Characterization of the study area

The proper management of the study population requires knowledge of the diametric distribution of the trees of which it consists (^{Cao, 2004}).

In order to make the floristic composition and diameter distribution of the analyzed parcels known, we present below a description of all the trees that exist in the plots, the diametric distribution of the measured individuals, and the grass species registered, as well as a general description of the observed characteristics.

The crowns of the carob trees cover, on average, 13 % of the total surface area of each plot; the spatial distribution of the individuals was random, with a tendency to form clusters with an average distance of 9 m between one another. These data are important because they make it possible to identify the level of influence of the shade provided by the canopy on the other components of the system, such as the soil, the grassland and the livestock.

The diameter classes indicating the distribution of individuals are the following: Class 1: DBH<10; Class 2: 10 to 19 cm DBH; Class 3: 20 to 29 cm; Class 4: 30 to 39. Figure 1 shows the diameter distribution of the 246 measured individuals.

Figure 1 Diametric distribution of all the measured carob trees. Central Chaco (December 2015). 

130 individuals (53 %) out of the total number of registered carob trees (245) fall into the category of regeneration, with a DBH<10 cm; the remaining 115 (47 %) had a mean DBH of 16 cm. An average of 31 specimens were measured in each plot.

As ^{Glatzle (2004)} has stated, allowing the establishment of carob tree (Prosopis alba Griseb., P. nigra Hieron. or Prosopis kuntzei Harms) regenerations with a density of 20 to 50 trees per hectare in degraded grasslands may bring ecological benefits, since the greater accumulation of organic matter beneath their crowns helps increase the yield of the grasslands for the livestock.

Basal area. ^{De Arruda Veiga (1984)} points out that knowledge of the basal area of a population is crucial for estimating the volume and determining the density of the population. In this regard, at the time of the measurement, the installed plots had an average of 31 individuals of Prosopis spp. per hectare, particularly of the Prosopis alba species (white carob tree), followed by Prosopis nigra (black carob). Figure 2 shows the values of basal area, with an average of 0.34 m² ha^-1; plot 6 had the minimum value, of 0.02 m² ha^-1, and plot 5 had the maximum value, which was 0.64 m² ha^-1. Furthermore, this figure depicts the average basal area of the carob trees per hectare.

Figure 2 Average basal area of the carob trees per hectare. Central Chaco (December 2015). 

35 trees were measured in plot 6; they were all regenerations, with a mean DNH of 2.6 cm. On the other hand, the 30 individuals of plot 5 had a mean DBH of 16 cm, i.e. they were all adult trees.

^{Senilliani and Navall (2005)} estimated similar values in a plantation of Prosopis alba trees aged 4.5 years, with a density of 555 trees per hectare and a basal area of 0.7 m ² ha^-1.

^{Kees et al. (2015)} cite higher values in the Argentinean Chaco, in a silvopastoral system consisting of a plantation of carob trees, with a density of 150 trees per hectare, associated with Panicum maximum Jacq. The mean basal area of the trees of this species aged 10 years was 5.62 m² ha^-1, and that of the trees aged 14 years was 8.07 m² ha^-1.

Volume. It is important to know the volume of timber produced in grasslands, forests or plantations in order to acquire technical knowledge on the development of individuals and in order for the producers to learn how much timber that can be obtained from these sources and marketed (Ordóñez et al., 2012).

The average estimated volume in the study plots was 1.59 m³ ha^-1; the maximum value was obtained in plot 2, made up in its entirety of white carob individuals, with a mean DBH of 13.15 cm, a height of 6 m and a volume of 2.75 m³ ha^-1; the minimum value, 0.035 m³ ha^-1, was estimated in plot 6, formed by regenerations of black carob, with a DBH of 2.66 cm and an average height of 1.76 m (Figure 3).

Figure 3 Volume of carob trees per hectare. Central Chaco (December 2015). 

In an assessment of silvopastoral systems with three carob tree species (Prosopis nigra, Prosopis affinis Spreng. and Prosopis vinalillo Stuck.) implemented on a of 125 m × 80 m plot at the Maroma ranch, in the Presidente Hayes Department of the Humid Chaco, the calculated volume averaged 2.79m³ ha^-1 (^{Arano y De Egea, 2014}).

Total biomass of the tree stratum. The aboveground biomass includes everything that is above the ground, such as the stem, the branches and the leaves (^{Yepes et al., 2011}). Thus, the average biomass of the tree component of the system was 2 t ha^-1 (Figure 4), a value higher than the reported by ^{Ibrahim et al. (2007)}, less than 30 individuals present per hectare, in tress with DBH ≥5.

Figure 4 Total biomass of the arboreal component. Central Chaco (2015). 

Grassland biomass

The values varied according to the degree of exposure to solar radiation; this refers to the canopy cover of the carob trees.

The grasslands most frequently identified in the plots were Gatton panic, Pangola and Buffel; stargrass and Urochloa grass were found in a lower proportion.

The carob trees influenced the development of the biomass of the grassland, as there were significant differences between the samples drawn beneath the carob tree canopy and those drawn outside of the crown, according to a comparison carried out using the Student t test method with data from plots that were not paired, at a confidence level of 95 %.

Thus, the biomass of the grassland located outside the projection of the carob tree crown had an average value of 0.46 t ha^-1; on the other hand, the mean biomass of the grassland beneath the crown amounts to 1.65 t ha^-1 (Figure 5).

Figure 5 Average grassland biomass under two different conditions of solar radiation. Central Chaco (December 2015). 

^{Valle et al. (2004)} cite higher values (3.5 t ha^-1) for the biomass of 3.5 month old Buffel grass associated with Gliricidia sepium (Jacq.) Kunth (a tree species belonging to the Leguminosae family) on a 500 m² plot located in Miacatlán, Morelos, Mexico.

^{Ibrahim et al. (2001)} stated that Panicum maximum had a higher content of biomass (2.98 t ha^-1 ) when associated with Erythrina poeppigiana (Walp.) O.F.M. Cook, compared to 2.07 t ha^-1 of biomass produced by grasslands associated with woody species.

Chemical properties of the soil

Organic matter content. The organic matter is made up of the remains of animals and plants (of fallen leaves, dead logs, tree roots or herbs and crop residues) in various degrees of decay and fulfills important functions; furthermore, it is an indicator of the quality of soil because it conditions its physical, chemical and biological properties (^{Plaster, 2000}; ^{Porta et al., 2014}).

The organic matter content of the soil in the studied plots did not show significant differences between sun and shade, but the differences are significant when the sampling depths are compared (Student t test at a confidence level of 95 %). Figure 6 shows the average biomass under the two solar radiation conditions.

Figure 6 Average organic matter content as a percentage under two different solar radiation conditions and at two different depths in silvopastoral systems of Central Chaco (2015). 

Samples drawn beneath the shade of trees, in the first 10 cm of ground, had the highest average value -3.38 %. This percentage is interpreted as a high content of organic matter. Under the sun, the value was 2.6 % in the first 10 cm, an indication that the organic matter content diminished under the sun. The values were also lower at a depth of 10-30 cm, i.e. 1.09 % in the sun and 1.43 % under the shade. As the depth increased, the organic matter content decreased to medium and low levels.

This coincides with the description by ^{Thompson and Throeh (2013)}, according to whom the organic matter covers the surface of the ground, then decays and is mixed and incorporated to the first 5-15 cm of mineral soil thanks to the action of the mesofauna that lives at these depths.

^{Glatzle (1999)} cites an organic matter content of 3.3 % beneath the carob tree crowns, and of 2.4 % in areas without a cover of carob trees in a study carried out in the Central Chaco of Paraguay.

Organic carbon. The soil is a great reservoir of carbon, its concentration has high variations, even within small areas, due to the heterogeneity of soils, the climatic conditions and the geomorphic elements (^{Yepes et al., 2014}).

The total organic carbon content was determined in the laboratory using the Walkley-Black method; the percentage value was extrapolated to kilograms and tons, considering a soil bulk density of 1.24 g cm^-3 and 1.25 g cm^-3 at the depths of 0 to 10 cm and 10-30 cm, respectively. Table 3 shows the average values found under different conditions.

The total content of soil organic carbon in the first 30 cm was 44.85 t ha^-1 under the direct influence of the carob tree crown, and 34.03 t ha^-1 outside it.

^{Lok et al. (2013)} reported similar values in a silvopastoral system with Panicum maximum and Leucaena leucocephala (Lam.) de Wit aged 8 years, with an organic carbon content of 38.8 t ha^-1 in the first 35 cm of ground.

Total nitrogen. Nitrogen is the main element that provides organic matter for plant growth; it is considered to be a primary macronutrient because it is used in large quantities by plants and is not always available in the soil in sufficient quantities to allow them a better growth (^{Plaster, 2000}).

The nitrogen content was estimated taking into account that the content of this element amounts to 5 % of the organic matter. Table 4 presents the average values found under each condition.

The total nitrogen content in the first 10 cm of soil outside the influence of the carob tree crown was 1.6 t ha^-1; 2.03 t ha^-1 under its shade, and 1.34 t ha^-1 and 1.75 t ha^-1 at a depth of 10 to 30 cm under the sun and shade, respectively.

^{Silberman et al. (2015)} reported a similar nitrogen content in the first 15 cm of ground covered with Gatton panic (0.828 t ha^-1), and a higher value (4.14 t ha^-1) in a silvopastoral system based on its association with Ziziphus mistol Griseb. (Mistol) (22 to 27 trees ha^-1) with Gatton panic, in a study carried out in Santiago del Estero, in the arid Chaco of Argentina.

Carbon/Nitrogen Ratio. The organic matter of the plots presented values of 11.8 and 12.07 in soils outside the direct influence of the carob tree crowns, at a depth of 0 to 10 cm and 10-30 cm, respectively; as for the ground under the influence of the carob trees, a value of 11.6 in the first 10 cm, and of 11.67 at a depth of 10 to 30 cm.

No significant differences were obtained in paired plots through the Student t test (with a confidence interval of 95 %) between conditions of sun and shade or for different depths. The calculated value indicates that, according to the description of the organic matter by ^{Porta et al. (2014)}, the organic matter is stable when the value of the C/N ratio is 10 to 14 (Figure 7).

Figure 7 C/N ratio in the plots at two different depths and under two different conditions of solar radiation. Central Chaco (2015). 

A C/N ratio is between 10 and 14 favors the proliferation of microorganisms that bring about the decay of organic matter, because it provides them with sufficient carbon to use as a source of energy and with enough nitrogen to synthesize their proteins, and stimulates the mineralization of the nitrogen made available to the plant components of the system.

These results are consistent with the records of ^{East and Felker (1993)}, for the C/N ratio, of 12 to 14, beneath Prosopis glandulosa Torr. var. glandulosa in the open land; the first result can be attributed to the increase in microbial activity that led to an increased release of nitrates.

According to ^{González (2009)}, there were no significant differences in the C/N ratio between silvopastoral systems and natural grasslands located in the province of Chimborazo, Ecuador; the ratio was 11.6 and 11.3, respectively.