18 Afforestation of saline soils in South Asia-a case study - Shams-ur-Rehman^[25]

ABSTRACT

Several countries in Africa, Asia and Latin America are facing acute problem of poverty as the majority of the population are living below the poverty line. In view of scarce resources, there is a need for some practical and economically sound strategies that could be developed to efficiently utilize the natural resources for the welfare of the rural poor. However, large tracts of land are still lying barren and unutilized which could be made productive to raise the living standard of the people. Lack of awareness among the masses and mismanagement seem to be the key reasons in several developing countries leading to little or no progress in this direction. Involving local communities in reforestation and selection of suitable species in combating salinity seem to be among the economically viable approaches to reduce poverty. This paper describes the screening/selection of species following the use of some soil amendments to successfully establish trees on these marginal and very low nutrient lands. It is anticipated that the rural communities could be greatly benefited if they are provided some minor incentives like provision of quality germplasm of suitable species along with short-term training in nursery and planting techniques. Simultaneously, growing nitrogen-fixing tree species on these problematic sites in the region would not only improve fertility level but could also reduce the problem of fodder and fuelwood to a certain extent.

INTRODUCTION

Large areas of agricultural land have been lost to cultivation due to the twin menaces of soil salinization and waterlogging in several countries of southern Asia. Recent estimates show that in Pakistan, 5.33 million ha of land out of 20.8 million ha cultivable land have been affected by salinity and 1.55 million ha by waterlogging. Continuous seepage from the network of unlined irrigation system over years and lack of natural drainage have resulted in the rise in groundwater-table. The phenomenon has led to waterlogging of large tracts of soils along rivers, canals and distributaries and evaporation of excess water from the surface of adjoining soils has resulted in their salinization.

A lot of effort has been put to eliminate the problem of soil salinity and waterlogging through sinking tube wells and development of surface and subsurface drains under several drainage projects throughout the country. However, the effort has not been successful to the desired degree due to several reasons. The drainage/reclamation approach, besides being costly and energy extensive, also suffers practical implications such as environmental degradation and disposal of brackish drainage water. Therefore, other options, which are relatively less expensive and environmentally sustainable, need proper consideration.

Plants, particularly trees, are commonly referred to as ‘biological pumps’ and play an important role in the overall hydrological cycle in an area. No doubt, the rate of evapotranspiration in an area will depend primarily on the vapour pressure deficit in the atmosphere; however, the importance of vegetation cover cannot be ignored as plants have significant influence on the water-table underneath the plantation and also in the surrounding area. A few researchers have made efforts to find out suitable species that can withstand waterlogging and salinity in Pakistan.

Altaf Hussain and Pazir Gul (1991), for example, recommended local tree species like Tamarix aphylla, Acacia nilotica, A. modesta and some exotics such as Acacia ampliceps, A. stenophylla and Casuarina obesa for planting under saline and sodic conditions provided the soil is properly worked and weed competition is decreased. It was concluded that planting tree species could reduce salinity level by improving soil conditions and leaching of salts to deeper layers of soil and thus assist in lowering the water-table. Later on, Mohammad Arif and Altaf Hussain (1993) studied the effect of sodium chloride on growth of some leguminous forest tree species at the nursery stage. The results indicated that although the growth of Acacia ampliceps was reduced, the plant did show a fair degree of tolerance to the highest concentration of sodium chloride (0.878-1.580 percent) compared with Albizia lebbek, Acacia nilotica and Leucaena leucocephala. Sheikh and Malik (1983), working on a planting site with pH varying 8.2-9.5 and electrical conductivity ranging 2.7-28.0 mmhos cm-1, recommended species like Tamarix articulata, Eucalyptus camaldulensis and Prosopis juliflora with survival of 53, 44 and 43 percent respectively after one growing season. Yadav (1980) observed that the success of plantations and improvement in soil depend to a great extent on the kind of planting technique and soil working methods. Species like Prosopis juliflora, Eucalyptus hybrid and Acacia nilotica could be grown successfully by treating the soil of the planting pit with gypsum and farmyard manure along with application of a small dose of nitrogenous and phosphate fertilizers. Sahibzada (1993) and Marcar (1996) recommended a few salt-tolerant species in Pakistan. Zafar Iqbal (1990) concluded that E. camaldulensis was the most salttolerant and salt-resistant species compared with Acacia nilotica, Prosopis juliflora and Albizia lebbek.

Some workers have also studied the effect of these species on water-use efficiency and water-table control. Kapoor (2000) reviewed and compared the results of three studies conducted in India, Australia and USA regarding the use of tree plantations to control groundwater-table and reduce the volume of drainable water. In India a draw-down to 8 m at a distance of 500 m from the edge of plantation was observed during a period of 6 years. In Australia the water-table underneath the plantations was 2-4 m lower than the adjacent pasture. The draw-down impact was restricted to about 20 m from the edge of plantation in 1984 and to 40 m in 1991. On the other hand, in the USA there was no appreciable draw-down of water-table despite high evapo-transpiration (115 cm) by Eucalyptus plantation during the 220 days of study in 1990. This was because a cumulative irrigation of 121 cm was applied during the same period. However, the potential of tree plantation to suck large quantities (115 cm) of shallow ground water was clearly evident. Benyon et al. (1999) compared water use between seven-year-old E. camaldulensis and six-year-old E. occidentalis and found that E. camaldulensis on saline soil used more water per tree. On moderately saline site, E. occidentalis used twice as much water per tree as E. camaldulensis trees.

In other studies (Morris and Collopy 1999), the annual water use of E. camaldulensis and Casuarina cunninghamiana over a two-year monitoring period was similar (about 350 mm). These researchers concluded that the continuous use of groundwater by these trees suggests that plantations may contribute controlling shallow water-table and salinity. Therefore, enhanced water use could be achieved by plantations irrigated with pumped groundwater or drainage water that would lead to more productive plantations as well as more efficient disposal of excess water. Masrur (1998) summarized that water consumption of pure stands of different species ranged as: Prosopis (1000 mm), Poplar (1800 mm) and Tamarix (2200 mm). The results on a range of sites and species in Pakistan, Thailand and Australia suggest that the differences between species at a given site are mainly attributable to differences in their growth rate (i.e. the sapwood area per hectare). Annual water use by the plantations studied ranged from 300 to 2100 mm (Morris and Collopy 1999).

Studies conducted by the Nuclear Institute for Agriculture and Biology (NIAB), Pakistan, showed that, on a sodic site, annual water use by Eucalyptus microtheca (1050 mm) was relatively lower than that by E. camaldulensis (1400 mm). Eucalyptus camaldulensis, dependent on saline groundwater at highly saline site, also transpired about 1100 mm of water per year. Acacia ampliceps used appreciably low water (625 mm) despite a basal area similar to that of E. camaldulensis. The lowest water use was recorded in an understocked natural stand of Prosopis juliflora. The higher water use by E. camaldulensis than E. microtheca was due to the more rapid growth and larger sapwood area in the former species. The question of what species to grow for biological drainage at a particular site is the key point for desired success. The choice of species is important for optimal water use but only in terms of maximizing growth and hence the sapwood area. Studies at the NIAB (Anonymous 1999) have shown that E. camaldulensis had similar sap flux density (SFD) and annual water use under low and high salinity conditions. However, higher annual water use (2225 mm) by A. nilotica at low salinity site compared with 1248 mm at high salinity site was attributed to the greater sapwood area at the former site, despite no evidence of reduction in SFD by high salinity (Khanzada et al. 1998). Although this paper describes the installation of piezometres to monitor watertable, the results are not presented, as these are very preliminary. However, a few salt-tolerant species tested at two sites in Pakistan are being monitored for growth according to the following method.

MATERIALS AND METHODS

A three-dimensional approach was followed to work out the best species for small-scale farmers to combat salinity and to improve the living standard of the rural poor. Two extremely saline sites were selected for rehabilitation and to undertake the following operations (Figure 1).

Figure 1. Experimental sites in Pakistan

Soil and water analysis

Soil samples were collected from 0-30 cm depth following a grid design system for two planting sites, viz. Pindi Bhattian and Sahiwal, Punjab Province, Pakistan. Samples of tube-well water used for irrigation by farmers were also collected for analysis. These were analysed for particle size, pH, electrical conductivity (EC), organic matter, phosphorus, carbonate, bicarbonate, chloride and soluble cations. Altogether 52 soil samples were collected from both sites. Due to the high salinity, there were no trees or crops and the land had virtually been abandoned by the users.

Establishment of field trials

Efforts were made to procure seed of proven provenances/seed sources of various salt-tolerant species, both locally and abroad from CSIRO, Australia. The planting stock was raised in the nursery at Peshawar and transported to the two sites, viz. Pindi Bhattian and Sahiwal, for establishment of species trial in Punjab Province. Six months before planting, the sites were deeply ploughed and six tractor trollies of farmyard manure (FYM) and 50 kg sulfuric acid/acre were applied to improve the soil structure followed by deep ploughing and levelling. A 10-species trial with 30 replications in single tree plot design was established in 2001 at Pindi Bhattian. Another 6-species trial (Acacia ampliceps, Casuarinas glauca, C. obesa, C. junghuhniana, Eucalyptus camaldulensis and E. microtheca) was established at Sahiwal, Punjab. The planting was done in blocks of 10 x 12 groups per species at 2 x 2 m spacing. A total of 720 plants were field planted in February 2002. A small quantity of gypsum (500 g) mixed with green manure was applied to each plant to improve the growth of seedling. The area was fenced to protect the experiment from grazing and human interference.

Installation of piezometers to monitor water-table

Twelve piezometers, five at Pindi Bhattian and seven at Sahiwal, were installed right in the centre of each field/ block. These included one peizometer installed in the unplanted area by fixing 2.7 m long pipe in the soil and leaving about 0.3 m in length above the soil. The plastic pipe of 7.5 cm diameter had 0.25 cm diameter holes up to 1 m and was placed in the ground. Gravel and sand were poured around the outside blank area allowing clean groundwater to enter the pipe. The bottom end was permanently sealed before fixing in the soil with plastic cap to avoid direct entrance of muddy water into the pipe and to avoid chocking. The upper end was also closed to eliminate the possibility of entry of rainwater into the pipe. The monthly water-table is being monitored. The paper describes the results of early growth of some species at the two sites and recommends these for large-scale planting on problematic areas in Pakistan to reduce poverty in the region.

RESULTS AND DISCUSSION

The soil samples including tube-well water were analysed for pH, EC, Na, K, Ca, Mg, CO3, HCO3, Cl and SAR for two sites. The results are given in Table 1.

Table 1. Preplanting chemical characteristics of soil and tube-well water at two saline sites in Pakistan

The data on soil analysis revealed that both the sites were strongly saline-sodic affecting the physical conditions of the soil, which was highly degraded to the extent that salt was accumulating and blackish colour was visible on the surface. In view of the severe problem of salinity-sodicity, addition of organic manure followed by deep ploughing to break the underneath hard crust has been recommended. Acidification of the soil with H2SO4 at the rate 100-120 kg ha^-1 was also conducted. The quality of tube-well water was marginal having the problem of salinity and sodicity. The soil features and analysis further confirmed that agricultural crops could not be grown without intensive management and reclamation methods. An integrated approach, e.g. chemical amendment, preferably acidification and application of gypsum followed by deep ploughing, addition of organic matter and biological measures such as growing of salt-tolerant species, was adopted. Results on the survival and growth of a few species grown at the two sites are discussed as follows.

Pindi Bhattian site

Survival of species, six months after planting:

The above data indicate that Tamarix aphylla and Casuarina glauca exhibited the highest survival (> 94 percent) at the initial establishment stage under saline and waterlogged conditions. Moreover, height- growth data, two years after planting at the same site, yielded encouraging results following the analysis of variance (Table 2).

Table 2. Results of analysis of variance of 10 species trial at Pindi Bhattian

There were significant differences in height growth at age two among the 10 species tested under severe saline conditions in Pakistan. Duncan Multiple Range Test was applied to find out significant differences among means of these species. Ranking for height presented the following picture:

The test showed that E. camaldulensis (Australian Source No. 15441) and Tamarix aphylla could be used for planting as these showed better survival and height growth than all other species tested at the same site. However, use of a proven seed source to combat salinity in Pakistan cannot be overemphasized.

Mass afforestation of E. camaldulensis under the Forestry Planning and Development Project (1985- 1995) has created some doubts in the minds of foresters as well as among farmers in Pakistan that it pumps groundwater at a higher rate than any other species and secondly it affects crop yield. This is a myth with no scientific data available in the country. The water requirements of the species do vary with the site, but the results of E. camaldulensis are comparable with others as shown in Table 3.

It can be concluded that the water requirements of Eucalyptus do not differ much from those of other species. It has been observed that crop yield especially of wheat is little affected compared with that of cotton. There is, however, a need that more data be collected to arrive at some conclusions. One thing that is quite clear is that saline areas, which are lying unutilized by the communities, could be rehabilitated following proper soil amendments, preparation and choice of suitable species. Once the trees are well established, data on water-use efficiency would bring about useful information on these trials.

Table 3. A comparison of annual water use by different tree species

Sahiwal site

Mean survival (percent) and height (cm) of one-year-old seedlings of six species at this site:

Acacia ampliceps was rated best at the above saline site in Pakistan as it exhibited 96 percent survival with good growth habit and form. This should be grown in combination with E. camaldulensis to meet the demand of pole/firewood and fodder. In areas where fodder is scarce, Acacia ampliceps is recommended which can coppice easily with excellent leaf biomass and early fruiting/seed production characteristics at age two.

Almost all rural communities in these areas are still using animal dung cake for fuel because of non-availability of any other source of energy. It has been estimated that an average family of 5-6 persons consumes a minimum of 8 tonnes of fuelwood per annum for cooking only, which is equivalent to 18 tonnes of animal dung cake. One tonne of animal dung cake is estimated to supply 3.5 kg N2, 1.5 kg P and 2 kg K. Therefore, 18 tonnes of animal dung cake would yield 63 kg N, 27 kg P and 36 kg K. As per current market rate, the price of these three fertilizers comes to Rs.1200 or US$20. Reclaiming saline soils and reforestation is an income generation activity as it costs less and pays more. The farmers therefore can easily improve their land by adding animal dung to the fields and could also meet their fuelwood requirements by growing E. camaldulensis on marginal and unutilized lands. The fertility level of soil could also be improved by planting Casuarina as it fixes atmospheric nitrogen into the soil. All such efforts would not only make unproductive area more productive but would also reduce poverty in Pakistan. Before such areas are reclaimed, one thing is important to note, i.e. use of proper provenance or source. It is therefore highly recommended that seed orchards of true-to-type seed source be established over larger areas to overcome the seed demand of local people. An orchard already exists but a lot more needs to be done to combat salinity at the national level.

A spacing of 3 x 3 m between rows and plants of E. camaldulensis with A. ampliceps in between would be an ideal form of planting to meet the problems of timber/poles and fodder in the area. There would be approximately 4000 seedlings per ha of both the species if 10 percent mortality is taken into consideration because of harsh soil and climatic conditions. A minimum net annual income of US$230 per ha is expected from at least 1000 trees at age six or seven when these would attain acceptable diameter for a paper mill.

CONCLUSION AND RECOMMENDATIONS

Biosaline technology advocates an enhanced role of suitable trees to combat salinity and waterlogging vis-à-vis helping rural communities to meet their needs. Reforestation of salt affected soils is possible given proper site preparation, choice of species and development of nursery and planting techniques. On account of specific location, nature of ownership, paucity of good quality water, refractory nature of soil, etc., several areas cannot be put to production of profitable agricultural crops and hence offer a good opportunity for reclamation, i.e. improving the fertility level by growing nitrogen-fixing trees. In view of these facts and in the light of the success story to combat salinity, the following important recommendations are made:

• Depending upon salinity level, addition of sulphuric acid at 100-120 kg per acre uniformly mixed with irrigation water and applied 4 to 6 months before the planting is recommended. Later on the application of six tractor trollies of farmyard manure with at least three deep ploughings and levelling would improve the soil structure. For trees, a sandy clayey soil is preferred. Gypsum at 3 tonnes/acre may be used if the land is highly saline.

• Seedlings in the nursery should be grown in a salt-free media and planted in the field when they attain a minimum size of 25-30 cm.

• Monsoon planting season, i.e. August/September, is preferred over the spring season.

• A pit of 60 cm depth and 25 cm diameter with seedlings placed at the top of the ridge of a furrow is recommended. Silt may be added up to 30 cm before planting any seedlings into the pit. The seedlings may be flood irrigated soon after planting taking care that water should be completely drained off the field after irrigation.

• A spacing of 3 x 3 m of Eucalyptus camaldulensis with seedlings of A. ampliceps in between would bring about higher income to the rural poor and would also help to meet their fuelwood and fodder requirements.

• Weeds may be removed manually up to two years after field planting.

• The best timber species like Casuarina obesa and the best fodder species like Acacia ampliceps may be grown on saline soils to improve the fertility level. An agrosylvopastoral system is encouraged.

• Additional income can be obtained though thinning and pruning at age three or four.

• A minimum net income of US$90.00 per acre/year from unutilized piece of land could improve the living standard of a small-scale farmer by providing the stock to a pulp mill when the trees would attain a diameter of 12-15 cm.

• Once the land is improved, suitable agricultural crops in conjunction with desirable trees may be grown to fetch better income from problematic sites.

• Farmers must be trained in nursery techniques and be advised to add animal dung to their fields to improve the fertility level and to increase productivity.

ACKNOWLEDGEMENTS

The financial support provided by the UNDP, Aus AID and the International Waterlogging and Salinity Research Institute, Lahore, is acknowledged. The author is also grateful to the staff of the Pakistan Forest Institute, Peshawar, for their assistance in planting and data collection of the experimental sites.

BIBLIOGRAPHY

Altaf Hussain & Pazir Gul. 1991. Selection of suitable tree species for saline and waterlogged areas. Pak. Jour. For. 41 (1): 34-43.

Anonymous. 1999. Productive use of saline lands and nuclear techniques. Ann. Progress Report. Nuclear Institute of Agriculture and Biology. 225 pp.

Benyon, R.G., Marcar, N.E., Crawford, D.F. & Nicholson, A.T. 1999. Growth and water use of Casuarina camudulensis and Eucalypthus occidentalis on saline discharge site near Wellington, NSW, Australia. Agr. Water Management 39: 229-244.

Kapoor, A.S. 2000. Biodrainage - a case study. Proc. Workshop ICID. New Delhi, India, 31 Jan-4 Feb 2000, 12 pp.

Khanzada, A.N, Morris J.D., Ansari, R. & Collopy, J.J. 1998. Groundwater uptake and sustainability of Acacia and Prosopis plantations in southern Pakistan. J. Agric. Water Mngt. 36: 121-139.

Marcar, N. 1996. Tree growing options for saline lands. Newsletter ACIAR No. 21: 4.

Masrur, A. 1998. Working paper for biodrainage programme of NDP. Lahore, National Drainage Programme. 13 pp.

Mohammad Arif & Altaf Hussain. 1993. Sodium chloride stress studies on growth of some leguminous forest tree seedlings. Pak. Jour. For. 43 (1): 21-27.

Morris, J.D. & Collopy, J.J. 1999. Water use and salt accumulation by Eucalyptus camaldulensis and Casuarina cumminghamiana on a site with shallow saline groundwater. J. Agric. Water Mngt. 39: 41-47.

Sahibzada, M.H.1993. Identification of fast growing salt tolerant tree species. Pak. Jour. For. 43 (4): 216-220.

Sheikh, M.I & Malik, M.N. 1983. Planting of trees in saline and waterlogged areas. Pak. Jour. For. 33 (1): 3-8.

Yadav, J.S.P. 1980. Salt affected soils and their forestation. Indian Forester 106: 259-272.

Zafar Iqbal. 1990. Effect of soil salinity on the early development of three important species and two provenances of Acacia nilotica. Proc. Workshop on Tree Production from Saline and Water-logged Soils, December 1990, Faisalabad, pp. 6-23.