Environmental stressors in coastal areas threaten the sustainability of marine resources and reduce their resilience to climate change impacts. Accelerated land erosion is a major stressor that leads to increased turbidity and sedimentation on downstream coral reefs and the degradation of ecosystem functions. Volunteers from a community-based initiative in Guam installed 130 tree seedlings and 54 m of sediment filter socks in eroding hillsides above Fouha Bay, to reduce erosion. A soil probing method for measuring soil depth was developed and used to evaluate the effectiveness of the watershed restoration tools. The trees and socks trapped 111.8 tons of sediment on land after 21 months. In heavily eroding portions of the restoration plot, where socks and trees were used in combination, the mean sediment trapping efficiency was 44 kg mأ¢ث†’2 yrأ¢ث†’1. Previous studies indicate a 75% reduction in sedimentation rate is required to bring Fouha Bay below severe-catastrophic sedimentation stress (>50 mg cmأ¢ث†’2 dayأ¢ث†’1). Based on the observed sediment trapping efficiency of restoration tools in this study, an estimated 0.05 km2 of severely eroding hillsides must be treated with 19 km of socks and 11,000 trees to trap 2121 tons of sediment and achieve the necessary reduction. If sediment input into the bay is controlled, existing sediment will clear out with storm-driven swells. As shown in other high islands, coral reefs are resilient and can recover after sedimentation stress is reduced. Data generated on the efficiency of watershed restoration tools in this study can be used in watershed management plans to promote the sustainability and resilience of coastal areas in other tropical islands.
Interventions discussed fall under the umbrella of “Watershed management and erosion control”. A community initiative called the Humåtak Project (humatakproject.org) coordinated the watershed restoration activities evaluated in this study. The initiative formed in 2001 after community members became concerned about the diminishing quality of nearshore marine resources, which resulted from poorly executed road construction between 1988 and 1990 (Richmond, 1993). Volunteers from this initiative in Guam. installed 130 tree seedlings and 54 m of sediment filter socks in eroding hillsides above Fouha Bay, to reduce erosion. Acacia auriculiformes, Acacia confusa, and Acacia mangium are the main tree species propagated for watershed restoration in Guam. The trees are fast growing and able to thrive in infertile badland soils due to the presence of nitrogen-fixing bacteria present in their roots. Acacias act as “nurse” plants, allowing other species of plants to grow after facilitating improvements in soil quality (Yang et al., 2009). Over time, acacias improve soil quality by increasing water absorption, regulating temperature through canopy shading, adding organic matter through leaf litter, and increasing nutrient content. Although these acacia trees are exotic (non-native) species to the island, there have been no signs of invasiveness in Guam (Space and Falanruw, 1999). Along with Acacia auriculiformes, three native tree species were used for reforestation efforts in the La Sa Fu’a Watershed; Artocarpus mariannensis, Premna obtusifolia, and Calophyllum inophyllum.
|Climate change impacts||Effect of Nbs on CCI||Effect measures|
|Soil erosion||Positive||To test effectiveness (compare treatments & controls) they compared differences in mass of sediment accumulation (sediment trapping efficiency) between plots kg m^-2yr^-1; see text for details|
In Fouha Bay, the outlet of the La Sa Fu’a Watershed, GUAM