Revista Mexicana Ciencias Agrícolas   volume 11   number 4   May 16 - June 29, 2020

DOI: https://doi.org/10.29312/remexca.v11i4.2467

Article

Forage behavior of the herbaceous stratum in different tree densities
of low deciduous forest

Raúl Ramírez-Contreras1

Alejandro Lara-Bueno1

Miguel Uribe-Gómez1§

Artemio Cruz-León2

Dante Arturo Rodríguez-Trejo3

Guadalupe Montserrat Valencia Trejo1

1Master of Science in Agroforestry for Sustainable Development-Chapingo Autonomous University. Mexico Texcoco highway km 38.5. Chapingo, Mexico. (ingraulramirezc@gmail.com; alarab-11@hotmail.com; gmvalenciatrejo@gmail.com). 2Postgraduate in Regional Rural Development-Chapingo Autonomous University. Mexico-Texcoco highway km 38.5, Chapingo, Mexico. (etnoagronomia1@gmail.com). 3Postgraduate in Forest Sciences-Chapingo Autonomous University. Mexico-Texcoco highway km 38.5, Chapingo, Mexico. (dantearturo@yahoo.com).

§Corresponding author: migueluribe123@gmail.com.

Abstract

The shade of the trees projected on the herbaceous stratum that grows under the canopy represents an undesired effect for livestock farmers for forage production in the silvopastoral system. In order to determine the dry matter production and the floristic composition of the herbaceous stratum with different amounts of photosynthetically active radiation, three plots with low tree density (BDA; 50 to 200), moderate tree density (MDA; 201 to 350) and high tree density (ADA; 350 to 500) trees ha-1 were selected. In each plot, photosynthetically active radiation (RFA) that affects the herbaceous stratum was estimated and the dry matter production (DM) of the forage herbaceous plants present during 15 weeks of the rainy season was determined. The intensity of light under the canopy and the production of herbaceous forage biomass in BDA, MDA and ADA were 8 594, 6 437 and 3 801 MJ m2 year-1 and 2 859, 1 988 and 1 679 kg of DM ha-1, respectively. The number of herbaceous species in BDA, MDA and ADA was 38, 48 and 27, respectively, although the dominant species, with more than 80% of the botanical composition, were only six. It is concluded that the tree density modifies the amount of light that falls on the herbaceous stratum, affecting the biomass production and the floristic composition of forage herbaceous plants that grow under the tree canopy in a traditional silvopastoral system in the Low Deciduous Forest.

Keywords: forage biomass, herbaceous stratum, light intensity, Sierra de Huautla, silvopastoralism.

Reception date: February 2020

Acceptance date: April 2020

Introduction

The need to reduce agricultural inputs without significant loss of productivity requires the redesign of production systems (Jamar et al., 2015). Agroforestry systems can contribute to feeding the growing population in a sustainable way Groeneweg (2018). The benefits of mixing herbaceous and woody species in agroforestry systems will depend on the specific interactions between the components of the system (Lovell et al., 2018).

Tree and herbaceous plant species frequently compete in time and space for natural production inputs: light, water and nutrients (Yu et al., 2015; Hong et al., 2017). Solar radiation is the main factor that affects photosynthesis and the performance of agroforestry systems (Jose et al., 2008). The quantity and quality of the solar radiation that affects the herbaceous layer are affected by numerous factors: tree species, tree height, canopy architecture, leaf area index, time of day and day of the year, location (latitude, longitude) and sun exposure of the surface according to the slope (Burner et al., 2018).

By optimizing the pasture-tree interaction, the productivity of the land can be improved, although the management of the system can be more complex because the interactions between the components of a silvopastoral system include changes in the microclimate associated with the pasture, of which the most notorious are the light intensity and the quality of the solar radiation that reaches the herbaceous component through the tree canopy. Sunlight that intercepts tree foliage affects the quantity and forage quality of herbaceous species that grow under the influence of the tree canopy and modify the botanical composition of paddocks (Cruz et al., 1999; Burner et al., 2018).

Tropical pastures, with a predominance of C4 plants, reach their maximum production with high levels of light intensity (Jose et al., 2008). However, comparable or even higher dry matter values have been reported when grasses are associated with trees, compared to pure grass grasses (Cruz et al., 1999).

These increases have been obtained in conditions of moderate shade in the range of 30 to 40%. Although, the availability of the total DM and the nutritional value of the forage herbaceous are often lower under the tree canopy (Ribaski, 2000). In other cases, dry matter production is affected by shade, but not the nutritional value of forage (Pentón et al., 1998). Trees in paddocks fulfill important functions within livestock systems and provide eco-systemic products and services (Ribaski and Menezes, 2002).

The most important thing for the ranchers is the shade that the trees provide to the animals, as well as, to obtain firewood and economic resources by selling wood or fruits. However, the shading of the trees can affect the productivity of the grasses, mainly, under ADA conditions, when the woody ones are dense crowns because they have a greater capacity of interception of the solar radiation necessary for the optimal growth and development of the plants that make up the herbaceous layer (Jose et al., 2008).

Silvopastoral systems have been developed to improve the quality and performance of grasslands through the interaction between herbaceous and forage tree species (Bellow and Nair, 2003). In this way, the association of grasses with woody plants contributes to improving the productivity of the land in both meat and milk production (Clavero, 2011). However, in silvopastoral systems the productivity of the herbaceous stratum may be conditioned by the canopy cover of the trees, which, by interference, limits the amount of light that reaches the herbaceous stratum (Gargaglione et al., 2014).

In this sense, as an alternative to improve the DM production of the herbaceous stratum, it is convenient to establish grass species tolerant to a certain level of shading (Gargaglione et al., 2015). The partial interception of sunlight by the tree foliage modulates the photosynthetically active radiation and the intensity of light that reaches the herbaceous stratum, affecting the morphology and growth of the plants that grow under the canopy, Fassola et al. (2006) they reported that the interception of the RFA in the canopy, promotes an increase in the maximum forage accumulation of the pastures when there is 40% shade and then descends until reaching critical levels from 70% shade. Ovispo et al. (2008) demonstrated that both production and forage quality are affected by the amount of shade that the tree canopy casts on the herbaceous layer, so that too much shade cast by the tree negatively affects pasture yield.

Thus, the shadow cast by the tree canopy on the herbaceous layer constitutes one of the most important negative effects of the presence of trees in silvopastoral systems (Pentón, 1998). However, the botanical composition of herbaceous plants can be similar in both production systems of grasses with and without trees; likewise, the percentage of C4 plants increases and that of C3 decreases when they grow at a greater distance from the base of the tree (Ruiz et al., 2001).

Several studies have addressed the interaction of trees on pastures that have reported higher growth, productivity and nutritional quality of herbaceous plants that grow in the shade (Cruz et al., 1999; Fassola et al., 2006; Jose et al., 2008; Piñeros et al., 2011; Burner et al., 2018). However, there are differences in the information regarding the benefits and limitations of the shade of trees on the herbaceous layer. This heterogeneity of results is determined by the diversity of conditions in which the investigations have been carried out, as well as by the different experimental methodologies used.

The herbaceous layer that grows during the rainy season under the canopy of the trees of the traditional silvopastoral system of the Sierra de Huautla, Morelos, is of great importance for the economy of the peasant communities that inhabit that region, however, little information is available to improve the management of the components of the ecosystem.

The objective of this study is to analyze the productive behavior and the floristic composition of the herbaceous layer that grows below the canopy as a function of the tree density and the photosynthetically active light intensity in the traditional silvopastoral system of the Low Deciduous Forest of the Sierra de Huautla, Morelos, Mexico.

Materials and methods

The study was carried out during the rainy season in the communities of Los Sauces and El Limón, in the municipality of Tepalcingo, Morelos, Mexico, in the region called Sierra de Huautla, located to the south of the entity. The coordinates of the study area are 18° 20’ north latitude and 98° 51’ west longitude (Dorado et al., 2005).

The dominant climate, reported by Koppen and modified by García (2004), is Aw0(w)(i’)g, the driest of sub-humid climates, with a rainfall regime in summer and winter precipitation of less than 5%; mean annual precipitation of 900 mm, and mean annual temperature of 25 °C, with annual fluctuation of monthly mean temperatures between 5 and 7 °C. With two climatic seasons during the year: the rainy season from July to October and the dry season that includes the other eight months of the year.

The predominant soils are the Haplic Feozems and the Luvic Feozems; where chromic Luvisols and Lithosols are more abundant presenting a lithic phase, with a rocky bed between 10 and 20 cm deep (INIFAP, 1995). Land use is given by semi-extensive cattle farming and seasonal agriculture with annual crops linked to livestock feeding (Uribe et al., 2015).

The dominant vegetation is the Low Deciduous Forest (Miranda and Hernández, 1963) characterized by species, arborescent and arboreal with 4 to 10 m in height. The dominant plant families are: Fabaceae, Euphorbiaceae, Burseraceae and Bombonaceae (Rzedowski, 2006). For the selection of experimental plots, visits were made to the study area in the common territory ‘Los Sauces’ and ‘El Limon’. Using the photointerpretation technique, areas of high, moderate and low tree density were identified with the help of Google Earth Pro software (Guerra, 2003).

With field information, wooded areas were delimited with (ADA; 351 to 500), (MDA; 201 to 350) and (BDA; 50 to 200) trees ha-1. To determine the tree density, the sampling sites were divided into quadrants of 50 x 60 m using the diameter of the stem at the height of the first fork (> 5 cm) and height at the crown (> 2.5 m) of the arboreal ones, identifying the dominant species and botanical family. The experimental area considered nine 1000 m2 sampling plots (three plots for each tree density).

Herbaceous plant material was collected every 14 days using a 1 m2 quadrant. Four 50 x 50 cm sampling sites were located in each experimental plot, where herbaceous plant material was collected to determine floristic diversity. To determine the floristic composition of the herbaceous species present in the forage, the collected herbaceous material was separated and identified by visual comparison with the species described by (Dorado et al., 2005).

The collected plant material was weighed with a digital scale separating the botanical species present. Subsequently, a sub-sample of 20% of the collected material was taken, which was placed in brown paper bags and dried at a constant weight in a forced air stove at 60 °C, to determine the dry weight of each sample. Thus, the daily biomass production of the herbaceous stratum was calculated for each tree density in each sampling period, dividing the total weight of the biomass of each sample between the days of the sampling period.

To measure the photosynthetically active radiation entering the herbaceous stratum through the tree canopy, photographs were taken using a digital camera with a hemispherical lens. Randomly, three photographs were taken at each sampling site, placing the camera 50 cm from the ground, obtaining nine photographic images for each of the evaluated tree densities. The images were obtained by aiming the camera lens up from a point under the tree canopy.

Using a 180° ‘fish eye’ lens with a high resolution digital camera following the protocol of use established by Ramírez-Contreras y Rodríguez-Trejo (2006). For this, uniform clear sky conditions were considered, just before sunrise or sunset or when the sky was cloudy but uniform. To estimate RFA, each digitized photographic image was analyzed with the HemiView V.2.1 program, using the simple molar model to obtain RFA in Mj m2 year-1 (Ramírez-Contreras y Rodríguez-Trejo, 2006).

The experimental design was completely randomized, taking tree density as treatment, with three replications and nine sampling sites per treatment, with each sampling site as the experimental unit. Forage yield data was analyzed by analysis of variance using the SAS GLM procedure (Park, 2009).

Results and discussion

The richness, abundance and density of trees scattered in paddocks in the traditional silvopastoral system in the study region is a product of the decisions taken by producers to conserve those multi-purpose tree species in the pastures. Generally, producers leave trees in the pastures that provide services and benefits to their paddocks and their families. In the experimental area, 37 multipurpose tree species corresponding to 18 botanical families were identified, with the Fabaceae family standing out (Table 1).

Table 1. Trees present in the study area of the traditional silvopastoral system of the Sierra de Huautla, Morelos.

The behavior of solar radiation in the canopy varied with the density of trees present. The highest RFA in the herbaceous stratum was found at the BDA sites, with this radiation being 25.1 and 55.8% higher than those registered at the sites with MDA and ADA, respectively (p< 0.05; Table 2). The production of forage biomass from the herbaceous stratum was directly proportional to RFA and inversely proportional to the tree density, the higher the radiation and the lower tree density, the greater the production of forage biomass from the herbaceous stratum.

Table 2. Photosynthetically active radiation (RFA), forage biomass production (PBF) and forage production rate (TPF) of the herbaceous layer in the traditional silvopastoral system of the Sierra de Huautla, Morelos.

abc= means with different literal in the same row show significant differences (p> 0.05); ADA= high tree density (sites with 351 to 500 trees ha-1); MDA= moderate tree density (sites with 201 to 350 trees ha-1); BDA= low tree density (sites with 50 to 200 trees ha-1).

The BDA sites, but with higher RFA in the herbaceous stratum, produced 30.5 and 41.3% more forage biomass than the sites located in MDA and ADA, respectively (p< 0.05; Table 2). The behavior of solar radiation in the canopy varied with the density of trees present. The highest RFA in the herbaceous stratum was found at the BDA sites, with this radiation being 25.1 and 55.8% higher than those registered at the sites with MDA and ADA, respectively (p< 0.05; Table 2).

The production of forage biomass from the herbaceous stratum was directly proportional to RFA and inversely proportional to the tree density, the higher the radiation and the lower tree density, the greater the production of forage biomass from the herbaceous stratum. Thus, the BDA sites, but with the highest RFA in the herbaceous stratum, produced 30.5 and 41.3% more forage biomass than the sites located in MDA and ADA respectively (p< 0.05; Table 2).

The maximum accumulation of forage biomass in the herbaceous stratum for the different tree densities was around the sixth sampling period, approximately 84 days after the start of the rainy season. Likewise, the forage production rate of the herbaceous stratum was 32.4 and 38.0% higher in the BDA sites, compared to the MDA and ADA sites, respectively (p< 0.05; Table 2).

Similar behavior was observed in the dry matter content of the forage biomass of the herbaceous stratum. Forage samples harvested at BDA sites had 14.8 and 26.7% more DM content compared to forage harvested at MDA and ADA sites, respectively (p< 0.05; Table 2).

Similar results to those obtained in the present study were reported by Acciaresi et al. (1994); Cruz et al. (1999); Fassola et al. (2006); Jose et al. (2008); Piñeros et al. (2011); Burner et al. (2018), who found higher forage yields in places without shade and in places with low levels of shade. Under ADA conditions, forage biomass production was affected due to less solar radiation reaching the herbaceous layer.

In contrast, in sites with BDA, where the incidence of solar radiation is higher, grasses and other forage that grow under the tree canopy show higher yield due to the higher rate of photosynthesis, since the flow of solar energy has direct relationship with forage production (Hernández and Guenni, 2008). Likewise, the RFA increased in the sites with the highest solar radiation, similar to that reported by Bernal et al. (2006) with the growth rate of native grasses in the oak forest of the state of Mexico.

Ribaski et al. (2002) report similar results when studying the availability and quality of buffel grass (Cenchrus ciliaris) in a silvopastoral system with carob (Prosopis juliflora) in the Brazilian semi-arid region. The forage produced under the carob tree presented higher nutritional value, characterized mainly by higher levels of crude protein. However, forage yield was lower. Ovispo et al. (2008) studied the effect of shading on the production and quality of guinea grass (Panicum maximun) in a silvopastoral system and found that the production and quality of the biomass of Panicum maximum was negatively affected by the level of shading. Different results were found by Piñeros et al. (2011) when studying the response of vidal grass (Bothriochloa saccharoides) to different simulated shade intensities in the Magdalena warm Valley in Tolima, Colombia.

The objective of these Colombian researchers was to analyze the biomass production and nutritional quality of vidal grass under shadow simulators, with percentages of 0% 30% and 50% shade, however, they did not find significant differences with respect to the percentages of shade in the yield and quality of the forage. This reflects that the productive behavior and nutritional quality of vidal grass are not affected by shading and confirm the tolerance to shade of some grasses described by other researchers (Bernal et al., 2006; Piñeros et al., 2011; Gargaglione et al., 2015).

The results of the identification of herbaceous plants that make up the forage mass that grows under the canopy in the different tree densities are presented in Table 3. For the BDA condition, 38 herbaceous species were recorded, although only 12 of them occupied 90.2% of the botanical composition of the study area, highlighting Rhynchelytrum repens, Atheropogon radicosa, Brachiaria brizantha, Oplismenus burmanni and Andropogon citratus corresponding to the Poaceae family, all of which are C4-type grasses that require higher light intensity.

Table 3. Floristic composition of the herbaceous layer in sites of low tree density BDA in the traditional silvopastoral system of the Sierra de Huautla, Morelos.

It is evident that the intensity of solar radiation directly influences the composition and richness of herbaceous species in paddocks, herbaceous plants tolerant to a certain shade condition and those that require a higher incidence of light developing more easily. The first three herbaceous species Rhynchelytrum repens (38.1%), Atheropogon radicosa (16.3%) and Brachiaria brizantha (10.08%), are important foragers in the SBC of the study region, since they are used for livestock feeding.

For the MDA range, 48 herbaceous species were recorded that grow under the tree canopy (Table 3). Of these, only 25 species represent 90.2%, among which stand out: Brachiaria brizantha, Rhynchelytrum repens, Tithonia tubaeformis, Oplismenus burmanni, Atheropogon radicosa and Commelina dianthyfolia, belonging to the Poaceae, Asteraceae and Commelinaceae families; however, the percentages of grasses are lower than those observed in the BDA condition, which is explained by the lower incidence of sunlight reaching the herbaceous layer and allowing the growth of other herbaceous species other than grasses that are tolerant to certain levels of shade.

In the condition of (ADA; Table 3) 27 herbaceous species were registered under the tree canopy, of these, 90.7% correspond to Oplismenus burmanni, Tithonia tubaeformis and Setaria geniculata, belonging to the Poaceae and Asteraceae families. These results indicate that the different tree densities generate conditions for the development of species such as Tithonia tubaeformis (family Asteraceae), which changes the composition of herbaceous vegetation in the SBC (Larenas et al., 2004). Larenas and De Viana (2005) suggest that Tithonia tubaeformis is an important herbaceous as a remediator of contaminated soils, although its forage value is low.

The results indicate that the tree density, through modifying the light intensity that affects the herbaceous layer, imposes conditions for the development of the dominant herbaceous species that grow under the canopy of the trees present in the SBC (Larenas et al., 2004; Larenas and De Viana, 2005). Results similar to those of the present study in the Tuxtla region in the state of Veracruz, Mexico were reported by Lira et al. (2007); also, research carried out by Ruiz et al. (2001), determined the botanical composition of herbaceous species in a livestock production system with and without trees. These researchers found that the botanical composition of herbaceous plants under the canopy did not vary in paddocks with and without trees, however, they found that the greater the distance from the base of the tree, the percentage of type C4 grasses was higher, while the percentage of herbaceous broadleaf type C3 was greater at a shorter distance from the base of the trees.

In this way, the solar radiation that affects the herbaceous stratum influences the composition and floristic richness of the herbaceous plants in the pastures of the traditional silvopastoral system of the SBC of the Sierra de Huautla, Morelos, promoting greater abundance of herbaceous species tolerant to the shade cast by the trees present. These herbaceous species are of importance in livestock feeding, forming the mixture of forage plants with high nutritional value that satisfy the requirements in the daily diet of animals during the rainy season (Rosales, 1999).

Although the shading of the trees offers conditions for the greater diversity of species in the herbaceous stratum, it is also true that the forage yield decreases with increasing tree density in the traditional silvopastoral system of the Sierra de Huautla. This encourages producers to clear existing vegetation for the establishment of pure pastures, with serious consequences for the stability of the SBC, due to decreased biodiversity, soil erosion, deterioration of hydrological basins and degradation of pastures.

Conclusions

There was a marked difference in the amount of RFA reaching the herbaceous stratum in the different tree densities of the traditional silvopastoral system of the Low Deciduous Forest of the Sierra de Huautla, Morelos, which affected the floristic composition and growth of the herbaceous species that grow under the canopy of trees. Forage biomass production of herbaceous species growing under the tree canopy was higher in the BDA condition compared to dry matter production.

Obtained under MDA conditions, due to the greater amount of RFA available for the growth of herbaceous plants. The maximum yield of forage herbaceous plants that grow under the tree canopy is around 84 days after the start of the rains, which can contribute to establishing rational grazing systems with two or three periods of occupation during the rainy season in the cattle ranches of the traditional silvopastoral system of the Sierra de Huautla, Morelos.

The study of the shade produced by the trees presents questions that do not allow us to reach definitive conclusions about the phenomenon. It is suggested to delve into the subject studied to develop a higher quality herbaceous plant layer for livestock feeding in the traditional silvopastoral system of the Sierra de Huautla, Morelos.

Cited literature

Acciaresi, H.; Ansín, O. E. y Marlats, R. M. 1994. Sistemas silvopastoriles: efectos de la densidad arbórea en la penetración solar y producción de forraje en rodales de álamo Populus deltoides Marsch. Agroforestería en las Américas. 1(4):6-9.

Bellow, J. G. and Nair, P. K. R. 2003. Comparing common methods for assessing understory light availability in shaded-perennial agroforestry systems. Agricultural and forest meteorology. 114(3-4):197-211. https://doi.org/10.1016/S0168-1923(02)00173-9.

Bernal, F. Á.; Hernández, G. A.; Pérez, P. J.; Herrera, H. J. G.; Martínez, M. M. y Dávalos, F. J. L. 2006. Patrón de crecimiento estacional de pastos nativos en un bosque de encino en el Estado de México, México. Agrociencia. 40(1):39-47. http://www.scielo.org.mx/pdf/ agro/v40n1/1405-3195-agro-40-01-39.pdf.

Burner, D. M.; Ashworth, A. J.; Laughlin, K. F. and Boyer, M. E. 2018. Using SketchUp to Simulate Tree Row Azimuth Effects on Alley Shading. Agronomy Journal. 110(1):425-430.

Clavero, T. 2011. Agroforestería en la alimentación de rumiantes en América Tropical. Revista de la Universidad del Zulia 3ª época Ciencias del Agro Ingeniería y Tecnología. 2(2):11-35. http://www.revencyt.ula.ve/storage/repo/ArchivoDocumento/revluz/v2n2/art02.pdf.

Cruz, P.; Sierra, J.; Wilson, J. R.; Dulormne, M. and Tournebize, R. 1999. Effects of shade on the growth and mineral nutrition of tropical grasses in silvopastoral systems. Annals of Arid Zone. 38(3-4):335-361.

Dorado, O.; Maldonado, B.; Arias, D. M.; Sorani, V.; Ramírez, R.; Leyva, E. y Valenzuela, D. 2005. Programa de conservación y manejo de la reserva de la biosfera de Huautla. Comisión Nacional de Áreas Naturales Protegidas-Secretaría de Medio Ambiente y Recursos Naturales. México, DF. 143 p.

Fassola, H. E.; Lacorte, S. M.; Pachas, N. y Pezzutti, R. 2006. Efecto de distintos niveles de sombra del dosel de Pinus taeda L. sobre la acumulación de biomasa forrajera de Axonopus compressus (Swartz) Beauv. Revista Argentina de Producción Animal. 26(2):101-111. https://pdfs.semanticscholar.org/e8dc/e8d584190b0055072118be03e6e5df2cf56b.pdf.

García, E. 2004. Modificaciones al sistema de clasificación climática de Köppen (para adaptarlo a las condiciones climáticas de la República Mexicana). 5ta edición, Instituto de Geografía. UNAM. México, DF. 97 p. www.publicaciones.igg.unam.mx/index.php/ig/catalog/view/ 83/82/251-1.

Gargaglione, V.; Peri, P. L. and Rubio, G. 2014. Tree-grass interactions for N in Nothofagus antarctica silvopastoral systems: evidence of facilitation from trees to underneath grasses. Agroforestry systems, 88(5):779-790. https://link.springer.com/content/pdf/10.1007/ s10457-014-9724-3.pdf.

Gargaglione, V.; Peri, P. L.; Sosa L. S.; Bahamonde, H.; Mayo, J. P. and Christiansen, R. 2015. Mejora del estrato herbáceo en sistemas silvopastoriles de Nothofagus antarctica. Evaluación de especies forrajeras. In: 3° Congreso Nacional de Sistemas Silvopastoril y VII Congreso Internacional de Sistemas Agroforestales (3-7 pp.). Iguazú, Misiones. Argentina. INTA ediciones. 734 p. 

Groeneweg, D.; Vischedijik, F.; Appelman, J.; Van Buiten, G.; San Giorgi, X. and Hautier, Y. 2018. Polycultures in agroforestry. In: Ferreiro, D. N and Mosquera, L. M. R. Editors. Proceedings of the 4th European Agroforestry Conference “Agroforestry as Sustainable Land Use”. Nijmegen, The Netherlands. 452-456 pp. https://www.repository.utl.pt/ bitstream/10400.5/18629/1/EURAFIVCon-Groeneweg-D-et-all-page-452-456.pdf.

Guerra, P. F. 2003. Las doce principales reglas de la interpretación fotogeológica y las bases fundamentales que se derivan. Investigaciones geográficas, Boletín del Instituto de Geografía, UNAM. 50:42-66.  www.scielo.org.mx/pdf/igeo/n50/n50a8.pdf.

Hernández, M. y Guenni, O. 2008. Producción de biomasa y calidad nutricional del estrato graminoide en un sistema silvopastoril dominado por samán (Samanea saman (Jacq) Merr). Zootecnia Tropical. 26(4):439-453.

Hong, Y.; Heerink, N.; Jin, S.; Berentsen, P. Zhang, L. and van der Werf, W. (2017). Intercropping and agroforestry in China–Current state and trends. Agriculture, ecosystems & environment. 244:52-61. https://doi.org/10.1016/j.agee.2017.04.019.

INIFAP (Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias). 1995. Mapa edafológico, escalas 1: 250000 y 1: 1000000. INIFAP-CONANP. México.

Jamar, L.; Rondia, A.; Lateur, M.; Minet, L.; Froncoux, A. and Stilmant, D. 2015. Co-design and establishment of innovative fruit-based agroforestry cropping systems in Belgium. In: International Symposium on Innovation in Integrated and Organic Horticulture (INNOHORT) 1137. 347-350 pp.

Jose, S.; Allen, S. C. and Nair, P. R. 2008. Tree-crop interactions: lessons from temperate alley-cropping systems. Ecological Basis of Agroforestry. 15-36 pp.

Larenas, P. G. y De Viana, M. L. 2005. Germinación y supervivencia del pasto cubano Tithonia tubaeformis (Asteraceae) en suelos contaminados con hidrocarburos de petróleo. Ecología Austral. 15:177-181.

Larenas, P. G.; De Viana, M. L.; Chafatinos, T. y Escobar, N. E. 2004. Relación suelo-especie invasora (Tithonia tubaeformis) en el sistema ribereño del río Arenales, Salta, Argentina. Ecología Austral. 14:19-29.

Lira, N. A.; Guevara, S.; Laborde, J. y Sánchez, R. G. 2007. Composición florística en potreros de los Tuxtlas, Veracruz, México. Acta Botánica Mexicana. 80:59-87.

Lovell, S. T.; Dupraz, C.; Gold, M.; Jose, S.; Revord, R.; Stanek, E. and Wolz, K. J. 2018. Temperate agroforestry research: considering multifunctional woody polycultures and the design of long-term field trials. Agroforestry Systems. 92(5):1397-1415.

Miranda F. and Hernández-X, E. 1963. Los tipos de vegetación de México y su clasificación. Boletín de la Sociedad Botánica de México. 28:29-179. https://doi.org/10.17129/ botsci.1084.

Obispo, N. E.; Espinoza, Y.; Gil, J. L.; Ovalles, F. y Rodríguez, M. F. 2008. Efecto del sombreado sobre la producción y calidad del pasto guinea (Panicum maximun) en un sistema silvopastoril. Zootecnia tropical. 26(3):285-288.

Park, H. M. 2009. Regression models for binary dependent variables using STATA, SAS, R, LIMDEP, and SPSS. Working paper. The University Information Technology Services (UITS) Center for Statistical and Mathematical Computing. Indiana University.  IN, USA. 62 p. http://www.indiana.edu/~statmath/stat/all/cdvm/index.html.

Pentón, G.; Blanco, F. y Soca, M. 1998. La sombra de los árboles como fuente de variación de la composición botánica y la calidad del pastizal en una finca silvopastoril. In: Memorias. III Taller Internacional Silvopastoril “Los Árboles y Arbustos en la Ganadería”. EEPF “Indio Hatuey”. Matanzas, Cuba. 32 p.

Piñeros, R., Delgado, J. M., y Holguín, V. A. 2011. Respuesta del pasto Bothriochloa saccharoides ([Sw.] Rydb.) a diferentes intensidades de sombra simulada en el valle cálido del Magdalena en el Tolima (Colombia). Ciencia y Tecnología Agropecuaria. 12(1):42-50.

Ramírez-Contreras, A. y Rodríguez-Trejo, D. A. 2006. Evaluación del uso de plantas nodriza en una plantación de Pinus hartwegii Lindl. Tesis de Maestría en Ciencias Forestales. Universidad Autónoma Chapingo. Chapingo, Edo. México. México. 122 p.

Ribaski, J. (2000). Influência da algaroba (Prosopis juliflora (SW.) DC.) sobre a disponibilidade e qualidade da forragem de capim-búfel (Cenchrus ciliaris) na região semi-árida brasileira. Tese de Doutor em Ciências Florestais. Curso de Pós-Graduação em Engenharia Florestal, Universidade Federal do Paraná. Curitiba, Brasil. 165 p.

Ribaski, J. y Menezes, E. A. 2002. Disponibilidad y calidad del pasto buffel (Cenchrus ciliaris) en un sistema silvopastoril con algarrobo (Prosopis juliflora) en la región semiárida Brasileña. Agroforestería en las Américas. 9(33-34):8-18.

Rosales, M. M. 1999. Mezclas de forrajes: uso de la diversidad forrajera tropical en sistemas agroforestales. FAO animal production and health paper. 201-230 pp. http://www.fao.org/ag/aga/agaP/Frg/AGROFOR1/Rosales9.PDF.

Ruiz Fonseca, C. J.; Maradiaga, W. y Cuadra, N. 2001. Estudio de los recursos forrajeros, bajo el dosel de árboles en potreros, estudio de caso, finca Las Mercedes, Managua, Nicaragua. La Calera. 1(1):39-44.

Rzedowski, J. 2006. Vegetación de México. 1ra. Edición digital, Comisión Nacional para el Conocimiento y Uso de la Biodiversidad, México. 504 p.

Uribe, G. M.; Cruz, L. A.; Juárez, R. D.; Lara, B. A.; Romo, L. J. L.; Valdivia, A. R., y Portillo, V. M. 2015. Importancia del diagnóstico rural para el desarrollo de un modelo agroforestal en las comunidades campesinas de la sierra de Huautla. Revista Científica Ra Ximhai. 5(11):197-208.

Yu, Y.; Stomph, T. J., Makowski, D. and van der Werf, W. 2015. Temporal niche differentiation increases the land equivalent ratio of annual intercrops: a meta-analysis. Field Crops Research. 184:133-144. https://doi.org/10.1016/j.fcr.2015.09.010.