Types of Wetland

According to the fluctuation of the water table, wetlands can be classified into three major types:

1. Moist land
2. Waterlogged lands
3. Immerged lands

i. Moist land: The water table fluctuates from the surface down to 30 to 40 cm in the ground. The ground may occasionally be flooded during heavy rains. Such lowlands are commonly located along river-banks, lake shores and sea beaches. The vegetation commonly consists of moist tolerant and hydrophillous trees, shrubs and herbs.

ii. Waterlogged lands: The water table is close to the ground surface and often flooded during the rainy season. Soils are always water saturated except in a very dry climate and are rich in organic matter.

iii. Immerged lands: Most of the ground surface is under water for a considerable time of the year. The depth of water fluctuates from several cm simple and of almost pure seeds.

Measures of Soil Improvement in AF Practices

In wetlands too much water in the soil is the major obstacle to land use for either forestry or agricultural purposes. Several measures are generally practiced to improve the site conditions.

1. Ditching: In moist land, ditching is commonly used to lower the water table by draining the excess water. In some countries tractors are used to turn over the subsoil for raising dykes on one or both sides of the ditches, which are then planted. The depth and width of the ditches mainly depend on the water table and its fluctuation; the higher the water table with greater fluctuation; the deeper and wider the ditches. The water table and its fluctuations are generally controlled below 50 cm from the ground surface.

2. Terracing:In waterlogged lands the water table is closed to or even above the ground surface. Just ditching is not enough to drain the excess water. The soil removed from the water logged and immerged lands is used to raise spaces (terraces) on one side and to create ponds on the other. The size of the terrace and pond varies greatly in accordance with the water situation and agroforestry practice. In general, water and land are such proportions as 0.5:1.5,1.0:1.0 and 1.5:1.5.In immerged land, terraces should be higher and ponds should be bigger and deeper. For planting trees with a deep root system the terraces should be much larger and higher. If fishery is considered as a part of the agroforestry system, bigger ponds with deeper depths are needed.

Selection of Tree Species

Ecological adaptation and economic benefits are two important factors governing the selection of tree species for a wetland agroforestry system. Despite terracing, excess water is still a major problem for tree planting and crop cultivation in wetlands. Tree species for wetlands should have the following characteristics:

- Moisture tolerant with a relatively shallow root system and in particular capable of survival in water logged soil for a certain length of time.

- In view of growth and production of inter-or underplanted crops, tree species should be deciduous with a narrow and thin crown.

- For agroforestry purposes fast-growing multipurpose trees should be selected.

Tree species for Wetlands

According to the ecological requirements and economic conditions the following tree species are commonly used for wetland agroforestry practices: Salix babylonica (water willow), Salix xuchonensis (basket willow), Alnus cremastogyne (long peduncled alder), Alnus trabeculosa (trobeculate alder), Paulownia tomentosa (hairy paulownia), Morus alba (mulberry), Taxodium distichum (swamp cypress) and Taxodium scandens (pond cypress). Of these, Taxodium scandens is the best. It is a deciduous conifer with a small and thin crown. Its root system is shallow and water tolerant and is able to survive in immerged soils for several months or year-round. Its stem is straight, fast growing and good in wood quality. It sheds its needles in late October and renews its foliage in April, providing full light for underplanted winter crops.

The next in water tolerance, growth rate and timber quality are: water fir, swamp cypress and peduncled alder. These four species are very popular for wetland agroforestry.

Similarly jackfruit, monkey jack and crambola are fruit species which can tolerate waterlogged conditions to some extent.

Selection of Crops:

To obtain full utilization of environmental resources and to increase economic benefits, an appropriate crop should be interplanted with or underplanted after the establishment of a tree plantation in accordance with site and climatic conditions. Wheat, barley, oat, rape or broad-bean are commonly planted in late autumn and harvested by the end of the next spring. Soyabean, cotton and watermelon are seeded in late spring or early summer and harvested in autumn or in late summer.

Vegetables such as radish carrot, cabbage etc. can be cultivated between the major crops. In some areas strawberry and mint are also planted. As the trees grow and their crown enlarges, causing considerable shade on the floor, especially during the growing season, some shade tolerant crops or medicinal plants such as ginger, edible mushrooms, etc. are selected for underplanting. In shallow ponds and streams, aquatic plants such as rice, lotus, water-chestnut can be planted as crops. For deeper plants water-lily, water-lettuce, water-caltrop are cultivated as floating crops, Ponds with a water depths of more 1 m are good for breeding fishes.