Cactus pear has both productive and protective potential, and it can play an important role in enhancing the productivity of lands to meet the demand of ever-growing human and livestock population.
Future prospects for expansion of cactus pear in semi-arid and arid regions of India exist in 55.6 million ha degraded arid and semi-arid region. Systemic research on cactus pear has clearly shown that it can contribute significantly to meet the deficits of fodder, accelerate economic growth, and help in poverty alleviation, women empowerment, and livelihood support.
The major environmental functions of cactus pear plantation can be enumerated as:
1. Control of soil degradation and rehabilitation of problem soils
2. Control of desertification
3. Drought moderation
4. Increasing biodiversity in the farming system and watershed scale
Cactus pear can play a major role in maintaining the resource base and increasing overall productivity of rangeland, thus helping in building climatic resilient agriculture.
1. The Role Played by Cacti in Agroforestry:
The cacti plantations may be profitably supplemented by planting other fodder trees and shrubs, particularly those which provide nitrogen rich forage such as Atriplex spp., Acacia spp., and Prosopis spp. For instance, Prosopis spp. are phreatophytes that may make use of deep (15 m and more) brackish aquifers that would otherwise not be available; this is still more so with Atriplex spp. which can make use of aquifers up to 10 m deep, having a salinity of up to half that of sea water. Nitrogen-rich fodder shrubs and trees add to the necessary nitrogen requirements of stock and wild life. The mixed plantations are not amenable to a rational management in a practical and economic manner.
2. Role of Cacti in Land Rehabilitation and Desertification:
Land degradation is a combined effect of several natural and anthropogenic land degradation processes such as wind erosion/deposition, water erosion, salinisation, and physical disintegration, chemical, biological and natural. Among natural process climate and neotechtonic activities play an important role in the land degradation.
However, anthropogenic factors, such as cultivation on marginal lands, cutting of shrubs and trees, over grazing and use of highly saline and sodic waters for growing crops are the principal causes of degradation/desertification of different fragile arid ecosystems depending upon their degree of inherent vulnerability.
Under hot arid region where annual rainfall is less than 300 mm, wind erosion/ deposition is more serious and widespread due to low rainfall and highly vulnerable sandy terrain. Overgrazing and indiscriminate felling of trees resulted in the degradation of vegetative cover and decrease in biomass production. However in more than 300 mm rainfall zone, water erosion and wind erosion/deposition are the serious problems affecting the biological productivity of different ecosystems.
In India 55.6 m ha (36.3%) area of total hot arid and semi-arid regions is suffered from water erosion, wind erosion and mining and industrial waste. Planting shrubs and particularly cacti, is one of the easiest, quickest and safest ways to rehabilitate them, if not the cheapest. Given the high drought-tolerance and rain-use efficiency of cacti, the productivity of degraded rangeland converted to cactus groves can be increased by a factor of as much as 1 to 10 with respect to degraded rangelands and still a factor of 1 to 5 when rangeland in good condition is concerned.
Degraded Mediterranean rangelands have a RUE of 1 to 3 kg DM ha -1 year -1 mm-1, whilst rangelands in good condition exhibit a RUE of 4 to 6, desertised rangelands, in turn, may have a RUE factor as low as 0.1-0.5. Land rehabilitated with cacti or salt bushes, in contrast, exhibit a RUE of 10 to 20 kg of above-ground DM ha -1 year -1 mm -1 in the 200-400 mm range of mean annual rainfall and 15 to 30 in the 400-600 mm range.
Moreover, cacti, to a great part because of their easy establishment by vegetative propagation, are amenable to the rehabilitation of lands that could not be reclaimed through conventional agricultural methods because of their steep slope, stoniness or rockiness. Few plant species are able to provoke an increase in land productivity of the magnitude, particularly where marginal lands are concerned. Marginal lands often represent fragile ecosystems. Ploughing and indiscriminate vegetation removal can result in large-scale degradation and destruction of vegetation cover.
The scarcity and even disappearance of several plant species indicate the magnitude of genetic and edaphic losses. To reverse the desertification trend and to restore vegetation cover in such areas, appropriate integrated programs are needed for rangeland monitoring, livestock control, and conservation of natural resources.
3. Role of Cacti in Runoff and Erosion Control:
Cactus plantations also play important role in runoff and erosion control and watershed management. Cactus hedges also play a major role in land-slope partitioning, particularly when established along contours. Moreover, hedges are a physical obstacle to runoff, favouring temporary local runoff accumulation and silting, thus preventing regressive erosion. Spineless cactus (O. ficus-indica), which is drought and erosion tolerant, may be useful to slow the direct sand movement, to enhance the restoration of the vegetation cover, and to avoid the destruction of the land terraces by water.
In central and southern Tunisia, cactus plantations play a key role in natural resources conservation as well as provide a large amount of fodder for livestock. The strong rooting system of cacti helps to stabilise land terraces. One or two rows of cladodes are planted on the inner side of the terraces.
The rooting system is enhanced by the collection of the water at the base of the terrace. Roots are widespread on the elevated part of the terrace and go deeply into the soil, ensuring stability of the terraces. Platyopuntias are also used in combination with cement barriers or cut palm fronds to stop wind erosion and sand movement. In arid lands subjected to wind erosion, cactus hedges are an easy, cheap and efficient way of prevention and control of top soil loss and for the accumulation of wind-borne deposits.
4. Role of Cacti in Soil Conservation and Amelioration:
Opuntias have great potential for biomass productivity and could even be used for sequestering massive amounts of carbon, an international goal for mitigating the increasing atmospheric CO2 concentrations and the accompanying global climatic change. Opuntias also help in maintaining soil fertility via their geobiogene and trace element cycling activities, enriching the top-soil in organic matter and improving its structure and the stability of its aggregates, hence permeability and water intake budget and balance; this is in a large part due to the thick mat of shallow roots which they develop that later decomposes in the soil with an apparently rapid annual turnover.
Soil physical properties and organic matter content are considerably improved under opuntia hedges and immediate adjacent areas, as regards enrichment in organic matter and nitrogen, as compared to sub adjacent open-field conditions: 40-200% increase in organic matter and nitrogen have been reported as compared to sub-adjacent cultivated fields. Top soil structural stability of the aggregates is thus enhanced; hence sensitivity to surface crusting, runoff and erosion are reduced, while permeability and water storage capacity are increased.