Leaf decomposition of exotic and native tree species: rates and effect on soil

Decomposition of organic materials on the forest floor is a vital link between the various components of the forest ecosystem. Through this process, mineral nutrients bound to the biomass are released into the soil and then subject to uptake by plants, fixation by soil components, and losses through leaching and erosion. Decomposition can have considerable influence on the biological and chemical properties of the forest soil depending on the kind of organic material, soil properties, climate, and the availability of decomposers (e.g. Gartner and Cardon, 2004).

Exotic tree species are introduced species from other regions. They are widespread in tropical and subtropical countries since they are popular as reforestation species even in harsh environments (Nyland, 1996) due to their ability to adapt easily to variable site conditions (Weidelt, 1976). Many are considered economically viable because of their fast growth characteristic. Farmers value exotic species more than the native ones because of forestry extension recommendations and desirable cultural attributes (Cromwell and Bradie, 1996). In the Philippines, the most well-known exotic tree species belonging to this group are Mahogany (Sweitenia macrophylla King), Gmelina (Gmelina arborea Roxb.) and Teak (Tectona grandis Linn.).

Native tree species are species which originated from the region where they are growing. Among the more commonly known Philippine native tree species are Bagtikan (Parashorea plicata Brandis), Hagakhak (Dipterocarpus validus Blume) and Narra (Pterocarpus indicus Willd.). The first two species belong to Dipterocarpaceae family, the latter to the Fabaceae.

Presently, there is widespread notion that the use of exotic tree species for reforestation causes negative ecological effects such as soil degradation (Sawyer, 1993). Lindsay and French (2005) cited early studies showing that there are strong positive feedbacks between plant species composition and soil properties such that introduction of a new species can change nutrient cycling and soil properties. It is also believed that native tree species have positive effects on the site. However, very little data exist to support these claims.

We evaluated the effects of incorporation and subsequent decomposition of leaves of exotic tree species (Gmelina arborea, Sweitenia macrophylla and Tectona grandis) and native tree species (Pterocarpus indicus, Dipterocarpus validus and Parashorea plicata) on the quality of forest soil. Forty-two pots filled with an acidic and clayey forest soil and added with fresh leaves of the different tree species were set-up in an open area in Mt. Pangasugan. Retrieval of the first three pots for each treatment was done after two months and the remaining three pots, five months later. Soil samples were collected from each pot and analyzed for pH, OM, total N, available P, and respiration rates.

Our main findings were:

1. Decomposition of the leaves of exotic tree species generally did not change soil pH except that of S. macrophylla which increased soil pH after 5 months. In contrast, the leaves of the native species tended to decrease soil pH particularly in the first two months of decomposition.

2. There was no considerable difference between the effects of the leaves of exotic and those of native tree species on the organic matter and total nitrogen contents of the soil.

3. Available phosphorus content of the soil was significantly increased by the decomposition of leaves of both exotic and native species.

4. The leaves of exotic tree species decompose faster than those of the native species. This finding agrees with that of a separate litter decomposition study by litterbag method conducted at the same site by Aragon (2004).

Source:

Batistel CC and VB Asio. 2009. Effects of leaf decomposition of selected exotic and native tree species on forest soil quality. Annals of Tropical Research (in press)

References

Aragon JA. 2004. Leaf litter decomposition of Dipterocarpus validus Brandis (Dipterocarpaceae) and Gmelina arborea (Verbenaceae) in two forest sites of Mt. Pangasugan. Undergrad Thesis, Leyte State University, Baybay, Leyte. 50 pp.

Cromwell E and A Bradie. 1996. Germplasm for Multipurpose Trees: Access and Utility in Small-farm Communities. ODI London.

Gartner TB and ZG Cardon. 2004. Decomposition dynamics in mixed species leaf litter. Oikos 104: 230-246.

Lindsay EA and K French. 2005. Litterfall and nitrogen cycling following invasion by Chrysanthemoides monilifera ssp. Rotundata in coastal Australia. Journal of Applied Ecology 42: 556-566.

Nyland R 1996. Silviculture (Concepts and Application). McGraw-Hill Co. Inc. Singapore.

Sawyer J 1993. Plantations in the Tropics: Environmental Concerns. IUCN/UNEP/WWWF, Gland, Switzerland.

Weidelt H A 1976. Manual of Reforestation and Erosion Control for the Philippines. German Agency for Technical Corporation LTD (GTZ) Germany.

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