Ingenieria Forestal

USDA Forest Service RMRS - Moscow, Idaho > Microbial Processes Project > Publications

The Effects of Forest Management on Erosion and Soil Productivity*
William J. Elliot, Deborah Page-Dumroese, and Peter R. Robichaud (1999).

INTRODUCTION

In forest conditions, surface runoff and soil erosion are generally low because of the surface litter cover. Hydraulic conductivities are in excess of15 mm/hr, and erosion rates are generally less than 0.1 mg ha−1. If the litter layer is disturbed, then runoff and erosion rates can increase by several magnitudes. Disturbances can be natural, such as wildfire, or human induced, such as harvesting or prescription burning for ecosystem management, where conductivities can drop to under 5 mm/hr and erosion rates can exceed 20 mg ha−1. Roadsadversely impact forest soil productivity by directly reducing the productive area and by causing the greatest amount of soil erosion. Conductivities of roads have been measured to be less than 1 mm/hr, and erosion can be in excess of 100 mg ha−1. Harvesting activities reduce surface cover, and compact the soil, leading to increased runoff and erosion. Erosion generally decreases productivity of forestsby decreasing the available soil water for forest growth and through loss of nutrients in eroded sediment. The Water Erosion Prediction Project (WEPP) model is shown to be a useful tool in predicting the erosion impacts of different levels of vegetation removal at harvest and different levels of compaction. WEPP predicted that the nutrients lost through the organic matter in sediments aresignificant, but less than nutrient loss through tree removal. Work is ongoing to collect long-term site productivity data from numerous sites to aid in the analysis of forest management on soil erosion and site productivity.

For many years research has related soil erosion to productivity, with most activities focusing on agricultural or rangeland conditions. The Pierce (1991) overview includes over60 references to research on impacts of erosion on agricultural production. Pierce concluded that exact relationships between erosion and productivity are unclear and that to define any such relationship, considerable research is necessary over a wide range of soil and plant conditions.

Research is ongoing into the effects of management practices on forest soil productivity. Table 12.1summarizes some of this research. Relationships between disturbance and productivity are not simple but rather are extraordinarily complex, reflecting interactions among disturbance levels, soil water-holding capacities, nutrient cycling properties, and climate. Therefore, the effect of a given disturbance is highly dependent on site-specific soil properties and microclimate and may also be influenced byyear-to-year variation in climate. Table 12.1 shows that generally disturbances reduce long-term productivity, but there are cases where short-term productivity has increased following disturbances (e.g., Harvey et al., 1996; Corns, 1988). Research on the impacts of soil erosion on forest productivity is limited. This chapter provides an overview of current knowledge on the influence of forestmanagement activities on soil erosion and related on-site impacts and the subsequent effects of those impacts on forest productivity.

Soil erosion in an undisturbed forest is extremely low, generally under 1 mg ha−1 yr−1 (0.5 ton/acre/year). Disturbances, however, can dramatically increase soil erosion to levels exceeding 100 mg ha−1 yr−1 (50 tons/acre/year). These disturbances include naturalevents such as wildfires and mass movements and human-induced disturbances such as road construction and timber harvesting. Soil erosion, combined with other impacts from forest disturbance, such as soil compaction, can reduce forest sustainability and soil productivity.

|TABLE 12.1 |
|Typical Effects of...