in order to restore those areas with soil degradation problems through the use of conifers.

Materials and Methods

The restoration potential to reduce soil degradation in the Germplasm Movement Zones (GMZ) of Mexico was analyzed (^{Conafor, 2016}), based on comparisons between priority areas (production areas and restoration areas) and effective reforestation efforts (percentage survival of planted seedlings, number of nurseries, germplasm production units and germplasm banks).

Production areas are forest lands that, according to the structure and composition of vegetation, are subject to forest exploitation (^{Semarnat, 2015}); while the restoration zones are forest areas with degradation evidence, with different degrees of progress and that constitute a risk from the loss of the forest resource that they may represent (^{Semarnat, 2015}). The germplasm production units are areas established in natural stands, plantations or nurseries, with individuals belonging to a forest species, selected by their genotype or phenotype, whose origin is well identified, and which are used for the production of fruits, seeds or vegetative material (^{Conafor, 2016}).

From ^{Conafor data (2019a)}, the most commonly produced conifer genera and species were defined, and their average values of total seedlings planted from 2016 to 2018. This database has information on reforestation and conservation programs at the national level, which is used annually to write government reports. The conifer taxa were chosen because they cover most of the GMZs and produce different services for the population; for example, environmental services, timber production (^{Díaz-Núñez et al., 2016}), and organic carbon storage (^{INECC, 2015}).

Based upon the information of the National Forest and Soil Inventory, corresponding to the 2004 - 2007 period (^{Conafor, 2017}), the surface land of the production and restoration areas was estimated using the production and restoration maps of ^{Conafor (2020)}. This institution classified six types of forest land productivity: High production forest land (II.A); Medium production forest land (II.B); Low production forest land (II.C); Land in arid zones (II.D); Land for reforestation (II.E); and Land for forestry activities (II.F). In addition, five types of soil degradation: Forest lands with high soil degradation (III.A); Severely eroded forest land (III.B); Forest lands with average degradation (III.C); Forest lands with low degradation (III.D); and Degraded forest land with management for restoration (III.E). Of these, II.C, and III.C and III.D were chosen as priority areas, because they could be restored in a short time (^{Flores et al., 2019b}).

The effective reforestation efforts in each GMZ were assessed using the percentage of seedling survival defined by ^{Conafor (2010)}, as well as the number of established nurseries (N), defined Germplasm Production Units (GPU) and installed germplasm banks (GB), according to Conafor's records. This information was used because it directly supports the production of coniferous seedlings in the country. In each GMZ, the area that can be reforested with 1 100 plants ha^-1, and the average survival rate were estimated based on their registered percentages (^{Conafor, 2010}). GMZs with a high planted seedling survival rate and the highest amount of N, GPUs and GBs were considered the areas with the most effective reforestation efforts.

Finally, the priority areas for restoration were compared with those with the most effective reforestation efforts in order to define the restoration potential sites.

Results

Effective reforestation efforts

For the seedling survival percentages per GMZ, 10 zones were identified with values of 60.38 to 74.95 %; eight, with 46.99 to 58.55 %; five zones with 34.81 to 37.55 %, and four zones with values of 11.85 to 14.72 % (Figure 1).

Figure 1 Survival rates (green colors) and nurseries (yellow circles), germplasm production units (blue circles), and germplasm banks (pink circles) in the GMZs. 

Pinus oocarpa varied in all survival rates, while P. pseudostrobus and P. teocote were the most frequent pines, with higher, medium, and lower survival rates; P. devoniana occurred more frequently in high and medium percentages; in contrast, P. ayacahuite, P. cembroides, P. devoniana, P. douglasiana, P. durangensis, P. engelmannii, P. hartwegii, P. leiophylla, P. maximinoi had medium percentages, and P. cembroides, P. douglasiana registered low values.

As for the number of nurseries, one zone had the most (72), five had a considerable number (13 to 19), 17 zones had few (1 to 8), and four zones had none (Table 3). One zone accounted for the largest number of GPUs among the established units (14), while 17 zones had one to six, and nine zones had none. On the other hand, seven zones had very few GBs (1 to 5), and 20 zones lacked germplasm banks altogether (Figure 1, Table 3).

Table 3 Number of nurseries, germplasm production units and germplasm banks by GMZ. 

GMZ	Nurseries	GPUs	GBs
I.1	5	2	-
I.2	2	-	-
III.1	5	3	-
III.2	5	6	-
III.3	15	6	1
III.4	7	3	-
IV.1	-	-	-
V.1	1	1	-
V.2	2	1	-
V.3	8	5	1
VIII.1	4	-	-
VIII.3	-	-	-
VIII.4	-	-	-
IX.2	2	1	-
X.1	17	3	2
X.2	16	5	1
X.3	72	14	5
XII.1	1	-	-
XII.2	1	2	-
XII.3	13	5	-
XII.4	7	4	-
XII.5	7	1	1
XIII.1	4	-	-
XIV.1	19	3	-
XIV.2	-	-	-
XIV.3	2	-	-
XV.1	8	3	-

GPU = Germplasm Production Unit; GB= Germplasm Bank.

Discussion

This research evaluated the potential of certain Mexican conifers to reduce forest land degradation in the GMZs, through the reforestation program, in order to provide a basis for the implementation of a restoration strategy for temperate forests. The results of this study showed that the number of seedlings produced in nurseries, land degradation and survival rate were different for the selected conifers, and their restoration potential varies among the GMZs. Consequently, during the reforestation process in the GMZs, the species studied were able to restore many forest lands with medium (III.C) and low degradation (III.D).

Pinus, Abies, Callitropsis, Cupressus and Taxodium were the main genera used in nurseries and reforestation programs. Pinus was the most produced during the 2016 - 2018 period; while the lowest figures corresponded to Taxodium; there was little production of Abies, Cupressus and Callitropsis. The number of seedlings in nurseries depends on the seeds available and collected in state stands (^{Conafor, 2019b}).

Pinus species are the most widely distributed in the country, compared to other conifers (^{Farjon and Filer, 2013}); therefore, they have been the most commonly used by nurseries. For example, P. cembroides, P. oocarpa and P. pseudostrobus are distributed along different temperate mountain ranges as pure conifer and mixed forests (^{Rzedowski, 1979}; ^{Flores et al., 2011}; ^{Farjon and Filer, 2013}; ^{Flores et al., 2019a}) and are the most widely used for the production of seedlings. On the other hand, some pines are very important for obtaining wood (^{Sánchez-González, 2008}), for the sawmill industry and for resin production (^{Fuentes et al., 2006}), as well as for the establishment of commercial plantations (^{López-Upton et al., 2005}); therefore, they are quite frequently grown in nurseries each year.

Land degradation (II.C, and III.C and III.D) varied among the GMZs and presented different species of conifers; that is, the productivity of forest lands in II.C registered a smaller area of degradation, with 24 species (except P. jeffreyi), than land degradation types III.C and III.D with 16 species (Cupressus sempervirens, P. greggii, P. hartwegii, P. jeffreyi, P. lawsonii, P. maximartinezii, P. montezumae, P. patula, T. mucronatum were absent). This proved that the areas of II.C can be restored in a short time, but they need a great investment; for example, between the 2002 - 2007 period, almost 157 653 ha year^-1 (27.85 % of the national forest area) were deforested in Chihuahua, Durango, Coahuila, Guerrero, Nuevo León, San Luis Potosí, Zacatecas and Tamaulipas (^{Masek et al., 2011}), and large investments have been required for their restoration.

The efforts made to implement the reforestation program have been significant in the restoration areas, but are still insufficient for some states, despite the fact that reforestation and soil improvement activities have been developed since 1999 (^{Ceccon et al., 2015}).

The reforestation rate in the country is not enough; it is estimated that, in order to recover 43.5 million ha of degraded soils 400 000 ha must be reforested per year and approximately 68 million U.S. dollars must be invested; however, the Mexican government reforests only around 193 000 ha per year (^{Ceccon et al., 2015}) and invests merely 32 million U.S. dollars (^{Sánchez-Velásquez, 2009}).

The survival rates of the evaluated conifers varied among the GMZs. Most species had different survival values; P. pseudostrobus and P. teocote were the most frequent, with higher, medium and lower survival rates. This shows that species with high percentages of field survival ―e.g., P. cembroides with 81 % (^{Gómez-Romero et al., 2012}); P. pseudostrobus with 86 to 62 % (^{Gómez-Romero et al., 2013}); P. devoniana with 71 % (^{Blanco-García et al., 2008})― exhibit a high production of seedlings in nurseries and could be used for restoration works. Species with medium or low production also have an important survival rate, as indicated by ^{Gómez-Romero et al. (2012)} for P. hartwegii (89 to 82 %) and P. devoniana (80 %).

The species selected for nursery production should be tolerant to water deficit or even drought ―as it happens with P. cembroides, which is resistant to adverse conditions of rainfall, soil, frost, drought and high temperatures (^{Flores et al., 2018}; ^{Gutiérrez-García et al., 2015})― as, due to climate change, they are likely to experience drier conditions and water stress during their growth in the field. In addition, the selected taxa must have the ability to grow in substrates that limit their establishment, as is the case of P. leiophylla, a taxon whose seedlings reach significant height when produced on mine booty substrate, while P. devoniana has appreciable growth (^{Osuna-Vallejo et al., 2017}). For eroded areas, soil formation through the use of conifer taxa is another aspect to consider in species selection.

The restoration potential of conifers was different within the GMZ. The results clearly suggest that in a relevant number of zones (eight) their priority areas could be reforested, since both the production of seedlings and their different survival percentages indicate it. In this regard, the number of planted seedlings (1 100 plants ha^-1) with their percentages of survival in the field are sufficient to cover these areas, although they only represent 8.80 % of the total of areas II.C, III.C and III. D.

Pinus devoniana, P. oocarpa, P. pseudostrobus, P. halepensis, P. teocote, P. ayacahuite, P. douglasiana, P. lawsonii, P. maximinoi, P. montezumae, P. patula and A. religiosa were the taxa with the greatest presence in the GMZs.

In order to propose a program to restoration degraded areas, it is necessary to define different densities and species which support the restoration process, for example P. pseudostrobus, P. engelmannii, P. montezumae, P. greggii, P. arizonica and P. durangensis could be used to restore III.C and III.D areas (^{Flores et al., 2019}).

Seedling survival is an important factor to consider when reforesting. It is estimated that, in Mexico, reforested areas reach a low (36 %) (^{Wallace et al., 2015}) or medium (50 %) (^{Burney et al., 2015}) average of seedling survival after their first year, due to poor seedling quality and drought. Therefore, it is suggested that local seedlings be used in reforestation projects to increase the potential for acclimatization (^{Sáenz-Romero and Guries, 2002}) and reduce the risk of death from drought.

The X.3 zone was the most important for the restoration potential, because it includes many nurseries, GPUs and GBs; this shows that X.3 has a good effort within the reforestation program (^{Flores et al., 2019b}).

For forest owners, conifers are interesting trees to use in reforestation areas; thus, in the region surrounding the Monarch Butterfly Biosphere Reserve, their restoration potential has been an important factor in the decision to use them to reforest agricultural plots and degraded forests (^{Honey-Rosés et al., 2018}). In order to promote soil conservation practices with conifer taxa, the government has implemented the Forest Soil Conservation and Restoration Program, which pays a subsidy to landowners. This action aims to reduce the estimated land degradation in the country by 45 % (^{Semarnat-Colegio de Postgraduados, 2002}).

In the restoration areas, it is necessary to increase the efforts of the reforestation program (nurseries, GPUs and GBs); furthermore, restoration failures ―i.e., the high initial mortality, deficient growth and susceptibility to biotic and abiotic stressors, due to the misuse of the source and the genetic quality of the forest reproduction material― must be avoided (^{Godefroid et al., 2011}). Appropriate attention to the genetic quality of germplasm is important for a forest restoration that seeks to adapt tree species to changing conditions (^{White et al., 2007}).

Conclusions

In recent decades, the surface area of Mexico's temperate forests has been reduced due to increased land degradation. The reforestation program is an ongoing strategy to increase forest areas and reduce forest land degradation with Pinus, Abies, Callitropsis, Cupressus and Taxodium. Pinus cembroides, P. pseudostrobus, P. oocarpa, P. devoniana, P. engelmannii, P. montezumae and P. greggii, which add up to 76.36 % of the total production in nurseries during the analyzed period. In addition, these taxa are distributed in 27 GMZs, which have 7 418 975.30 ha of low-production forest land (II.C) and 9 389 577.70 ha of medium- and low-degradation forest land (III.C and III.D).

In the GMZs, 10 zones are identified with higher survival rates: eight with medium values, five with low values, and four with lower values. As for the number of nurseries, one area contains the majority of the nurseries; five areas include a considerable amount of them; 17 include few, and four areas include none.

One zone includes most of the established GPUs, while 17 zones have few units, and nine zones have none at all. In seven zones there are very few GBs, and none in 20. Eight GMZs have restoration potential, since they can be fully reforested, but zone X.3 alone includes more nurseries, GPUs and GBs, compared to the others.