Factors affecting runoff and soil erosion: plot-level soil loss monitoring for assessing sustainability of forest management

Abstract

The assessment on key ecological factors affecting runoff and soil erosion and the usefulness of plot-level monitoring of soil erosion was conducted by collecting runoff and soil loss records from 14 runoff plots. The runoff plots were set up in two catchments in Central Kalimantan, Indonesia, where conventional logging and Reduced Impact Logging (RIL) took place. Runoff plots were set up in forest areas with different levels of logging disturbances, i.e. harvesting areas (four plots), skid trails (six plots), and undisturbed/control areas (four plots). The magnitude of runoff and soil loss from skid trail plots were found to be the highest, followed by control plots and harvest plots. Canopy cover, sapling density, litter depth and woody debris appeared to be important ecological factors that determine the magnitude of soil loss. Tree canopy determines the size and erosive power of the raindrops. Sapling, litter layer, and woody debris protected soil surface, thus preventing soil detachment, and provided surface roughness that minimised soil particle movement down the slope. The roles of these ecological factors were less significant compared to rainfall in determining the magnitude of runoff.

Canopy cover, sapling density, litter depth and woody debris can be measured quantitatively or qualitatively without complicated equipment and methods. Furthermore, they are sensitive to logging disturbance which make them suitable verifiers of soil erosion. Forest managers need to limit disturbance to these factors in order to minimise soil erosion in their logging operation areas. Monitoring of soil loss using runoff plots was cost-effective and provided valuable information about soil erosion risks caused by logging operations. Runoff plots clearly demonstrated site disturbances where the plots are located. Monitoring allowed more direct linkages to be made between management practices and their impacts on runoff and soil erosion, thereby enabling forest managers to identify problems and take appropriate preventive measures to improve their management practices.

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.