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Rates of revegetation of gullies in coastal British Columbia : implications for sediment production Pellerin, Diane

Abstract

Steep forested terrain in coastal British Columbia is typically dissected by V-shaped debris flow channels, or gullies. These steep, unstable slopes are important sources of sediment. Debris flows scour gully sidewalls, strip vegetation cover and leave mineral soils vulnerable to erosion. Accelerated sediment production persists for several years following a debris flow. However, little is known about the time frame for gully sidewall re-stabilisation or the associated decline in sediment production over time. The objectives of this study were to compare rates of revegetation between logged and unlogged gullies, to estimate fine sediment discharge from gully sidewalls at different stages of vegetation recovery, and to examine the role of environmental factors in determining plant succession and fine sediment production. Eleven gullies were studied in the coastal western hemlock forest of Coquitlam basin, southwestern Coast Mountains. Vegetation quadrats were established at 92 sites and sediment yield was monitored at 18 of the sites for one year. Community analysis suggests that land management influences the availability of colonising species and gully light regimes. Consequently, logged and unlogged gullies show different community associations and vegetation recovery rates. Six years after a disturbance, vegetation cover was significantly lower on unlogged compared to logged sidewalls (58% and 78%, respectively). After 19 years vegetation cover was similar in both environments (111% and 114%, respectively). The logged environment was dominated by shrubs such as Rubus parviflorus and Thuja plicata, and likely will become a Thuja plicata community. Initially, the unlogged environment was dominated by ferns and forbs; however, 19 years later it showed signs of future dominance by Thuja plicata. Results suggest that debris flow disturbance in gullies promotes higher regional plant species diversity. Sediment yield declined over time in most gullies following revegetation. Sediment yields varied between 0.1 to 72 m /ha/yr in logged gullies, and 0.3 to 24 m /ha/yr in unlogged gullies. Sediment yields showed a similar rate of decline in both environments over time. Results indicate that plant cover on gully sidewalls accounted for most of the variation in sediment yield. Other variables such as percent bare soil, slope angle and adjacent vegetation stands (which intercept rainfall reducing rainsplash erosion) were also important determinants of sediment yield. Consequently, by determining composition of adjacent stands, land management significantly influences sediment yield from gully sidewalls.

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