Forest and livestock in the Arid Chaco ecosystem: A Review

Ricardo Ayerza (h)

The University of Arizona, Tucson, Arizona 85706, U.S.A
La Rinconada, Santa Elena, Ecuador
rayerza@newcrops.org

Abstract

Deforestation of the native forests of South America occupies the first place in the world ranking of this scourge. Although deforestation was initially attributed to soybean plantations, it was later demonstrated that the removal of the forest is dominated by its replacement with pastures for beef cattle production. This creates a huge antinomy between those who defend the tree as the lung of the planet and those who support livestock as source of quality protein. However, this discussion should not exist if the people on both sides knew that the tree and the cattle can not only live together, but benefit from it. This has been long been demonstrated in South America, in the Argentine Chaco Forest belonging to the South American Gran Chaco ecosystem.

These works showed that pastures that grew in the Arid Chaco under the influence of trees produced a greater amount of a higher quality fodder than those that grew on totally cleared surfaces, benefits that were maintained over time, increasing the difference between the two of them. Thus, it was also shown that the influence of the tree replaces the need to use chemical fertilizers in the pastures to maintain forage production in quantity and quality.

Keywords: silvopastoral, Arid Chaco, deforestation, Cenchrus ciliaris, cattle, Prosopis

Introduction

Deforestation of the South American native forests occupies the first place in the world ranking of this scourge. This process implied for the 2000-2010 period a decrease of the forest masses of almost four million hectares in the region (Chakravarty et al 2012). Today its progress continues in all forest ecosystems, especially in the Amazon and the South American Gran Chaco (Garret et al 2017). Deforestation is attributed to the expansion of agriculture, mainly soybean and the planting of pastures for animal production, especially cattle for meat (Barona et al 2010). The story in South America is that livestock pasture often comes first, followed by soy. Soybean farms are typically planted on old cattle pastures, and as soy encroaches, pasture is forced into new frontiers (Kimbrough 2021; WWF 2018).

The deforestation process contributes to climate change or global warming, not only producing carbon dioxide with burning but, more seriously, drastically reducing carbon sequestration as trees disappear (Bennett, 2017; FAO 2018). This creates a huge antinomy between those who defend the tree as the lung of the planet and those who support livestock as a source of quality protein. Today, this discussion is in the newspapers and forums of the world and everything indicates that it is a growing process (Arguelles-Gonzalez 2022; Havlick et al 2014). What is not debatable is that both positions are right, the food quality and the atmosphere are basic elements of life as we know it and both are equally necessary. However, this discussion should not exist if the people on both sides knew that the tree and the cattle can not only live together, but benefit from it.

The South American Gran Chaco

The possibility that the tree coexists with livestock has been demonstrated since the end of the last century. This has long been demonstrated in South America, in the Argentine Chaco Forest belonging to the South American Gran Chaco. This immense tropical and subtropical forest ecosystem is shared by Argentina, Paraguay, Bolivia and Brazil with percentages of 45.9 - 32.1 - 14.7 and 7.3, respectively. The South American Gran Chaco, with an area of ​​114 million hectares is the largest forest in South America after the Amazon and comparatively, represents a fifth of the Brazilian Amazonia surface being equivalent to the sum of the territories of France, Spain and Portugal (Ayerza (h) 2017; Goulding et al 2003; Metz and Wessling 2006; Vallejos et al 2015). The Gran Chaco has a very important biological diversity including 3,400 plant species, 500 bird species, 150 mammal species, 220 reptile species and amphibians, most of which will not be able to survive indiscriminate deforestation (WWF 2019).

Like the Amazon, the advance of agricultural production that year after year is stealing surface in an accelerated process is decimating the South American Gran Chaco (Casas and Puentes 2009; Hansen et al 2013). In vast areas of this ecosystem, many species have disappeared with no remaining specimens that can serve as seedbeds for the regeneration of the ecosystem (Ayerza 2020). The protection laws do not represent an efficient brake on this predatory action because the regional governments are all enthusiastic about making the exceptions, a constant that allows the continuity of these deforestation practices. With small variations, this situation is valid for all countries that share this immense ecosystem, as well as for the other South American forest ecosystems including the Brazilian Amazonia (dos Santos et al 2018; le Polain de Waroux et al 2019).

The South American Gran Chaco comprises five regions defined according to their rainfall, in: Humid Chaco (1200-1400 mm), Sub-humid Chaco (750-1200 mm), Semi-arid Chaco (500-750 mm), Arid Chaco (300-500 mm) and Highlander Chaco (500-900). It is common to bring together the Arid and Semi-arid Chaco under the name of Dry Chaco. Rainfall decreases from east to west from 1400 mm to 300 mm. The Highlander Chaco extends from north to south over the Sub-Andean and Pampean Sierras, interrupting the gradient of increasing aridity towards the Andes. It occupies the lower slopes of hills and ravines (Metz and Wessling 2006).

Map 1. Location of the ecosystems of the South American Gran Chaco and the Brazilian Amazon (Ayerza (h) 2020)

The Arid Chaco

In the southwestern portion of the South American Gran Chaco is the Chaco Arid, with an area of 9.6 million hectares (Karlin et al 2013), equivalent to the territory of Portugal (Map 2). This region is heavily desertified, forming one of the most impoverished regions of the South American Gran Chaco (Photo 1), due to deforestation and indiscriminate overgrazing. (Ayerza (h) 1982; Ayerza (h) et al 1988; Karlin 2013c; Santa-Cruz and Quiroga 1998; Vera et al 2003).

Map 2. Location of the ecosystems of the Arid Chaco, Semi-arid Chaco and Monte Desert, Argentina (Bronstein and Karlin 1986)	Photo 1. Representative photo of the desertified Arid Chaco in winter (Ayerza (h), unpublished)

Only traces of the climax plant cover, reconstructed by observing the no so altered areas, remain in some relicts of the areas protected by the provincial states; they occupy 14.41% of the total surface (Reati 2013). The predominant original vegetation is as follows: a xerophytic tree layer, short (7 to 12m high) and not dense, made up mostly of Aspidosperma quebracho-blanco, and Prosopis spp. There is no abundant tree species, such as Ziziphus mistol and Celtis tala. Herbaceous vegetation is dominated by C₄ grasses, mostly species of genus Chloris, Digitaria, Setaria, and Trichloris (Díaz et al 1984).

The original vegetation changed to a continuous shrub land with isolated trees and small spots dominated by grasses. Most of the new structure is formed by shrubs from the genus Larrea, Cercidium,Cassia, and Cactaceae as Opuntia spp., and Cereus spp., and annual grasses of the genus Aristida, and Boutelaua (Ayerza et al 1988). This bush cover competes strongly with the herbaceous stratus and with second-growth trees, precluding rapid recovery (Díaz and Karlin 1987). Therefore, forage and consequently animal production have decreased dramatically.

The vast majority of the soils of the Arid Chaco correspond to Entisols, with little participation of Aridisols in low areas and Mollisols in soils that border the Semi-arid Chaco (Karlin 2013b). The Arid Chaco has high temperatures in the summer and moderate temperatures in the winter, with occasional frosts (5 to 10 days / year). Rainfall is concentrated in summer (80% of the total rains occur between November and April, during the warmest months), being almost nil during the winter. Mean annual rainfall ranges from 500 mm in the East to 300 mm in the West (Anderson et al 1980; Morello et al 1977; SECYT 1986).

Livestock is the most widespread economic activity in the region. The breeding and rearing of cattle and the production of kids predominate throughout the region (Calella 1986; Rossi 1998). In the middle of the 20th century, with the increase in the degradation of the ecosystem, the number of cattle began to decrease while the number of goats began to increase. (Karlin 2013d). Goat breeding is of great importance in the most degraded areas invaded by woody shrubs such as Larrea divaricata, great invader practically not consumed by cattle, and with allelopathic effects that prevent the birth of pastures. However, it is one of the preferred bushes for goats throughout the year (Cora et al 2005).

Since the middle of the last century and the beginning of the current one, animal production in the Arid Chaco has been based essentially on the elimination of woody material and the implantation of monophytic pastures of Buffel Grass (Cenchrus ciliaris) -an exotic grass naturalized in South America- (Ayerza (h) 1991; Seia-Goñi 1985). Silvopastoral systems are currently spreading in the region; these are established using the mechanical treatment called low intensity roller chopping (called RBI by its acronym in Spanish) or with a bulldozer, eliminating the invasive bushes and leaving the trees (Ayerza (h) 2010; Blanco et al 2001, 2013; Ferrando et al 2013; Metz and Wessling 2006; Rossi 2005). The mechanical removal with a bulldozer of invasive shrubs from the native forest means a 50% lower cost compared to the total clearing to implant a traditional pasture (Ayerza (h) 2017).

There are numerous works, including researches carried out in the Chaco Arido, which highlight the important contribution of tree species towards the improvement of the chemical properties of the soil and fodder quality (Ayerza (h) 2010; Ayerza (h) et al 1988; Calle et al 2013; Diaz 2003; Díaz-Lezcano et al 2021; Karlin and Ayerza (h) 1982; Ludwig et al 2008; Mazzarino et al 1991; Montagnini et al 2013; Pezo et al 2019; Soler et al 2017). Through leaf fall and a favorable microclimate under the treetops, soil microbial processes and the subsequent release of nutrients necessary for grass growth have been reported and the accumulation of nutrients, much more important compared to sites without trees (Galicia et al 2002; Karlin 1983; Ribaski and Menezes 2002; Treydte et al 2008).

The objective of this review was to analyze the works on the relationships between pastures and trees with respect to animal production and to show the advantages of silvopastoral systems over those that involve total logging in the Chaco Arid ecosystem.

Materials and methods

The bibliographic review present in the databases of Google Academic, Science Direct, Scopus, Redalyc, ResearchGate, and Sitio Argentino de Producción Animal, using the terms agroforestry-silvopastoral-Arid Chaco was consulted. The keywords for the search were used in Spanish, French, English and Portuguese; the works not digitized on the internet and included in the South American Chaco section were obtained from the author's library and also from empirical information collected during 40 years of work in the region

The search for information on the internet was carried out without restriction on the date of publication. To many of the works in which the author intervened at the end of the last century, additional statistical analyzes and comments were added, which thus help to explain the results obtained in a timely manner.

Result and discussion

Although the search did not provide a large number of documents (87), it did show the existence of technical and scientific works in this regard, from the second half of the twentieth century, with a significant increase from the end of the twentieth century and the beginning of the 21st century. In some cases, studies carried out that demonstrate the influence of the tree on the productivity of pastures in the Semi-arid Chaco (500-750 mm) and other arid and semi-arid regions were used in order to support the results of the researches carried out in the Arid Chaco.

Almost seventy years ago, Morello and Saravia-Toledo (1956, 1959) pointed out the benefits of controlled silvopastoral systems in the regeneration of the Chaco forest. Twenty years later, these benefits were quantified with the works that studied the association of cattle, pastures and trees in the Arid Chaco including demonstrative plots in commercial farms. These researches carried out by scientists and technicians of the College of Agricultural Sciences from the National Universities of Córdoba and Catamarca, Argentina, together with private farmers and within the Algarrobo Program framework of the latter University (Karlin and Ayerza 1982), set the bases of intensive silvopastoral systems in the region. Science and experience have clearly demonstrated that the Chaco Forest can become an efficient producer of animal protein if we keep the original trees and return to the primary forest (Saravia-Toledo, 1984).

Returning to the primary forest implies eliminating the bushes that invade the spaces that were left free of trees removed for their wood and of pastures overgrazed by cattle, thus allowing the re-implantation of the original plant species. These predatory actions were performed indiscriminately due to the lack of adequate knowledge for the management of the forest as a whole (Saravia-Toledo 1984).

These early works showed that pastures that grew under the influence of trees produced a greater amount of a higher quality fodder than those that grew on totally cleared surfaces, benefits that were maintained over time increasing the difference between the two of them. It was also demonstrated that the influence of the tree replaces the need to use chemical fertilizers in the pastures to maintain forage production in quantity and quality.

Table 1 shows the comparative results of the organic matter and nitrogen contents of the soils located in the open range and under the tree canopy of the Algarrobo Blanco (Prosopis alba), at different depths. The soils under the tree canopy presented 103%, 67% and 13% more organic matter than those located in the open range at the surface level, between 2-10 cm and 10-20 cm deep, respectively. A similar trend appeared in the nitrogen content; the difference in the influence of the tree decreases with depth. Standard Deviation values showed for both organic matter and nitrogen, a lower dispersion of the contents of the three sampling depths outside the Algarrobo canopy than under the canopy. These same observations were reported in soils under and outside the tree canopy of Black Algarrobo (Prosopis flexuosa) and Quebracho Blanco (Aspidosperma quebracho-blanco), also in the Chaco Arid ecosystem, in Chancani, province of Cordoba (Hang et al 1995; Oliva et al 1993), and the Acacia senegal and Balanite aegyptica in Ethiopia, in an area with a mean rainfall and temperature of 650 mm and 21°C, respectively (Mussa et al 2016).

Table 2 compares the dry matter production of a seven years old pasture of Buffel Grass cv. Texas 4464, in the open range and under the influence of the tree canopy. The forage yield was measured as dry matter per hectare. The grass grown under the tree canopy produced 115% and 67% more (P<0.05) than that in the open range, in 1983 and 1984, respectively. Standard Deviation values show less dispersion under the canopy than outside it, suggesting less variability over time.

This higher forage production is supported by the results obtained under and outside the tree canopy of Prosopis flexuosa and Prosopis alba determined in the Paraguayan Semi-arid Chaco, verifying a greater significant difference (P<0.05) in pastures of Buffel Grass, Panicum maximum cv. Gatton and Pasto Pangola (Digitaria decumbens) grown under the canopy tree (Gamarra-Lezcano et al 2018).

Figure 1. Evolution of crude protein production of Buffel Grass cv. Texas 4464 throughout the year, in the open range and under the canopy of the Algarrobo Negro in the Arid Chaco (adapted from Diaz et al 1984)

The Standard Deviation of the Fig. 2, 0.10 and 0.98 (data not shown in Fig. 1) under and outside the canopy of the Algarrobo Negro tree, respectively, shows an important concentration of crude protein values, which confirms the little variation throughout the year in both sites. The average through the year was significantly (P<0.05) higher for forage crude protein content under tree influence than outside it. These results are consistent with the levels of crude protein obtained in Buffel Grass growing under and outside the tree canopy of Algaroba (Prosopis juliflora) in the arid ecosystem of the Brazilian Catinga (Ribaski and Menezes 2002).

The crude protein contents of Buffel Grass cv. Texas 4464 that grows under the canopy of the Algarrobo Negro - fig. 1 - are sufficient to cover the needs of an adult cow without a nursing calf, -dry pregnant mature cows- during its entire gestation period. However, the content of the Buffel Grass that grows outside the tree canopy was insufficient in all the determinations, according to the National Research Council (1980). It should be noted that the National Research Council tables have been developed essentially on the basis of Bos taurus animals, while there is strong evidence that Bos indicus cattle require significantly less protein among other nutrients, than the first ones (Ayerza (h) 2019; Frisch and Vercoe 1977; Mudgal and Ray 1965; NDRI 1980; Patle and Mudgal 1975; Rodriguez 1983; Vercoe 1970).

The influence of the Algarrobo Blanco on pastures during the hot and rainy season was also determined in the protein production of native pastures (Table 3), with 23% (average of three species) more (P<0,05) protein being determined under the tree canopy than in the same pasture species in the open range. The lower Standard Deviation of the grasses in the open range shows that under conditions of lower productivity, the three grasses showed greater uniformity than under the tree canopy.

The percentages of crude protein in Table 3 are sufficient to meet the requirements of cows nursing calves (average milking ability; first 3-4 months postpartum; 5.0 kg milk/day). However, outside the tree canopy, crude protein contents were not sufficient to reach their maximum potential (National Research Council 1980). With the available information, these last contents are sufficient for physiologically equivalent cows of Bos indicus breeds or their crosses (Ayerza (h) 2017; Mudgal 1981; NDRI 1980).

Table 4 compared the quantity and quality of pasture biomass produced in the open range and under the tree canopy during the hot and rainy season. In the open range there was a decrease of 53% in the quantity of grass measured as dry matter, and 36% in the quality measured as protein content. Both differences were significantly different (P<0.05).

Photo 2. Silvopastoral system in winter with Buffel Grass cv. Nueces and native trees, Arid Chaco of Villa Dolores, Cordoba, Argentina (Ayerza (h), unpublished)

The importance of the tree influence on the pastures biomass quality was clearly demonstrated when it was determined how its nutritional quality decreases dramatically as the grasses move away from the tree canopy (Table 5). The grass protein content was measured in an eight years old pasture of Buffel Grass cv. Texas 4464. This was 36, 42 and 45% lower at distances of 2, 5 and 10 m from the edge of the tree canopy, respectively.

The influence of the tree on the quantity and quality of biomass produced by pastures allows saving up to just over 100 kg/ha/year of nitrogen (Table 5) equivalent to 222 kg of urea (45%). This chemical fertilizer is one of the most used worldwide in plant production and is responsible for the pollution of water resources (Skorupka and Nosalewicz, 2021). In Argentina, this also implies a fertilizer saving of approximately US$ 189/ha/year, at current values.

The influence of the tree is not restricted to the legume family, with the Algarrobo (Prosopis spp) different species. A decrease in the quantity and quality of the pastures in the open range compared with the influence of the non-legume tree Quebracho Blanco ( Aspidosperma quebracho-blanco) was measured. The results were 47% lower on the amount of biomass and between 28 and 44% lower on the protein content, depending on the year (Table 6). The results of this work allow us to infer that the percentages of crude protein determined in the Buffel Grass cv. Texas 4464 grown under the Quebracho Blanco canopy are sufficient to cover the requirements of pregnant adult cows Bos taurus, without a nursing calf, -dry pregnant mature cows-, while it does not grow outside the tree canopy (National Research Council, 1980). We must clarify that the latter not only meet the requirements of Bos indicus cattle, in the same physiological state as Bos taurus, but also of pregnant cows and with a nursing calf (Mudgal 1981; NDRI 1980).

Another very important aspect of the tree-grass relationship compared to grass grown in open fields is a small decrease in protein content over time. Table 8 shows a difference in favor of grass under the canopy of 8% one year after clearing, which is extended to 77%, eight years later.

The results of a research carried out by Karlin et al (2021) in the Arid Chaco of the province of Cordoba suggest “different effects on soil and microclimate when comparing the effect of canopies and litter cover of P. flexuosa. Canopy and litter accumulation tended to conserve soil moisture by reducing soil and air temperature and increasing air moisture compared to the intercanopy. These relations also favored the frequency of decreases, which produce better forage quality”. Comparing the effect of rainfall, Magliano et al (2016) determined that Eragrostis curvula and Cenchrus ciliaris pastures in open range versus under the canopies of Prosopis flexuosa, Aspidosperma quebracho-blanco and Larrea divaricata presented less direct interception (0.35±0.13 vs. 1.51±0.50 mm/ precipitation event), higher surface runoff (~28 vs. 0% runoff in large and intense events) and higher potential evaporation at ground level (7.0±0.9 vs. 4.4±0.9 mm/day in the summer). These results suggest that the establishment of pastures on sites originally covered by forests would potentially reduce the perspiration of rainfall and increase the potential risk of water erosion.

These works included the determination of beef production for the ecosystem of the Arid Chaco of Villa Dolores, Córdoba, with an intensive silvopastoral system. Up to 270 kg of meat production/ha/year was obtained with a stocking rate of 2 animals/ha/year (Table 8), using 60 trees/hectare of native trees, Prosopis flexuosa, Aspidosperma quebracho-blanco, Geoffroea decorticants and Cercidium praecox, in a proportion of 85,3%, 13,9%, 0,6% and 0,2% respectively, Buffel Grass cv. Nueces, Nelore (Bos indicus) and Braford (Bos indicus x Bos taurus) cattle, and an intensive rotational management (Ayerza (h) 2010). This beef production is relevant for the area, considering that in lands totally cleared and implanted with chemically fertilized Buffel Grass cv. Texas 4464, with heifers of these two same genotypes and intensive rotational managing, 226 kg/ha/year were obtained with a stocking rate of 1.7 animals/ha/year, in the same location (Ayerza (h), 1988).

Diffusion and intensive silvopastoral implementation

Silvopastoral systems properly managed not only allow the production of food, but also maintain the basic services of the forest, promoting biodiversity, carbon sequestration, water and ecological processes quality (Castro-Nuñez et al 2021; Giraldo et al 2011). In addition, silvopastoral systems have been shown to decrease the production of ruminal methane, forming animal production systems with a positive balance of greenhouse gases, which allows us to suggest that they are environmentally sustainable systems (Arcos-Dorado 2016; Naranjo et al 2012).

It is necessary to be clear on the concept that implementing intensive silvopastoral systems in completely deforested land is a slow and much more expensive process than resorting to its implementation in native forests (Ayerza (h) 2017). In addition, from an environmental point of view, native forests have been shown to maintain greater biological diversity than artificially forested forests (Murgueitio et al 2011).

This intensive silvopastoral system has recently been called ISPS by its acronym Intensive Silvopastoral Systems (Calle et al 2013). We should not confuse these technified systems with traditional silvopastoral systems that only involve the introduction of livestock into forests, without considering the physiology of each of its components that is, without proper administration of the different elements that make it up (management). In the Arid Chaco, 270 kg/ha/year of live weight (Table 8) were obtained with ISPS, while traditional silvopastoral systems only produce between 4-10 kg/ha/year (Anderson et al 1980; Ayerza (h), 2010; Bocco et al 2007; Calella 1986; Karlin 2013a).

A review work on the technical silvopastoral systems research carried out by researchers of the National Institute of Agricultural Technology of Argentina (Soler et al 2017) demonstrates the increase they had in recent decades in South America. They show the researchers interest on these systems in different forest ecosystems of the region, including the Arid Chaco. However, the generation of this information did not imply an important incorporation of these systems based on the trees conservation, such as stopping the incessant process of regional deforestation (Ayerza (h) 2019; Baumann et al 2017).

Reality in the Arid Chaco as in all South America shows that, deforestation goes faster than the implementation of intensive silvopastoral systems (ISPS) (Montagnini et al 2013). Everything indicates that the information on the comparative advantages of these systems did not spread fast and depth enough to allow raising awareness among those who must make the decisions properly, both private and state. Perhaps if the two conflicting opinion forces, environmentalists and productivists, joined and dedicated themselves to understanding and spreading this information among the actors of the process, that is farmers, politicians, rulers, administrators and all those who have to make the decisions to determine the future of the countries, this widespread ignorance could be overcome and the complete elimination of forest masses for animal production be avoided.

Conclusions

The presented review shows the close and beneficial relationship between the tree and the pastures, which translates into the fact that keeping the native trees, the pastures, both native and introduced, associated with livestock, can produce outstanding volumes of high quality protein food in the Arid Chaco ecosystem. Much work still needs to be done to determine these associations, but everything indicates that intensive silvopastoral systems (ISPS) come up on the horizon as a promising alternative to overcome this contemporary antinomy between productivists and environmentalists, stopping the scourge of deforestation.

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