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Assessing Constructed Forested Wetland Development Using Successional ( Performance ) Trajectories Susan M. Carstenn Hawaii Pacific University Kaneohe, Hawaii Why Reference Wetlands? Restoration Strategies Endpoint…target….goal Assessment metrics Reference Wetlands

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Assessing Constructed Forested Wetland Development Using Successional (Performance) Trajectories

Susan M. CarstennHawaii Pacific UniversityKaneohe, Hawaii


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Why Reference Wetlands?

  • Restoration Strategies

    • Endpoint…target….goal

    • Assessment metrics

  • Reference Wetlands

    • minimally adversely affected by anthropogenic activities

    • the wetland being destroyed as in the case of mitigation

    • old restoration project that has been deemed successful


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Constructed Forested Wetland

Constructed Forested Wetland

Natural Forested Wetlands


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Why Trajectories?

  • Wetlands are dynamic ecosystems; therefore, static metrics are inappropriate assessment tools.

  • Reference wetlands serve as the target, but can not assess incremental progress towards the target.

  • Trajectories provide incremental targets.


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Reference Wetland Approach

The performance of a constructed wetland (stars) is compared to the mean ( standard error(s)) of a set of reference wetlands (green).

Quality or Quantity

Time


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Reference Wetland Approach

  • Success is declared when the constructed wetland (stars) meets or exceeds the reference wetlands.

  • Wetlands developing at different rates are assessed with the same criteria.

  • The time required to replace wetland function is inconsequential.

Quality or Quantity

Time


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Trajectory Approach

  • A constructed wetland (star) is compared to the mean of all previously constructed wetlands (green).

  • Success is declared when a newly constructed wetland (stars) falls within the 95% predication intervals around the mean of all previously constructed wetlands (green).

Quality or Quantity

Time


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Trajectory Approach

A wetland demonstrates suboptimal development…then what?

Success is declared when a newly constructed wetland (stars) falls within the 95% predication intervals around the mean of all previously constructed wetlands (green).

Quality or Quantity

The orange line represents the timing of a redemediation action e.g., supplemental planting, fertilization, or understory seeding.

Time


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Trajectory Construction

  • Space for Time Substitution

    • A chronosequence of wetlands

    • Array of metrics measured once

  • Individual Wetland Trajectories

    • Many newly constructed wetlands

    • Metrics monitored over time


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Canopy Trees

Height

Diameter at breast height (dbh)

Size class distributions (dbh)

Community basal area

Canopy cover

Stem density

Species richness

Species diversity

Subcanopy Trees and Shrubs

Stem density

Diameter at breast height

Species richness

Species diversity

Herbaceous Species

Species richness

Species diversity

Understory Species

Canopy and subcanopy seedling richness and frequency of occurrence

Vine species richness and frequency of occurrence

Functional group richness and frequency of occurrence

Soils

Soil water content

Bulk density

Organic Matter

Particle size analysis

Assessment Metrics


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Assessment Metrics

  • Emerging Properties

    • Hierarchical size class frequency distributions of tree diameters

    • Changes in frequency of occurrence of vegetation structural categories with age

    • Increasing organic matter associated with increases in soil water content

    • Decreasing bulk density associated with increasing organic matter content


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Assessment Metrics

  • Emerging Properties

    • Hierarchical size class frequency distributions of tree diameters

    • Changes in frequency of occurrence of vegetation structural categories with age

    • Increasing organic matter associated with increases in soil water content

    • Decreasing bulk density associated with increasing organic matter content


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Results – Individual Metrics

  • Canopy Trees

    • height (r2 = 0.81; p < 0.05)

    • diameter at breast height (r2 = 0.80; p < 0.05)

    • stand basal area (r2 = 0.75; p < 0.05)

    • canopy cover (r2 = 0.77; p < 0.05)

  • Subcanopy Trees (Ilex cassine)

    • diameter at breast height (r2 = 0.64; p < 0.05)

    • stand basal area (r2 = 0.75; p < 0.05)

  • Shrubs and Understory

    • no significant trajectories


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Hydrogeomorphic Classes

  • Depressional

    • isolated

    • riparian

    • headwater

  • Lake Fringing

    • littoral

  • Stream Floodplain

    • bordering an incised channel

www.geog.psu.edu/wetlands/manual/image13.gif





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Results – Emerging Properties

5 Years Old

18 Years Old

Natural Wetland



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Discussion

  • Challenges

    • Selecting data collection methods to support comparisons

      • across developmental stages

      • with literature values


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Discussion

  • Canopy trees

    • height

      • approach values of natural wetland

      • fall short of literature values

    • dbh

      • sites > 12 years old met or exceeded literature values

    • stand basal area

      • only the oldest sites approached literature values

  • Subcanopy trees

    • Myrica cerifera andSalix caroliniana similar to literature values

    • Ilex cassine and Persea palustris fall below literature values


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Discussion

  • Shrubs

    • lower species richness

    • similar frequency, dbh, and density

  • Understory

    • similar to richness of cypress domes and bayheads, but less than hardwood swamps and marshes.

  • Soil

    • weak, but significant trends in organic matter


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Conclusions

  • Canopy tree development alone may not indicate restoration success; it suggests that the site is developing conditions indicative of a mature forest.

  • Subcanopy, shrub, and understory

    • community structure is approaching those of wetlands described in the literature.

    • richness and diversity is on the low end of the range of reported in the literature.


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Conclusions

  • The trajectory approach shows great promise.

    • In spite of the added variation inherent in the space-for-time substitution, highly significant trends were detected.

  • Uniform data collection methods should be established.

  • How much time should be allocated for a wetland to meet a particular assessment criterion?


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