Quantifying trail-related soil erosion at two sites in the humid tropics : Jatun Sacha, Ecuador, and La Selva, Costa Rica

The relationship between trails and soil erosion in temperate areas has received considerable study. In tropical areas, however, no comprehensive research has been done on the subject of trail-related soil erosion. This study is the first to quantify the effect of trails on runoff generation and sediment mobilization in the humid tropics. Field research was conducted at two ecotourism sites in the humid tropics: Jatun Sacha Biological Station, Ecuador, and La Selva Biological Station, Costa Rica. To gauge the difference in runoff and sediment production between adjacent on and off-trail sites, portable rainfall simulators were used. Forty-one rainfall simulation tests were conducted on trails and 44 tests were conducted in the adjacent, undisturbed forest. Total runoff volume was measured and samples were taken of both entrained sediment and rainsplash. To quantify trail soil compaction, soil bulk density and soil penetration resistance were measured. The results show that both runoff generation and sediment mobilization were significantly higher on trails than in off-trail areas. These results are similar to those of past studies undertaken with the same equipment in East Tennessee. Compared to the Tennessee trail sites, trails in this study produced a comparable, but slightly higher coefficient of runoff and an average of 66 to 84% less detached sediment. Bulk density measurement showed striking differences between trail and adjacent off-trail sites – trail bulk density was higher due to the loss of organic material and compaction due to trampling. The static cone penetrometer was found to be useful for measuring relative differences in soil compaction. Highly localized differences in soil characteristics made the penetrometer unusable as a predictive measure for bulk density. Neither bulk density nor soil penetration resistance was found to provide a good analogue for infiltration rate. Using trail length and sediment mobilization data, I determined an erosional potential for trails in the two reserves. At Jatun Sacha, the 1.9 km of trails on alluvial soils mobilize an average of 22 kg of sediment per millimeter of rain and the 12.7 km of trails on residual soils mobilize 212.4 kg. Thus, in a 15 mm precipitation event, 3,186 kg of sediment would be mobilized for possible transport on the trails of Jatun Sacha. At La Selva, a 15 mm rain event would mobilize 3163.5 kg of sediment on alluvial soils and 1552.5 kg on residual soils. Overall, this study shows that compacted trails in humid tropical environments act as partial area runoff and sediment sources, as in temperate environments. While their exact effects may differ somewhat (i.e., greater coefficient of runoff and less sediment detachment in tropics), they follow the same broad pattern of increased sediment and runoff production on trails.