14.6 FORESTRY

Flooded and riparian forests / Loss of forest cover: impacts on catchments and stream ecology /
Transboundary issues

The importance of forests to stream ecology--
The changing sequence of forest communities along the length of the Mekong River is an important source of biodiversity and biological resiliency. Forests provide a number of important functions for stream ecosystems:

• Shade and temperature moderation
• Bank stability and erosion control by providing solid root mass and ground cover
• Nutrient and food source

Stream physical features vary from small torrential headwater streams to the gently flowing, meandering rivers that pass through lowland swamp forests (see Section 3). Headwater streams usually have more water energy and are more erosive than the low energy depositional streams in lowland areas, with variations in between. Accordingly, forest streams can differ greatly in terms of their physical, chemical and biological characteristics, depending on their location in the watershed (see Section 6).

Plants and animals found in a around the water are adapted to the combination of conditions that exist in different types of streams. Removal of the surrounding forest will change conditions making them unsuitable for some species and more suitable for others. Significant changes in plant and animal community structure can result. Particularly on smaller streams, the forest canopy limits the amount of light penetrating to the water surface. Removal of riparian vegetation will decrease this shading effect, which in turn can result in elevated water temperatures.

The removal of forest cover can affect the quality, quantity and timing of stream flows. If trees are removed from a large portion of a watershed, flow quantity can increase substantially. The ultimate impact depends on the amount of vegetation removed and proximity of the removal site to the stream. Increases in flood peak can occur if vegetation in the areas closest to the stream is removed. Long-term loss of riparian vegetation can result in bank erosion and channel widening.

Wood that falls from the forest into streams can add to in-stream habitat diversity/complexity by providing shelter for fish and invertebrates. This woody debris can also helping to shape the form of the stream and control local flow characteristics by affecting sediment storage and transport processes. The amount of wood available to serve this function may be reduced or eliminated by removing forest cover.

Material dropping from the tree branches can be important: leaves and organic matter from trees add nutrients to the stream; and insects and other small animals supply food to fish and invertebrates in the stream (see Sections 5 and 6).


Flooded and Riparian Forests--
Riparian forests occur along riverbanks with shrubs and trees that are relatively tolerant to submergence located close to the waters edge and less tolerant varieties located further inland. It is widely acknowledged that the flooded forest and floodplain vegetation on the shoreline of the Great Lake and Tonle Sap River system in Cambodia are among the most important and diverse habitats in the region, and that these environments are critical to birds and wildlife in the area.

Most of the floodplain terrain (approx. 80%) is covered by semi-continuous, locally homogenous stands of short trees and large shrubs; however, tall (7-15 m) and dense gallery forests commonly skirt the lake shoreline and river ways of the floodplain. Key flooded forest species that are important for fisheries and waterfowl include Hydrocarpus anthelmintica and Homalium brevidans. Aquatic herbaceous vegetation, including floating mats of water hyacinth (Eichhornia crassipes), is also common throughout the floodplain area.

Flooded forests are sometimes located away from the water's edge but are in low elevation areas that may be inundated each year when floodwaters rise. Flooded forest in the Great Lake area provide habitat for many species specially adapted to the seasonal flood pattern. They are of particular importance as spawning and rearing areas for fish species that enter from the main river. Some fish species tend to need good water quality conditions and usually retreat quickly to the main river as flood water recedes (whitefish), while other species can endure harsher conditions and tend to spend longer times in backwater areas of the forests (blackfish). Current human pressures on flooded forests include fuel wood collection and charcoal production, as well as conversion to agricultural lands. It has been estimated that from an original one million ha of forest land around the Great Lake there is now approximately 360,000 ha of flooded forest and 157,000 ha of degraded forest and associated vegetation types.


Loss of forest cover: impacts on catchments & stream ecology--
Forestry practices have far-reaching impacts on the economic and ecological health of the basin. Thirty percent (30%) of the LMB catchments, due to their biophysical conditions, should be classified as hydrological protection forests (Class I areas), or protection and limited production forests (Class II areas).

Class I forests include headwater areas, which are usually at high elevations, have very steep slopes and should remain under permanent forest cover.

Class II forests are usually found at high elevations, with steep to very steep slopes, and landforms that require soil and water conservation restrictions.

Most of the Class I and II forests are in Lao PDR (69%), followed by Thailand (14%), Viet Nam (13%) and Cambodia (4%).

Forest cover is declining in all Mekong River Basin riparian countries. At present, losses of forest cover can be attributed to a number of activities including:

• Excessive and/or inefficient commercial logging;
• Shifting cultivation;
• Encroachment for human settlements;
• Farming and infrastructure development; and
• Intensive fuel wood collection.

Historically, large areas of forest along the Lower Mekong, including significant areas of wetland vegetation, were severely and in some cases irreparably damaged from heavy use of defoliants during the American/Vietnam War.

Recent forest cover losses (early 1980s to late 1990s) in three LMB countries are highlighted in the table below.

Loss of forest cover from logging and other clearing activities that are poorly planned and implemented can seriously affect stream ecology. Effects include:

• Erosion – unplanned deforestation can increase surface runoff rate increasing erosion
• High sediment loads – increased erosion will lead to increased sediment in streams
• Altered hydrology – faster runoff will cause more rapid and higher-volume flood events
• Altered nutrient regimes – loss of organic debris and food organisms
• Altered species composition and abundance – impairment of physical/chemical properties can result in reduced biodiversity

Transboundary issues--
Transboundary issues relate both to the physical/ecological consequences of forest clearing activities and to difficulties for institutions on different sides of borders to deal with the problems. Major issues relate to:

• Erosion and changes in river channel morphology
• Alteration in flow/flood patterns
• Pollution flows
• Institutional constraints.

Additional information on transboundary ecological effects is provided in Section 11.