Watershed restoration as a tool for improving coral reef resilience against climate change and other human impacts

Shelton, A. J. and Richmond, R. H., 2016. Estuarine, Coastal and Shelf Science

Original research (primary data)
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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.

Case studies

Basic information

  • Case ID: INT-184-1
  • Intervention type: Created habitats
  • Intervention description:

    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.

  • Landscape/sea scape ecosystem management: Yes
  • 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
  • Approach implemented in the field: Yes
  • Specific location:

    In Fouha Bay, the outlet of the La Sa Fu’a Watershed, GUAM

  • Country: Guam
  • Habitat/Biome type: Created forest |
  • Issue specific term: Community-based (general)


  • Notes on intervention effectivness: This study experimentally assesses an NBS (tree planting) vs a hybrid (tree planting + sediment socks) for erosion control. Effectiveness is assessed by comparing differences in sediment accumulation between plots. Both the tree planting and sediment sock plots/trees treatments were effective; and the “presence of trees influences soil beyond the area directly under their canopies”. They also assessed tree health, which shows that the sediment sock treatment actually helps trees establish and become healthy. In tree plots acacia trees survived better than native trees (they are more resilient in low nutrient environments) In terms of sediment trapping efficiency, the most effective is the hybrid intervention ‘the Sock Plot (SP) where a combination of trees and sediment filter socks’ “The highest trapping efficiency of sediment filter socks and acacia trees (two trees per 9 m2) observed in this study was 44 kg m2 yr1. Therefore, the necessary sediment yield reduction can be achieved by treating a total area of 0.05 km2 with 19 km of 8 in diameter sediment filter socks and 11,000 acacia tree seedlings” Note - The sediment sock plot actually also had planted tree seedlings, thus it represents a hybrid intervention. “Acacia tree seedlings were interspersed between the sediment filter socks. Casurina equisetifolia seeds were scattered into and behind the sediment filter socks” Experimental detail - Pedestal, sheet, and gully erosion were observed prior to restoration treatments. The plot was divided into four sections; Tree Plot (TP), Sediment Sock Plot (SP), Tree Reference Plot (TR), and Sediment Sock Reference Plot (SR). Photos were taken at the restoration test plot every few months to create a time-series record of restoration progress
  • Is the assessment original?: Yes
  • Broadtype of intervention considered: Other non-NbS approach(s)
  • Compare effectivness?: Yes
  • Compared to the non-NBS approach: Less effective
  • Report greenhouse gas mitigation?: No
  • Impacts on GHG: Not applicable
  • Assess outcomes of the intervention on natural ecosystems: No
  • Impacts for the ecosystem: Not reported
  • Ecosystem measures: n/a
  • Assess outcomes of the intervention on people: No
  • Impacts for people: Not reported
  • People measures: n/a
  • Considers economic costs: No
  • Economic appraisal conducted: No
  • Economic appraisal described:
  • Economic costs of alternative considered: No
  • Compared to an alternative: Not reported

Evaluation methodology

  • Type of data: Quantitative
  • Is it experimental: Yes
  • Experimental evalution done: In-situ/field
  • Non-experimental evalution done: Not applicable
  • Study is systematic: