THEME 3. LOSS OF WATERSHED FUNCTIONS

Posters on Watershed Functions

Hydrology of Coppiced Gmelina arborea Plantation in the Philippines

by Evangeline T. Castillo¹

The hydrologic response of a Gmelina plantation totally coppiced and allowed to regrow for four years was investigated in comparison with a seasonally-burned control grassland catchment in San Mateo, Norzagaray, Bulacan Central Philippines from 1994 - 1997 using the paired - catchment approach. The study was conducted by Dr. Evangeline T. Castillo, Supervising Science Research Specialist at the Ecosystems Research and Development Bureau (ERDB), Department of Environment and Natural Resources.

The coppiced Gmelina plantation in the watershed produced a mean annual runoff of 105 mm lower than the grassland catchment which is equivalent to a mean reduction of 19.4% over the control. Sediment yield was reduced by an annual average of 61.2%. Infiltration rate was much faster in the yemane catchment and a longer time to saturate (6.5 hrs vs 6 hrs in grassland catchment).

The runoff and sediment yield reduction effected by the coppiced Gmelina relative to the control catchment can be accounted to the quick vegetative growth both in the understorey and by the overstorey canopy as well as in the production of litter covering the soil layer. Coppiced Gmelina trees regrowth was relatively fast with mean growth rates of 1.8 m/yr for height, 1.92 cm/year for stem diameter 0.6 m/year for crown depth and 0.02 m/year for crown diameter. Gmelina trees produced mean height of 5.19 m on the first year and 12.3 m on the fourth year; mean stem diameter of 6.82 cm and 14.5 cm on the first and fourth year respectively; mean crown diameter of 2.04 m and 2.62 m on the first and last year; and mean crown depth of 4.28 m and 6.8 m initial and final values. After four years, the Gmelina plantation gave rise to 30 undergrowth species comprising grasses, herbs, vines, shrubs mosses and trees dominated by Chromolaena odorata. This is in contrast with the control catchment with only 19 species limited to grasses, sedges and vines. The initial unvegetated ground layer was 7.7% on the first year was finally reduced to 0.5% of the ground area. Litterfall accumulated at a rate of 358 g/m² /year with peak fall months in May and December and lowest in April and May.

Soil analysis indicated significantly higher cation exchange capacity and soil pH in the Gmelina catchment but significantly lower nitrogen and potassium content. Soil physical properties such as bulk density and porosity was initially higher in the grassland watershed but improvement in the Gmelina catchment was observed on the fourth year.

Results of this investigation inferred the positive utilization of Gmelina as industrial species owing to its quick growing coppicing potential but also for its protective capability of reducing surface runoff and sediment yield.

Footnote_______________

¹ Ecosystems Research and Development Bureau, College, Laguna, Philippines