There are numerous reasons behind the diffusion of cacti as forage or fodder around the world, such as the simple cultivation practices required to grow the crop, rapid establishment in a new area, easy multiplication practices (18). Also, they have the ability to withstand prolonged drought, high temperatures as well as soils affected by wind and water erosion (16). This aptitude has made the cactus plantations suitable as food supply and they have become strategic to mitigate the effect of drought over animal production systems in various arid and semi-arid areas of the world. Cacti have greater water-use efficiency due to Crassulacean Acid Metabolism (CAM) photosynthetic pathway (24) and this makes them especially suited for forage productions in arid lands.
Opuntia species are not a balanced feed: rich in energy, calcium and beta carotene, but poor in fiber and nitrogen (4). As evidenced by various studies, some efforts have been made with the purpose of increasing the nitrogen content in cactus for using as forage (8, 10, 17).
Opuntia spp. can be cultivated in a wide range of environments. The major limitation to cultivation of cactus in many areas of the world are severe cold winter temperatures such as happens in the region of Mendoza, Argentina (16), northern Mexico (5), the Mediterranean Basin (22), the arid highland steppes of western Asia (23) and the southwestern United States (25). O. ellisiana has a lower growth and productivity compared to other Opuntia species such as O. ficus-indica (L.) Mill. but it stands out for its resistance to sub-zero temperatures proper of the arid lands.
Prosopis spp. has the capacity of tolerating drought and unfavourable edaphic conditions, such as salinity, as well as being well adapted to the herbivory. All these are the main reasons of its dominant position in the silvopastoral systems of the arid and semi-arid areas of America. On the other hand, they generate spatial heterogeneity that affects the distribution of the shrub and herbaceous layers due to the modification of the microclimatic conditions under its canopy (29).
The improvement in the forage value of cactus under the Prosopis crown indicates the better condition of the site as a result of the higher nutrient content of the soil (26). In arid ecosystems, dominant woody plants are likely to cause changes in microclimate and soil properties by mitigating harsh environmental conditions such as high temperature and solar radiation (6, 19). Also, shrub species play an important role in arid land vegetation, since in several cases, they act as "nurse plants" improving microclimatic conditions, increasing water and nutrient availability, and offering protection against herbivory (13).
The objective of this work was to determine the effect of associating tree-cactus over the bromatological parameters and DM productivity per plant of Opuntia ellisiana Griffiths, implanted under and outside the Prosopis spp. canopy, considering the north and south location respect to the tree. It was hypothesized: a) productivity and nutrient content, mainly crude protein (CP), of cactus under the Prosopis will be greater than those planted outside the canopy and b) these parameters will be influenced by the location of cacti plantation, north or south, with respect to the Prosopis trunk.
Materials and methods
The experience was carried out in the campus of the CCT-CONICETMendoza (32°53'45" S; 68°52'28" W, 840 m a. s. l.) Mendoza, Argentina. The maximum and minimum absolute temperatures (2014-2015 period) were 38.0 and 0.9°C in summer and winter, respectively. Precipitation is characterized by high spatial and temporal variability, it occurs in summer mostly as intense precipitations and the mean for the 2014 and 2015 period was 324.2 mm year-1, during an exceptionally wet period given that the mean of the past 10 years was 226.7 mm year-1. Soils in the study area are of alluvial origin with heterometric clasts in a sandy matrix, normally non-structured, deep, well drained, belonging to the order of Torripsamments. Soluble salts are washed into the profile increasing the electrical conductivity between 25-40 cm deep.
In the soils, texture (Boujoucos), N (Kjeldahl), P (UV visible spectrophometry), K (Pratts), Na (flame photometry), Ca and Mg (complexometry with EDTA), pH and electrical conductivity were determined. Samples of soil for analysis were taken from the first 20 cm of the soil, bearing in mind that 91 to 100% of the roots of the O. ficus indica are found in the top 15 cm of the ground (27). To know the soil parameters at greater depth, other soil samples were also extracted at 40 cm depth.
O. ellisiana was used in this study due to its resistance to low temperature (12). This experience started in the autumn of 2014. Double cladodes, homogeneous in size, provided by the IADIZA experimental collection, were used for the plantation. They were cut before they started giving out new cladodes.
Sampling of cladodes took place at the end of the growing period, as from the month of April. Organic matter (OM), dry matter (DM), neutral detergent fiber (NDF) and acid detergent fiber (ADF) (Van Soest and Wine), crude protein (CP) by Kjeldahl, P (vanadomolibdophosphoric complex colorimetric, Spectronic 21 Bausch & Lomb.), K and Na (flame photometry, Metrolab 315), Ca and Mg (atomic absorption spectroscopy, Perkin Elmer AAnalyst 200) were determined in the cladodes. The samples were obtained from the south and from the north, under the Prosopis canopy; a similar sampling being carried out outside the influence of the canopy.
The experimental design included 18 and 23 plants located under and outside of the Prosopis canopy, respectively. Under the canopy of five trees, four plants were implanted, two to the north and two to the south, equidistant between the trunk and the edge of the canopy. Five plots with five double cladodes in each of them were also used as controls. Two plants under Prosopis spp. and control, respectively, did not thrive in their growth. After a year of the plantation, all the cladodes were collected, with the exception of the double cladodes planted when initiating the experience.
In order to determine the photosynthetically active radiation (PAR), a linear radiometer (Apogee MQ 300) was applied. To quantify the air temperature at 1 m height from the ground and 5 cm deep in the ground, a multichannel digital thermometer was used.
The data for PAR and temperature were registered in spring (September), summer (February) and winter (June), between 11 a.m. and 1 p.m. These reports were taken in the centre of the tree and outside the influence of the canopy; the orientation being north-south.
The data were analysed using multivariate analysis with InfoStat statistics software. As confirmation, multivariate analysis of variance (MANOVA) of the nutritional variables was performed. To compare mean vectors between the groups, a Hotelling test was used (21).
Results and discussion
In each sampling period, the air temperature had very similar values both outside and under the canopy. The values of radiation and soil temperature were higher outside the canopy (table 1, page 133). Although the influence of the radiation level on the growth and productivity of Opuntia has been demonstrated (1, 9, 11, 14), in this experiment the Prosopis’ effect on the cactus was more important than the higher radiation received by the cladodes outside the canopy. Shadow of Prosopis influenced the incident radiation with a decrease of 20% in summer, 15% in winter and 32% in spring, compared to outside the canopy. During the post-implant and the harvest of the cladodes the temperatures recorded were not low enough to evaluate any damage by frost, having registered -0.7 and 1°C as the coldest days of winter of 2014 and 2015, respectively.
In soil, under Prosopis canopy, the contents of N, OM and others elements such as P and K were greater than those found outside it (table 2, page 133).
The contribution of nitrogen by Prosopis mulch under its cover could be explained by the redistribution of the nutrients absorbed by the radical system (3, 28) and the biological fixation of nitrogen by Rhizobium (15), and in addition, through of the litter accumulated under the canopy.
Throughfall effects are generally attributed to leaching from leaves, but canopies also filter and capture atmospheric nutrients, and these processes contribute large amounts nutrients to the understory.
The high percentages of nitrogen, derived from atmosphere in wood and leaves, clearly indicate that Prosopis is actively fixing nitrogen (15).
There is evidence that canopy filtration of atmospheric nutrients provided K, Mg, Ca, Na and P (7).
In the principal component analysis (PCA), considering the data obtained from cladodes and soil, the first two axes of variation explain 78.4% of the total variability (figure 1, page 134).
The first component separates cladodes according to the nutrient contents and productivity; to the right, cladodes under the canopy with higher values, to the left cladodes outside the canopy and closely related to environmental variables.
No differences were found in the variables measured for cacti located in the northern and southern parts of the canopy.
Furthermore, to compare mean vectors between groups and confirm the information given by PCA, a Hotelling test was used.
Table 3 (page 135), presents the bromatological values and cladode quantity per plant of O. ellisiana planted outside and under the Prosopis canopy.
Dry matter productivity per plant (g) were 34.9 ± 16.1 and 48.1 ± 24.1 outside and under the canopy, respectively. Under the Prosopis canopy, the nutritional values of O. ellisiana were increased, the nitrogen content, to almost double its value. Productivity cladodes per plant and concentrations of moisture, OM, ADF, NDF, K and P in the cladodes were also significantly higher under Prosopis, while Ca and Na were higher outside the canopy. Magnesium values were not affected by the position, under or outside the canopy. Under the crown of the tree the effective precipitation is greater than outside of it, due to the runoff of the branches and trunk (20), which favors the leaching of Na in the soil; while an increase in N and P reduces the uptake of Ca and Na (8).
The improvement in the forage value of cactus under the Prosopis crown indicates the better condition of the site as a result of the higher nutrient content of the soil and the contribution of OM as mulch, resulting in the formation of fertility islands (26).
In arid and semiarid ecosystems, dominant woody plants are likely to cause changes in microclimate and soil properties by mitigating harsh environmental conditions (e.g., high temperature and radiation) and by modifying soil characteristics, resource availability (e.g., water and nutrients) and spatial distribution of nutrients (2, 6, 19).
The nurse effect of Prosopis improves the nitrogen content of cladodes in the same way as cattle manure applied to soil (10). The high doses of chemical fertilizers applied almost doubled the CP mean content of the 1-year-old cladodes when it was compared with the treatment in which no fertilizer was added: 7.8 and 4.3% DM, respectively (17).
There is no significant influence (p>0.05) of the north/south orientation under the canopy on the measured variables.
There are significant differences (p<0.05) in productivity and nutrient content for cactus planted under and outside the Prosopis canopy. The nurse effect of Prosopis improves the nitrogen content of cladodes in the same way as cattle manure or high doses of chemical fertilizers applied to soil. The enrichment of the pastoral system in arid environments by the introduction of Opuntia associated to Prosopis increases both the productivity and forage quality of the system.
This article was originally published in Rev. FCA UNCUYO. 2018. 50(2): 129-137. ISSN impreso 0370-4661. ISSN (en línea) 1853-8665.