Watershed Conservation Section, Royal Forestry Department, Thailand
Thailand has a wide range of different forest types. There are evergreen forests in the south where the average annual rainfall is about 1 600 millimeters. The central part of Thailand has a long dry season ranging from five-six months. This region receives less than 100 millimeters of rain per month and it is here that deciduous forests are located. In the mountainous and northern areas of Thailand, there are Pinus forests and dry evergreen forests. The mixed deciduous forests tend to have a high economic value. These forests contain species preferred for building construction and many other uses. For example, teak (Tectona grandis) and bamboo are found throughout the mixed deciduous forests. The dry dipterocarp forests tend to be airy, with relatively clean under-storey and semi-open canopies. Species belonging to the Dipterocarpaceae comprise the predominant trees found in these forests.
Annual forest fires are commonly experienced in the deciduous forest due to the dry conditions. Under these conditions, soil moisture during the dry season is reduced to around 50 percent of wet season levels. The yearly forest fires wreak havoc on both climax and pioneer tree species. If there are no forest fires for 10 years, regeneration will occur, but the original forest status will change. The number of new tree species per hectare is as much as nine times more than in the original forest (Sukwong, 1977).
Natural regeneration of deciduous forest
The deciduous forest has an inherent capability to recover from human disturbance. Roots of the deciduous forest remain viable after clear-cutting or fire. Within one year, these roots will produce new coppice growth. An average of 281 stumps per hectare will emerge. Within three years, up to 2 956 new stumps per hectare can be observed, comprising more than 47 species. This demonstrates the ability of the original plants in the deciduous forest to regenerate without having to rely on the seed-based reproductive systems of the trees (Sukwong, 1978).
Natural regeneration of dry evergreen forest
Studies have shown that in deforested sites formerly covered by dry evergreen forests, very few of the original species will be restored through natural regeneration. Species formerly found in the uppermost succession level do not recover easily. It is difficult and requires a long period of time to bring the forests back to their original status.
Normally, the hill evergreen forest passes through a gap phase lasting up to 50 years. Thereafter, another 50 years will pass comprising the building phase and an additional 100 years will go by before original conditions are restored. (Watershed Conservation Section, 2001). Studies have also been conducted in former dry evergreen forest areas that were cut over, converted to farms, and then abandoned for 18 years. The trees that regenerated naturally were pioneer species common to secondary forests. The original appearance of the dry evergreen forest was altered because the genetic pool was destroyed. If forest destruction continues due to human disturbance and fire, a scrub forest may develop, which indicates severely degraded forest.
Results of research suggest that to positively influence natural regeneration, it is advisable to apply appropriate treatments during the gap phase. Research results indicate that prevention of repeated trespass and forest fires helps to shorten the gap phase from 50 years to only 10 years. Studies also indicate that if root stock from original species with deep-rooting characteristics are planted, forest cover will return to very near its former status within 30 years. Veer (1996) elaborated methods for natural regeneration of forests that include both economic and social factors, in addition to bio-physical considerations. The potential to apply these methods should be explored when formulating plans to restore forests on degraded and denuded land.
ANR in Thailand
Assisted natural regeneration (ANR) is a well-recognized and normal phenomenon in the countryside, especially in areas having an average annual rainfall of 1 500 millimeters. Plant succession is high under these conditions. Regeneration is especially vigorous if communities participate actively in the process. For example, in 1975, the community of Ban Dong Yai in Ubon Ratchathani Province started tending a 400-hectare secondary forest that emerged after the land was converted to farms and then abandoned. Forest cover has since been restored. Similarly, some monks participated with the community and helped restore the forest at Boong Pra Temple, Nakornrachasima Province. Another example can be found in the lowland dry evergreen forest areas of northern Thailand, where communities in Nan and Lampoon worked together to bring back the forests. These experiences strengthened community collaboration and people empowerment. Motivation to work together was based on the desire to ensure stable supplies of water during the dry season. Communities were aware that the drinking water and tap water they used came from underground sources. They also recognized that forests increase rainfall infiltration and replenish underground aquifers, while also creating a sustainable natural storehouse of food, medicine, firewood and charcoal.
Rural poverty is the principal impediment to expansion of ANR in the above-cited communities. Most of the people are poor and farming is their major source of livelihood. Making use of the land for farming has more immediate relevance in their lives than the restoration of forest cover. As the population continues to increase, the pressure to convert forests into farms will continue. Another constraint relates to politics. Politicians often tend to focus on short-term measures that garner votes, rather than the long-term issue of forest conservation. Thus, creation of incentives to practice ANR is not part of their agenda.
Government plans to improve the forest
Several government initiatives aim at restoring forest cover and otherwise improving forest conditions. Some of the plans that include ANR techniques are summarized below.
Watershed improvement plan
This plan calls for tree planting and other measures to restore forest cover on denuded and degraded watershed catchments. The methods include reforestation with the original indigenous species, strict fire prevention and maintenance of pioneer trees and other vegetation that emerges from root stocks.
Construction of check dams
Small dams will be built across second order streams to reduce stream and rainfall run-off as much as possible, in order to increase infiltration that replenishes underground aquifers. The plan also aims to retain moisture and to hasten natural regeneration of stream-bank species, thereby reducing the risk of fire along waterways.
Farmers will be encouraged to plant elephant grass (Typha spp.) and Vetiveria zizanioides, both of which are deep-rooted species. Planting on horizontal contours will be promoted to prevent soil erosion and moisture loss. This will create favorable conditions for natural regeneration of tree cover, while also improving farm production.
Satetarak, A. 1978. A study comparable of forest fire in 3 years period. Royal Forestry Department, Bangkok
Sukwong, S. 1977. Some effects of fire on dry dipterocarp forest community. In BIOTROP - Kasetsart University Symposium on Management of Forest Production in Southeast Asia. Kasetsart University, Bangkok, Thailand.
Sukwong, S. 1978. Natural reproduction 3 years after clear felling in teak forest. Technical Paper No.8, Department of Forest Biology, Faculty of Forestry, Kasetsart University.
Veer, C. 1996. Forest restoration though natural regeneration in Asia. Kasetsart University, Bangkok.
Watershed Conservation Section, 2001. Guide for watershed management unit. Royal Forestry Department, Bangkok.