Abstract
This study examines vegetation succession and soil physico-chemical properties in the upper reaches of the O'Donell River, which have been affected by the seasonal lahar since the 1991 eruption of Mt. Pinatubo, central Luzon, the Philippines. Field surveys and physico-chemical analyses of lahar deposits and soil were carried out in areas including the living zone of the Aeta to understand the land recovery process along with land management practiced by the Aeta. Seedlings of Ararong (Trema orientalis) associated with root nodule bacteria are widely distributed on the lower lahar terraces and it was pointed out that Ararong is a pioneer plant. It is also the dominant species of tree and shrub layers on higher lahar terraces, and has a tendency to reduce its height in response to increases in coverage of herbs such as Talahib, Magkakamote, and Bureok.
Vegetation succession was divided into the following stages (I–IV) based on an interpretation of the development of the geomorphological surface and a vegetation survey. The former surface soil of the 1991 pre-eruption was tentatively defined as the goal of land recovery and was classified as Stage V.
A grain-size analysis of lahar deposits and soils in φ scale was carried out by the dry sieving method. Chemical properties of lahar deposits and soils were examined by analyzing total carbon (T-C), total nitrogen (T-N), total phosphorus content (T-P), C/N ratio, pH (H2O), cation exchange capacity (CEC), base saturation (BS), and total elemental composition.
Lower terrace of post-eruption lahar deposits with no vegetation (Stage I) and lower terrace of lahar with Ararong seedlings or herbs (Talahib, Bureok) had low T-C contents 0.2–0.58 g kg-1, and no significant differences were recognized between the two stages. The higher terrace of lahars with Ararong of 2–5 m height and herb coverage (Stage III) and ridge area affected by the 1991 pyroclastic flow and/or fall deposits, currently used as fields with shifting cultivation (Stage IV), had an increased T-C content of from 0.37 to 8.9 g kg-1 in response to the increased density of the vegetation canopy and number of vegetation species, which showed an exponential increase towards Stage V (21 g kg-1).
The T-N content showed a slight tendency of nitrogen accumulation occupied by Ararong and rhizobial nitrogen fixation. As far as we examined soil pH (H2O), T-N, CEC, and BS, the soil fertility of the shifting cultivated field and the secondary forest (stage III, IV) in the upper reaches of the O'Donell River is equivalent to that of the fields on plains of the Bambang and Pasig-Potrero rivers. Nitrogen fixation by root nodule bacteria of Ararong, and available phosphorus supply originating from apatite in the lahar deposits are both accelerated by the elution of organic acids from herbs such as Talahib. Potassium supply from the weathering of plagioclase and biotite, the major minerals of lahar deposits, is also the basic process responsible for land recovery on the upper slope of Mt. Pinatubo.
Based on analytical data from this study, it is found that the activity of the Aeta people of burning down secondary forests of Ararong to open small areas of arable land is not a regulating factor against land restoration. The soil-forming process, which is closely related to vegetation management from the activities of local people, may help us to understand the ongoing sustainable development in the upper reaches of the O'Donell River.
