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Introduction
Given its edaphic, topographic, and climatic characteristics, Mexico is rich in natural resources, mainly sustained by its diversity of tree species (Palma 2006). In some systems the use of trees as a forage resource is limited, even though it can represent a valuable food source for cattle and wild fauna (Olivares-Perez et al. 2013, Olivares et al. 2013, Rojas-Hernandez et al. 2015). A silvopastoral system is characterized by the interaction of trees, shrubs, grasses and pasture with animals and soil in time and space (Guerreiro et al. 2015, Olivares-Perez et al. 2011). In any silvopastoral system, the aim is to establish multipurpose trees which should be able to adapt to diverse soil and climate conditions (Reis et al. 2010, Olivares-Perez et al. 2011). Among the required characteristics should be the ability to fix nitrogen, which restores soil fertility, nutrient recycling, sequestering of carbon dioxide, and support of biodiversity (Nair et al. 2009, 2010, Lorenz and Lal 2014, McGroddy et al. 2015). Nowadays, it is important to reevaluate traditional knowledge and uses of native species, since these are the basis for their integration in silvopastoral systems. This procedure has been developed in several systems where several uses have been reported, such as fuel (firewood), wood, forage, human foodstuffs (flowers and fruits), medicine, industrial (dyes and resins), fences (posts and living fences), and shade for livestock (Olivares-Pérez et al. 2011, Leon-Castro et al. 2015).
One way to maintain balance in the ecosystem and preserve plant species is to create technological alternatives that help to increase productivity. One such method is to use non-leguminous trees and shrubs, given the diversity of functions that they can have within tropical production systems and their important role in ruminant feeding (Rojas et al. 2012, 2013, Olivares et al. 2013, Jimenez-Ferrer et al. 2015). The objective of the present study was to identify the foraging importance, uses, and density of non-leguminous trees in silvopastoral systems in the southern part of the State of Mexico.
Materials and methods
The study was carried out in the communities of Bejucos of Sanchez Colin and Llano Grande in the municipality of Tejupilco, State of Mexico (18° 45' 30" and 19° 04' 32" north, 99° 59' 07" and 100° 36' 45" west). The climate is tempered subhumid and semi-warm humid with rains in summer, with a mean annual temperature between 15 and 30 °C, an altitude of 1 340 m, and annual precipitation of 1 014 mm. Soil types are regosol (61.76 %), leptosol (22.11 %), cambisol (6.99 %), luvisol (4.22 %), phaeozem (3.34 %) and vertisol (0.7 %) (Prontuario de información geográfica municipal de los Estados Unidos Mexicanos 2009).
Characterization of the production units (PU)
The populations have a census of 83 producers; 69 LPU were characterized, 54 in Bejucos and 15 in Llano Grande. A questionnaire was used in which the following aspects were considered: non-leguminous foraging trees they know, uses and/or benefits they obtain from them, parts consumed by livestock (based on empirical observation by livestock farmers), place and season of consumption, spatial distribution within the LPU (Harvey and Haber 1999, Sosa et al. 2004).
Diagnosis of diversity, density, frequency, and relative abundance of tree species
Field measurements were taken in 878 ha, integrating six PU. In each of these, four 1 h transects were outlined to identify trees scattered in the pastures, and four 100 linear m transects were measured to count trees in living fences (Camacho 2000). Through visual verification of the transects we registered the density of scattered species in the paddocks (trees per ha−1) and in the living fences, and relative frequency (Fr), calculated by dividing the number of transects where each species was found by the total number of transects in the sample.
Dasometric diagnosis
The trees identified in the transects were measured dasometrically to determine diameter at breast height (DBH) and height (h). Five adult trees of each identified species were randomly chosen; their height was measured with a Haglöf electronic clinometer (HEC) and DBH was measured with a tape measure 130 cm from the ground (Camacho 2000, Sosa et al. 2004). For botanical identification, the common name was registered and branches (stem, leaves, flower and/or fruit) of each species were collected. Identification was performed in the botany laboratory of the Temascaltepec University Center of the Autonomous University of the State of Mexico, with the use of procedures and identification keys described by Pennington and Sarukhan (2005).
Statistical analysis
Relative abundance (Ar) was estimated by dividing the number of individuals of each tree species by the total number of non-leguminous trees present in each sample area (Camacho 2000). The information on uses, density, relative frequency, relative abundance, and variables related to the dasometric measurements of the trees were analyzed through descriptive statistics (Steel and Torrie 1988).
Results
Characteristics of the production units
The size of the 69 LPU (6 356.5 ha) varied from 2 to 500 ha. The PU are characterized as follows: meat production (62.3 %), dual-purpose (34.8 %), and sheep-goats (2.90 %). The inventory of the animals is: 4 273 cows which include beef breeds such as the European Swiss, Brahman, Simmental, Simbrah, Beefmaster and Charolais; 446 goats with production of creole breeds and their crosses with Nubians and Boers; and 187 sheep with a majority of Pelibuey breeds and their crosses with Dorper sires. Some 86 % of them supplement in the dry season. The feeding system in the LPU is undergrazing with use of native grasses in 78.7 % (4 854.4 ha) of the area devoted to livestock and with induced pastures in 21.3 % (1 315.1 ha) of the remaining surface, with predominance of the following grasses: Andropogon gayanus (1 256.6 ha), followed by Cynodon nlemfuensis (23.5 ha), Panicum maximun (23.5 ha), mulato hybrid (Brachiaria ruziziensis x Brachiaria brizantha) (10 ha), and Brachiaria brizantha (1.5 ha). The predominant foraging crops are maize (471 ha) and sorghum (43 ha); 28 % of these crops are silaged and 72 % used as ground hay to supplement animal fodder. The main labor force in the PU is the family (86 %), while over 54 % of the LPU hire temporary laborers and 52 % use permanent contracts.
Knowledge of the use of the trees in the LPU
Producers have ample knowledge of the consumption of tree species by their animals; despite this, only 4 % of them mentioned having harvested foliage or fruits from the trees to feed their confined animals. Although the trees provide foliage and fruits to feed livestock, only 26 % of the LPU plant trees to reforest and 64 % of the producers mentioned that their animals consume foliage and fruits in the dry season during grazing. The uses of trees in the LPU are as shadow (36 % of the surveys highlighted Ficus glabrata, Spondias purpurea, Crescentia alata, Magifera indica, and Guazuma ulmifolia by its greater crown diameter; as livestock forage (42 % of the surveys highlighted G. ulmifolia, C. alata, F. glabrata, and M. indica); and as live fences (29 % of surveys listed G. ulmifolia, C. alata, F. glabrata, Ipomoea murucoides, S. purpurea, and Mastichodendron capiri) (Table 1). Limitations of the trees were singled out as invasive species in 19 % of the surveys, and trees that cause economic problems in 7 % of surveys for I. murucoides and P. guajava for being species with high dispersal and seed production that require control practices of their density; The 3 % of livestock farmers mentioned that Licania arborea, Ficus cotinifolia and F. glabrata decreases the growth of pastures under the crown diameter. The distribution system of the trees in the LPU was established as follows: 93 % of the trees in fences, 96 % trees scattered in the grasslands, 19 % fruit trees, 1.4 % trees in living fences, and 55 % shrubbery as part of the original vegetation.
Parts of trees consumed by livestock
The frequency with which the producers mentioned foraging trees and the parts of the trees on which the animals feed allow them. The experience of livestock farmers, it facilitated to select the species with higher foraging potential. G. ulmifolia and C. alata are the ones that the producers report as having the highest foraging potential. Producers mentioned that the fruits are the morphological component most favored by livestock, which account for 66.7 and 52.2 % of fruits from the aforementioned trees (Table 2).
Uses of trees in production units
Non-leguminous trees, besides being an important source of forage for livestock, have other uses in the LPU. C. alata had nine local uses, especially as shade (62.3 %), living fences (49.3 %), and medicine (52.2 %). G. ulmifolia had seven uses, most importantly as shade (65.2 %), living fences (52.2 %), and firewood (44.9 %). P. guajava, F. glabrata, and S. purpurea had six uses. It is important to mention that seven species are used as medicinal plants, particularly C. alata and G. ulmifolia (Table 3).
Table 3. Local knowledge of producers on the use of non-leguminous trees in Llano Grande and Bejucos, State of Mexico.
*The total is more than 100 % because of multiple responses. Le: firewood, Po: posts, So: shade, Ac: living fence, Me: medicinal, Ch: human consumption, Ar: artisanal, Or: ornament, Ma: timber.
Density, frequency, and abundance of non-leguminous foraging trees
Five native species are reported to be the product of natural regeneration, with a population of 291 trees (24 ha) scattered in the grasslands at a mean density of 12.2 trees ha−1. The trees with the highest density were C. alata and G. ulmifolia, with 7.2 and 4.5 trees per ha−1, respectively (Table 3). The most abundant and frequent species were C. alata (59.1 and 83.3 %) and G. ulmifolia (36.8 and 83.3 %), respectively. In the fences, we found 46 trees (2 400 m) with a mean density of 1.75 trees. The species with the highest density is G. ulmifolia, making it the most important species in the LPU (Table 4).
Dasometric measurements of non-leguminous trees
In the two silvopastoral systems, scattered trees in pastures and living fences trees, the identified species were C. alata, G. ulmifolia, C. glabrata, F. cotinifolia, and S. purpurea (Table 5). For the scattered trees, the range of diameter at breast height (DBH) was 30.1 to 44.4 cm, and height was 6.3 to 9.5 m, while for living fence trees it was 28.9 to 117.3 cm DBH and height was 4.9 to 28.0 m (Table 5). The trees that had higher natural regeneration were C. alata and G. ulmifolia in the silvopastoral system of trees scattered in the grasslands with a DBH minor to 1cm and a total of 50 plantlets for C. alata and 19 plantlets for G. ulmifolia, respectively (Figure 1).
Table 5. Density, frequency, and abundance of non-leguminous trees scattered in pastures and living fences.
*Diameter Breast Height; ± Standard deviation.
Discussion
The results indicate that in most LPU the animals feed on the foliage/fruits of the trees in the field during grazing hours as much as they can, since only 4 % of the producers use a harvest and carry system with the trees. Similar studies report that the number of producers using the foliage and fruits of the trees in a harvest and carry system to feed livestock is limited (Olivares-Perez et al. 2011). Zamora et al. (2001) report differently that producers collect the foliage and collect and buy the fruits to feed the animals when gramineae lose their nutritious value, making the fruits and dry leaves the main forage source. In Colombia, during the dry season, 83 % of producers depend on foraging trees to feed their livestock, especially those producing pods and/or fruits (Cajas and Sinclair 2001). Therefore, the use of trees as forage is limited to browsing on living fences or trees in the paddock. It is necessary to train producers in the management and preservation of tree resources in their LPU to maximize their use. With regard to the benefits that trees have for the LPU, livestock feeding is of great importance (42 %) (Cajas and Sinclair 2001, Guerreiro et al. 2015, Jimenez-Ferrer et al. 2015); however, trees also favor the organic carbon entry and increased the nutrient content soil (Nair et al. 2009, 2010, Lanna et al. 2010, McGroddi et al. 2015). With regard to the limitations of trees in the LPU, the answers of the producers agree with data reported by Harvey and Haber (1999) in that 45 % of the LPU control tree density because they produce too much shade and invasive species are eliminated because of their high capacity to regenerate.
Parts of trees consumed by livestock
The higher foraging potential of G. ulmifolia and C. alata (Table 2) coincides with studies carried out in Jamaica and Honduras, where producers prefer to use the foliage and fruits of G. ulmifolia and C. alata with harvest and carry management to feed their livestock (Barrance et al. 2003). The foliage of G. ulmifolia is consumed during the rainy season; however, during the dry season, when the fruits ripen and dehydrate because of the sun, they fall off and are eaten by the livestock (Olivares-Perez et al. 2011). Producer preference for certain species depends on the services and products that the tree offers the LPU, which can be implemented and developed in silvopastoral systems (Olivares-Perez et al. 2011). This facilitates the producers becoming participants in the technological development of their LPU (Pinto et al. 2005).
Use of trees in the production units
Most of the trees identified have multiple uses and are important components of the natural vegetation in the area, with good adaptation to the local environmental conditions (Table 3). These results coincide with studies performed in other states in the country (Mexico); in Morelos (Solares 2004), Quintana Roo (Sosa et al. 2004), Tejeda, Veracruz (Couttolenc et al. 2005), Angostillo, Veracruz (Villa et al. 2009), Sierra of Tabasco (Grande et al. 2010), The Limon, Veracruz (Bautista et al. 2011); and in other countries, in Rio Frio, Costa Rica (Villacis et al. 2003), in the Caribbean, Colombia (Cajas and Sinclair 2001), in the Pacific, Costa Rica (Harvey and Haber 1999). This shows that the use of trees in different regions is similar; generally speaking, producers prefer to keep trees that offer more products and services to the LPU (Esquivel et al. 2011).
Density, frequency, and abundance of nonleguminous trees
Although the study was centered around nonleguminous trees (Table 4), the density coincides with studies done in Cañas, Costa Rica (Esquivel et al. 2011) and in the State of Mexico (Olivares-Perez et al. 2011). The low density of some trees in the grasslands and living fences may be to the result of damage and the mortality of plantlets, caused by the livestock in the paddock, the application of herbicides, and weeding done by the producers in their regular control practices (Olivares-Perez et al. 2011, Guerreiro et al. 2015). The predominant species show characteristics that facilitate their natural regeneration, abundant seed production and the ability to be spread by livestock after consumption. These aspects are important for the natural repopulation of these species (Villacis et al. 2003). The use of trees in living fences is beginning to become more important because of their durability and the economic saving to be made, especially in areas where the original vegetation has been disturbed by the multiple services provided by the trees (OlivaresPerez et al. 2011, Jimenez et al. 2015).
Non-leguminous foraging trees are found scattered in the grasslands and in living fences, and they play an important role in the preservation of biodiversity in silvopastoral systems by preserving habitats for wild animals, important for the natural spreading of tree seeds for natural regeneration (Olivares-Pérez et al. 2011, Gonzalez-Valdivia et al. 2014). At the same time, they act as biological corridors (Mastrangelo and Gavin 2014, Talamo et al. 2015).
Dasometric measurements of non-leguminous trees scattered in the grasslands and in living fences
The observed DBH ranges of 30.1 (+ ES12.3) to 44.4cm (+ ES 15.0) constitute an indicator of the low regeneration rate of the trees scattered in the grasslands and in living fences (Table 5). No trees were found with < 10 cm DBH for F. glabrata, F. cotinifolia, and S. purpurea (Figure 1). Studies performed in the dry tropics report DBH similar to those in this study (Tovar and Ibrahim 2010, Esquivel et al. 2011). Though natural regeneration of species was observed in the study, the livestock farmers control the density of the trees in their prairies. The high variation in DBH of some trees scattered in the grasslands and in living fences may be related to the handling practices of the producers regarding their grasslands and living fences (use of herbicides, frequency of trimming and weeding) that cause changes in growth (Tovar and Ibrahim 2010, Esquivel et al. 2011). This hinders the development of the trees and their production of abundant fruits for their natural regeneration. The above indicates that with time some desirable species in producers' systems might have low natural regeneration and, consequently, may be lost (Esquivel et al. 2011).
Conclusions
Non-leguminous trees are important in production units, both for their foliage and fruit supply for animal feeding in the low forage availability season and for their versatility, since they provide shade, firewood, posts and foliage in the LPU. The most important species, given their density and abundance, are C. alata and G. ulmifolia, which are selected by producers for their easy reproduction and because they are not invasive.
