Abstract
Forest operations can lead to increased runoff and soil loss on roads and skid trails. Best management practices (BMPs) aim to minimize erosion and water quality problems, but the efficacies of various BMP options such as water bars are not well documented. The aim of this study was to evaluate the effects of different densities of water diversion structures (water bars) on runoff volume and soil loss on different skid trail gradients on two soils with different textures in the Shenrood forest, Guilan province, northern Iran. The treatments included combinations of four densities of water bars (1, 2, 3 or 6 water bars per 150 m length of skid trail section [overland trail]), on two levels of trail gradient (≤20% and >20%) and two soil textures (clay loam and silt loam). Average runoff volume and soil loss per m2 of skid trail surface area were significantly greater (P≤0.05) on silt loam than on clay loam textured soils, and on slope gradients >20% (23–28%) than on gradients ≤20% (5–13%). Average runoff volume increased, and average soil loss decreased significantly (P≤0.05) with increasing density of water bars on both gradients and on both soil textures. On both soil textures, the lowest surface runoff volumes were observed with one water bar and the greatest volumes with six water bars installed. In contrast, the smallest amount of soil loss on both soil textures was observed with six water bars, and the greatest soil loss when only one water bar was installed. The installation of additional water bars led to significant differences in both responses at each level of density and led to reductions in soil loss of 77%, 57% and 27% in the clay loam, and 79%, 60% and 30% in the silt loam soil compared to the single water bar treatment. The reduced soil loss per unit of surface runoff volume is likely due to the reduced velocity of surface water runoff in the skid trail. The greater density of water bars appears to effectively divert more but slower flowing water from the skid trail, leading to reduced soil loss. While additional water bars thus better meet the objective of BMPs to minimize soil loss, managers need to balance the cost of the construction of additional water bars against the ecological benefits of reduced soil loss. An investment into additional water bars may be worthwhile if the additional structures are able to divert surface runoff more effectively to nearby vegetation and reduce the input of soil from skid trails to streams, thereby preventing the loss of water quality of these streams.
Highlights
The potential degradation of forest soils is a major problem in ground-based skidding in mountainous forests
This study evaluated the amount of surface runoff and soil loss using four densities of water diversion structures on two soil textures/sites to determine the efficacy of the water bars in reducing soil loss on skid trails immediately after skidding was completed in the late summer of 2018
The results of this study showed that slope length or the distance between water control structures is another important factor that influences unit area runoff and soil loss
Summary
The potential degradation of forest soils is a major problem in ground-based skidding in mountainous forests. Two major subcategories of skid trails are distinguished that differ in their potentials for soil loss: overland skid trails and bladed skid trails. Overland skid trails generally cause less soil disturbance and potential erosion than bladed skid trails (Sawyers et al 2012, Wade et al 2012). Both types of skid trails expose bare soil (Liu et al 2014, Cerdà 2007), whose lack of protective surface cover can generate significant amounts of soil loss during the rainy season (Arnáez et al 2004, Jordán-López et al 2009, Bochet et al 2009). Poor soil conditions make natural restoration of ground cover vegetation on skid trails difficult (Bochet and García-Fayos 2004)
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