Factors affecting runoff and soil erosion: plot-level soil loss monitoring for assessing sustainability of forest management
Introduction
The history of farming is inseparable from that of soil and water management and farmers have long recognised that they must manage the soil well if they are to expect good harvests. Although this is true also for forest management, the links were not as strong, possibly because forests, unless they are very heavily logged, continue to provide protection against soil erosion (Bruijnzeel, 1990). With increasing mechanisation of forest harvesting operations the impacts on soil have increased quite dramatically (Greacen and Sands, 1980, Binkley and Brown, 1993, Oyarzun, 1995). This is particularly true on the fragile Ultisols and Vertisols of the humid tropics (Bruijnzeel, 1990), where poor soil cohesion, high rainfall and high temperatures give rise to highly erosive soils that are very sensitive to the impacts of heavy machinery and clearance of vegetative cover (Lal, 1981, Hendrison, 1990, Huang and Laflen, 1996). This situation is further exacerbated on hill slopes (Zachar, 1982, Ross and Dykes, 1996). Sources of impacts include bank erosion, skid trails, logging roads, and timber extraction. Skid trails and logging roads have been identified as the major sources of sediment (Trimble and Sartz, 1957, Gilmour, 1971, Douglas et al., 1993). However, little information is available on the magnitude of sediments generated at those different sites (Bruijnzeel, 1990).
The impacts of logging operations can be limited to on-site soil loss, but can also be observed at a larger scale downstream. While on-site erosion is more quickly observed and falls within the area of the managers’ responsibilities, off-site erosion is not readily observed but can be very serious and generate greater public concern (Gilmour, 1971, Hewlett, 1982). On-site soil erosion affects chemical and physical fertility of the soil. The loss of top soil rich in nutrients and organic matter causes a decrease in soil fertility, land productivity to sustain plant growth, and land degradation (Ebisemiju, 1990). While off-site impacts of soil loss include increased sedimentation and turbidity, increased levels of nutrients and pollutants that diminish water quality, siltation of dams and irrigation channels (Vitousek, 1981, Riekerk, 1983, Hopmans et al., 1987, Craswell et al., 1997), and a decrease in fish abundance (Skarbovik, 1994 in Danielsen and Schumacher, 1997).
In most humid tropical areas, forest managers are often inadequately informed about the consequences of their harvesting and roading activities on the sustainability of the forests they are responsible for. The feedback loop after initial planning and implementation of forest operations back to the planning stage is usually not closed, thus management systems can be described as being out of control. Consequently there have been several initiatives to close this loop through appropriate monitoring activities (Smyth and Dumanski, 1993, Gomez et al., 1996). In recent years there has been a major initiative to develop criteria and indicators (C&I) for Sustainable Forest Management to promote effective monitoring. The Center for International Forestry Research (CIFOR) has been at the heart of one of the biggest international research efforts to develop effective and acceptable criteria and indicators at the forest management unit level.
Under the umbrella of CIFOR’s criteria and indicators project, we undertook to evaluate the effectiveness of monitoring methods for key ‘verifiers’ related to water and soil management. Verifiers are defined as data or information that enhance the specificity or the ease of assessment of an indicator. In other words, verifiers refer to the measurable elements of the indicator (CIFOR C&I Team, 1999). We considered verifiers related to stream water quality and quantity because of their potential to describe not only management practices within the catchment but also the response of that catchment to interventions. However, for the selected study site in Central Kalimantan, Indonesia, these off-site verifiers were not rated as highly against several attributes set out in Prabhu et al., 1996, Prabhu et al., 1999, as were verifiers that required on-site monitoring. The terms ‘on-site’ and ‘off-site’ are used with respect to the locus of forest harvesting operations.
Soil erosion monitoring can be carried out on-site (at plot level) and off-site (at sub-catchment and catchment levels). The advantages and disadvantages of these two monitoring approaches are currently the subject of debate. Many studies on soil erosion have been conducted at sub-catchment or catchment levels. Although this kind of approach can better describe the response of a catchment to certain management practices, instream monitoring is expensive and time-consuming as monitoring should include a calibration period. Upslope or on-site monitoring, on the other hand, is relatively simple to conduct and inexpensive. This type of monitoring is best suited to portraying soil erosion processes and soil disturbances on-site (Corner et al., 1996). As a result of these considerations, we selected on-site monitoring methods for the verifiers concerned.
This study investigated the impact of logging activities on on-site surface runoff and soil erosion at plot level, with the objectives: (1) to determine key ecological factors that determine runoff and soil loss; (2) to determine the potential suitability of these factors as criteria and indicators of Sustainable Forest Management in terms of their sensitivity, cost-effectiveness, etc.; and (3) to investigate how useful on-site monitoring of runoff and soil loss is in describing the impact of management practices.
Section snippets
Site description
This study was conducted in a tropical rainforest of Central Kalimantan, Indonesia. The study site is located at 1°17′46″S and 112°22′42″E, in the headwaters of Mentaya River, about 175 km NNW from Sampit as shown in Fig. 1. The soils are of red–yellow podzolics and sensitive to erosion.
The research area is a typical lowland primary rainforest, with the altitude ranging from 100 to 300 m above sea level. The area is quite hilly with a maximum slope of 35°. Kartawinata et al. (1981) have reported
Experiment design
Fourteen runoff plots were set up in two catchments for the purpose of this study. Logging operations took place in both catchments but they were of two different logging systems: Reduced Impact Logging in catchment 92 (Wanariset Sangai research site) and conventional logging in catchment 37. In Indonesia, the conventional logging system is referred to as Tebang Pilih Tanam Indonesia or Indonesia Selective Logging and Planting System (Bertault and Kadir, 1998). The size of catchment 92 and
Results
This study found a general pattern of runoff at the two catchments, i.e. runoff at harvest plots (1.11±0.07 mm in both catchments) were lower than runoff at control plots (2.50±0.12 mm in catchment 92, 2.50±0.13 mm in catchment 37), and control plots had lower runoff than skid trail plots (227.87±16.20 mm in catchment 92, 306.60±10.61 mm in catchment 37). Similarly, soil loss at harvest plots (0.57±0.05 g/m2 in catchment 92, 0.22±0.02 g/m2 in catchment 37) were found to be lower than soil loss at
Discussion
The results showed that plot locations (i.e. control, harvest, and skid trail) and logging systems (i.e. conventional logging applied in catchment 37 and RIL applied in catchment 92) had significant effects on runoff and soil loss. However, further analysis showed that there was a significant difference in the runoff among different plots, but not between the two catchments. There was a significant difference in soil loss among different plots and between the two catchments. This indicates that
Conclusion
We found that sapling density, canopy cover, litter layer, and woody debris are important ecological factors that determine soil erosion. Apart from these factors, rainfall played an important role in determining the magnitude of runoff and soil loss. At the control and harvest plots, the presence of organic forest floor materials, such as litter layer and woody debris, is very important in preventing soil detachment and providing surface roughness, thus reducing runoff and soil particle
Acknowledgements
The authors would like to thank Dr. Syaiful Anwar for his technical advise in the field, Mr. Ramadhani Achdiawan and Ms. Meilanie Buitenzorgy for their valuable inputs on the statistical analyses.
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