Introduction

The area of temperate forests in the Chihuahua State is 7.4 million hectares, of which 86 % are for collective or ejidal use (^{Inegi, 2014}). In these forests, the climax floristic composition is represented mainly, by species of the Pinus and Quercus genera (^{Martínez et al., 2012}). The timber harvest in the entity is 1.16 million m³ average, which is equivalent to 18 % of the national production (^{Semarnat, 2017}); these data highlight the socioeconomic and biological importance of these ecosystems, as well as the need to implement forest monitoring programs and design strategies for the sustainable management of natural resources (^{Kleinn and Morales, 2002}).

Before reviewing the evolution of monitoring in the state, it is necessary to specify the differences between forestry and forest. The first refers to the collection of periodic information to assess the response of the forest to the application of management and harvesting treatments (^{Martínez et al., 2012}); while the second, in addition to including forestry, incorporates the measurement of indicators that address the complexity of ecosystems, considering the structure and functioning of populations, communities, landscapes and socio-economic and cultural indicators (^{Palmer, 2011}).

Forest monitoring in the beginning focused on knowing the quantity and distribution of forests through indicators of ecosystem structure, such as forest area, botanical composition and timber stocks for forestry purposes (^{Päivinen et al., 1994}; ^{FAO, 2017}). Subsequently, with the growing discussion about environmental chaos and the incorporation of theories on sustainability and climate change, the need to generate knowledge about ecosystem functions, such as carbon sequestration, water production, biodiversity conservation, productivity and degradation of soils were recognized (^{Bugmann and Solomon, 1995}; ^{Wulder et al., 2004}; ^{Saarinen et al., 2018}). Currently, some variables related to social and cultural participation in forest management have been incorporated (^{FAO, 2017}).

One of the first forest monitoring schemes implemented in Chihuahua was the “Model Forest” network (Besseau et al., 2002). This system had an influence in Mexico through the establishment of monitoring plots in the forested areas of the state in 1995 (^{Martínez et al., 2012}). At the end of the 90’s, the international audit process for certification was promoted Forestry of the Forest Stewardship Council (FSC) (^{Ismail et al., 2011}) and recently, the Monitoring, Reporting, Verification (MRV) system as part of the mechanism of Reduction of Emissions of Effect Gases Greenhouse Caused by Deforestation and Forest Degradation (REDD +) (^{Palmer, 2011}; ^{Plugge and Köhl, 2012}).

However, the history of forest monitoring in Chihuahua dates back to the middle of the last century, with the first Permanent Silvicultural Research Site (SPIS) known as “El Poleo”, which was located in the municipality of Madera, in the year 1950 with the purpose of evaluating the effect of different cutting intensities on the timber increase (^{Mas and Pahua, 1989}). This was followed by a series of monitoring sites described in this document, until 2002, which were established by forestry technicians, with support from different agencies (^{Tena, 2000}; ^{Meléndez, 2001a}; ^{Meléndez 2001b}; ^{Martínez et al., 2012};^{Nava-Miranda et al., 2014}).

It should be noted that, during the years 2007, 2013 and 2014, 3 669 permanent sites were delimited, in 14 states of the Mexican Republic, which include those of Chihuahua, in whose temperate forests 857 sites were located. This effort was developed by the National Forestry Commission (Conafor), with the support of the Universidad Juárez del Estado de Durango (UJED) for the storage of information online, using the system called Monafor (^{Nava-Miranda, 2014}).

On the other hand, the influence of forest inventories in the monitoring of the forests of the Chihuahua State has also been important. The first inventory was carried out in the period 1961-1985, in which aerial photographs and field samples directed on the main wooded areas of the country were used as spatial references (^{SARH, 1992}). This set the pattern for the National Forestry Inventory of Great Vision (^{SARH, 1992}) to be implemented in 1991, in which for the first time the forest areas were captured on 1: 1 000 000 scale maps. In 1992, the National Inventory was initiated Periodic Forestry (^{SARH, 1994}), and as of 2004 the National Forest and Soil Inventory (INFyS, for its acronym in Spanish), at a scale of 1: 250 000, replaced it (^{Conafor, 2009a}). This concluded in November 2007, at which time the periodic remeasurements began in 10 % of the INFyS conglomerates, nationwide (^{Conafor, 2009a}).

Despite the local, regional and national interest in having optimal forest and forestry monitoring systems, there are few experiences that are considered well documented to form a reliable database that allows quantification of timber growths or increases, which is necessary to implement sustainable management systems in the local area of the forests of Chihuahua (^{Martínez et al., 2012}). This document shows a history of the SPIS established in the entity in the last 70 years. The information is useful for the development of projects that affect state ecological research, as well as historical evidence of silvicultural monitoring in the region.

Location of the SPIS

The history of forestry research monitoring in the Chihuahua State shows that between 1950 and 2004, 346 plots distributed in 16 Permanent Research Sites were established in different regions of the state (Table 1).

Table 1 Location of the SPIS in Chihuahua state.^* 

Name of the site	Location	Latitude	Longitude	Num. of plots
El Poleo	Mesa del Negro, municipality Madera	29º33’3.55”	108º 31’56.7”	9
Madera Experimental Area	El Largo Madera, South zone, Madera	29°06’45”	108°11’45’’	225
San Ignacio de Arareco	Ejido San Ignacio de Arareco, Bocoyna municipality, Chihuahua	27º40’21.6”	107º32’17.8”	16
San Juanito	Ejido San Juanito, Bocoyna municipality	28º0 47.7”	107º35’44.9”	16
Cusarare	Ejido Cusarare, Guachochi municipality	27º34’49.7”	107º28’13.3”	16
Retiro y Gumeachi	Ejido El Retiro y Gumeachi, Bocoyna municipality	27º59’5.6”	107º40’ 54.6”	16
Caseta de la Judicial	Ejido Chinatú, Guadalupe y Calvo	26º04’50.00”	106º45’15.00”	4
Rancho del Indio	Ejido Chinatú, Guadalupe y Calvo	26º01’15.00”	106º46’01.00”	4
Puerto El Sabinal	Ejido Chinatú, Guadalupe y Calvo	26º01’15.00”	106º47’55.00”	4
Llano Blanco	Ejido Llano Blanco, Guadalupe y Calvo, Chihuahua	26º03'34.0"	106º54'28.6"	12
El Zorrillo Air track	Ejido El Pinito, Guadalupe y Calvo	26º03’4.66”	106º58’42.51”	4
Las Cuevas	Ejido El Pinito, Guadalupe y Calvo	26º05’18”	107º00’24.4”	4
Regeneration area	Ejido El Pinito, Guadalupe y Calvo	26º05’38.5”	107º01’13.4”	4
Ojuelos o Aguaje	Ejido Redondeados y anexos, Guadalupe y Calvo	25º44’	106º51’	4
Junction at the Mesa de San Rafael	Ejido Redondeados y anexos, Guadalupe y Calvo	25º47’	106º47’	4
Regeneration area	Ejido Redondeados y anexos, Guadalupe y Calvo	25º05’	106º01’	4

* The location of 857 permanent plots established by Conafor (^{Nava-Miranda et al., 2014}) is not included, this information is protected by Universidad Juárez del Estado de Durango (UJED) on its page http://forestales.ujed.mx/monafor/inicio/creditos.php

For the documentary integration of the SPIS, revisions of the Permanent Forestry Research Sites projects were carried out since 1950. Existing databases were consulted in the archives of the Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias, as well as in the Forest Management Units of the Madera (0802), San Juanito (0805) and Guadalupe y Calvo, Chihuahua (0808) regions.

Figure 1 shows the spatial distribution of SPIS in the state, which includes the northern zone in the area of Madera, the center in the region of Bocoyna and Guachochi and the southern one in the municipality Guadalupe y Calvo. In the period from 2013 to 2014, 857 permanent plots were implemented in various state estates, with sizes ranging from 625 m² (25 × 25 m), 1 000, 1 500 or 2 500 m² in mixed masses (^{Nava-Miranda et al., 2014}).

Figure 1 Location of SPIS in Chihuahua State 

El Poleo Experimental Site

It was established in 1950; from then until 1980, the normal diameter, total height, height of the clean shaft and crown diameters of all trees were corrected every ten years, in nine plots of one hectare, each.

Initially El Poleo had two conditions: the so-called old growth, with mature structures, diameters greater than 30 cm with little presence of young trees; and the condition of young or second growth masses, with high densities per unit area. In both grow Pinus durangensis Martínez and Pinus arizonica Engelm., associated with different taxa of Quercus spp (^{Manzanilla, 1993}; ^{Tena, 2000}).

The structure of the plots is square, with 300 m per side and an area of nine hectares. Each one divided into nine experimental units of one hectare (100 × 100 m). In these plots four intensities of cut (IC) were applied, with four repetitions, with the intention of evaluating the effect of residual density on the increase in volume (Figure 2). Also, the effect of density on a control was evaluated (treatment without intervention) (^{Mas and Pahua, 1989}).

(Modified from ^{Manzanilla, 1993}).

Figure 2 Distribution of treatments at the El Poleo site. 

The volume and increment information for the 1950-1960 period shows evidence of the effect of residual density on the increases in the first ten years (Table 2). Higher annual values are observed in the control plots, 50 % and 75 % of cutting intensity, with 5.4, 5.2 and 4.2 m³ha^-1 of total tree volume (round), respectively; in the following decades, as well as in the average Annual Mean Increment (IMA, for its acronym in Spanish) evaluation, the treatments with less intensive cuttings (A, D, E and G) showed smaller increments compared to the treatments of greater intensity (B, H, I). The above is attributed to the effect of competition through density management, which considerably reduced the increase in biomass.

Table 2 Volume and increases in SPIS El Poleo for the period 1950-1980. 

Plot	IC %	VTA m3ha-1 and IMA in the period							Average IMA
		1950	1960	IMA	1970	IMA	1980	IMA
A	25	217.8	225.6	0.8	254.0	2.8	278.0	2.4	2.0
B	75	57.7	86.7	2.9	163.9	7.7	218.4	5.5	5.4
C	75	61.4	103.1	4.2	144.3	4.1	173.6	2.9	3.7
D	25	108.9	139.7	3.1	180.6	4.1	194.6	1.4	2.9
E	0	178.8	232.4	5.4	265.7	3.3	288.7	2.3	3.7
F	50	104	118.6	1.5	130.8	1.2	162.7	3.2	2.0
G	20	148.2	172.3	2.4	230.3	5.8	233.6	0.3	2.8
H	50	112.1	163.6	5.2	205.0	4.1	251.3	4.6	4.6
I	100	16.3	17.7	0.1	81.7	6.4	144.2	6.3	4.3

VTA= Total Tree Volume; IC= Cutting intensity; IMA= Annual Mean Increment in m³ha^-1 VTA; average IMA = average IMA for the studied period.

Discussion

The best decisions for the use, management and conservation of forest resources are based on the results of the analysis of reliable data series provided by forest monitoring, which bases its quality on the clear definition of its scale and aims; however, whatever these are, forest monitoring is usually an expensive process, especially because of the involvement of highly qualified scientific or technical personnel (^{Palmer, 2011}).

The SPIS of Chihuahua represent a good reference for forest monitoring in northern Mexico. Nevertheless, it is clear that this system has partially met its objective, due to the interruption in the periodic collection of information, a situation that can be related to the lack of financing, lack of operation of long-term projects and little involvement of local people or forest owners.

In recent years, efforts have been intensified for the establishment of Permanent Forestry Research Plots in Mexico, which including 857 plots in Chihuahua, as part of a monitoring strategy implemented by Conafor (^{Nava-Miranda et al., 2014}). The information on the measurements and remeasure of these sites, established in the last decade, is stored in a database called: National Forest Monitoring (Monafor), which is hosted on the home page of the Universidad Juárez del Estado de Durango (http://forestales.ujed.mx/monafor/inicio/).

On the other hand, at the international level it has been suggested to approach the collection of information, based on the scheme called community-based/locally based monitoring or Community Local Monitoring (^{Danielsen et al., 2005}; ^{García and Lescuyer, 2008}; ^{Conafor, 2009b} ). This monitoring involves the participation of the owners and local users of the forests, with the participation of the inhabitants, both in the decision of which indicators to measure and in the periodic collection of the data (^{Danielsen et al., 2009}).

This type of local intervention has been widely accepted by the MRV system of the REDD + mechanism; It is also considered an essential part of environmental governance and as a safeguard for forest management (^{Palmer, 2011}; ^{Skutsch, 2011}).

Local Community Monitoring has been implemented in the state of Oaxaca, in southern Mexico, through the system known as Community Forestry (^{Barton and Merino, 2005}). However, in Chihuahua, it has been difficult to introduce, probably, by the forms of social organization and the interpretation of the rights and obligations of local populations in the use, management and conservation of natural resources.

In the national and international scopes there is a growing interest in the collection of information to generate strategies to curb the degradation of ecosystems, take care of biodiversity and sustain the production of goods and services, as well as to evaluate social participation in the management of Forests (^{FAO, 2017}). These initiatives include the MRV System of the REDD + Mechanism (^{Plugge and Köhl, 2012}) and the certification mechanisms, national or international (^{Ismail et al., 2011}), which have promoted the establishment of monitoring sites with multiple objectives, which include variables not only environmental, but also social and economic.

With regard to the design of permanent plots of forest or forestry research in Chihuahua state, in the beginning, the establishment of Permanent Sites of Silvicultural Research was based mainly on the methodology proposed by Manzanilla in 1980. Such format has been used by some researchers, who have adapted the monitoring system to current forest conditions and new information needs (^{Corral-Rivas et al., 2009}). In addition, there is also the structure of the INFyS plots, with circular sites and subplots to measure the major and minor vegetation, regeneration, as well as the characteristics of the soil and fuels. This design involves a high value database for the knowledge of the forest ecosystems of Mexico (^{Conafor, 2018}).

It should be noted that to refer to forest and forestry monitoring in northern Mexico it is advisable to consider the monitoring system established in the state of Durango, whose data can be consulted in the Monafor system. Based on this digital effort, a process of remedies has been initiated, which has allowed the identification of relevant information in relation to timber increases and that tools for forest management have been generated, such as density guides and growth equations of the structures of the trees, as well as the validation of planning systems for the management of forests in the region (^{von Gadow et al., 2016}; ^{Corral-Rivas et al., 2019}; ^{Quiñonez-Barraza et al., 2018}).

On the other hand, the use of remote sensors, in the international arena, has become an easily accessible and reliable tool for monitoring forests (^{De León et al., 2014}; ^{Saarinen et al., 2018}). Among the main possible variables to monitor through the use of remote sensors are loss of forest cover, forest fragmentation, carbon sequestration, forest fires and pests, as well as changes in land use (^{Banskota et al., 2014}).

Both local community monitoring, as well as the use of geospatial tools and remote sensors are elements that will allow the implementation of a modern and inclusive forest monitoring system, which will help society to have greater access to forest resources and develop more assertive tools for according to the biological, cultural and socioeconomic complexity of the silvicultural activity in the ecosystems.

Conclusions

Since 1950, efforts began to establish a silvicultural monitoring system in the Chihuahua State. In the period 1950-2002, 346 SPIS were implemented and in recent years (2013 and 2014), 857 more plots were located in various state estates. However, forestry policy and local decision-making has not been assertive in the process of remeasurement of these plots, since most of them only have information on their establishment, which frustrates the monitoring objectives.

The lack of uninterrupted financing can be an important factor to limit monitoring, so alternatives could be incorporated, such as community monitoring and the use of modern tools, remote sensors, to ensure the continuous collection of information.

For the El Poleo site, average increases of up to 5.4 m³ ha^-1 of total tree volume were estimated, while for the south of the state in the ejido Llano Blanco they were up to 16.3 m³ ha^-1 vta. These data have not been documented before, and show the high potential of timber increases in the state's forests.

The monitoring carried out in Chihuahua State in the period considered has focused on the evaluation of the increment and growth in tree biomass, depending on the application of harvest treatments; therefore, it is suggested to incorporate monitoring systems that involve indicators of ecosystem function, as well as socio-economic and cultural variables.