Quantifying Functional Increases Across a Large-Scale Wetland Restoration Chronosequence

Jacob F Berkowitz

doi:10.1007/s13157-018-1103-9

Abstract

Over 300,000 ha of forested wetlands have undergone restoration within the Mississippi Alluvial Valley region. Restored forest successional stage varies, providing opportunities to document wetland functional increases across a large scale restoration chronosequence using the Hydrogeomorphic (HGM) approach. Results from >600 restored study sites spanning a 25 year chronosequence indicate that: 1) wetland functional assessment variables increased toward reference conditions; 2) restored wetlands generally follow expected recovery trajectories; and 3) wetland functions display significant improvements across the restoration chronosequence. A functional lag between restored areas and mature reference wetlands persists in most instances. However, a subset of restored sites have attained mature reference wetland conditions in areas approaching or exceeding tree diameter and canopy closure thresholds. Study results highlight the importance of site selection and the benefits of evaluating a suite of wetland functions in order to identify appropriate restoration success milestones and design monitoring programs. For example wetland functions associated with detention of precipitation (a largely physical process) rapidly increased under post restoration conditions, while improvements in wetland habitat functions (associated with forest establishment and maturation) required additional time. As the wetland science community transitions towards larger scale restoration efforts, effectively quantifying restoration functional improvements will become increasingly important.

Highlights

IntroductionWetlands provide a variety of well-established hydrologic, biogeochemical, and habitat functions linked to ecosystem services that prove beneficial to society (Smith et al 1995; Novitski et al 1996)
Wetland Restoration in the Mississippi Alluvial ValleyWetlands provide a variety of well-established hydrologic, biogeochemical, and habitat functions linked to ecosystem services that prove beneficial to society (Smith et al 1995; Novitski et al 1996)
Over 10 million ha of bottomland hardwood (BHW) once occurred in the region; extensive alteration to the landscape have resulted in a 70% decline in BHW habitat (The Nature Conservancy 1992; King et al 2006)

Summary

Introduction

Wetlands provide a variety of well-established hydrologic, biogeochemical, and habitat functions linked to ecosystem services that prove beneficial to society (Smith et al 1995; Novitski et al 1996). The current study returned to the chronosequence to further evaluate restoration conditions, with an increased focus on quantifying large scale wetland functional increases as restored BHW wetlands approach canopy closure thresholds. To support USACE wetland restoration initiatives in the region, Smith and Klimas (2002) developed a Hydrogeomorphic (HGM) wetland functional assessment method designed for the Mississippi Alluvial Valley. The HGM method addressed seven wetland functions provided by BHW wetlands, and included approaches to determine conditions in both mature and restored areas (Smith et al 2013). Despite numerous publications addressing afforestation and restoration within the Mississippi Alluvial Valley, questions persist regarding the mid- to long-term success of large scale restoration efforts designed to improve BHW wetland function (Stanturf et al 2001) and the applicability of existing assessment approaches designed to quantify those functions (Cole 2016). Trajectories incorporating the oldest available restored BHW wetlands in the region, and 3) compared the levels of restored wetland function with mature BHW forested wetlands

Methods

Results and Discussion

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