Toward a mechanistic understanding of "peat collapse" and its potential contribution to coastal wetland loss.

Lisa G Chambers,Havalend E Steinmuller,Joshua L Breithaupt

doi:10.1002/ecy.2720

Abstract

Coastal wetlands are susceptible to loss in both health and extent via stressors associated with global climate change and anthropogenic disturbance. Peat collapse may represent an additional phenomenon contributing to coastal wetland loss in organic‐rich soils through rapid vertical elevation decline. However, the term “peat collapse” has been inconsistently used in the literature, leading to ambiguities regarding the mechanisms, timing, and spatial extent of its contribution to coastal wetland loss. For example, it is unclear whether peat collapse is distinct from general subsidence, or what biogeochemical changes or sequence of events may constitute peat collapse. A critical analysis of peer‐reviewed literature related to peat collapse was supplemented with fundamental principles of soil physics and biogeochemistry to develop a conceptual framework for coastal wetland peat collapse. We propose that coastal wetland peat collapse is a specific type of shallow subsidence unique to highly organic soils in which a loss of soil strength and structural integrity contributes to a decline in elevation, over the course of a few months to a few years, below the lower limit for emergent plant growth and natural recovery. We further posit that coastal wetland peat collapse is driven by severe stress or death of the vegetation, which compromises the supportive structure roots provide to low‐density organic soils and shifts the carbon balance of the ecosystem toward a net source, as mineralization is no longer offset by sequestration. Under these conditions, four mechanisms may contribute to peat collapse: (1) compression of gas‐filled pore spaces within the soil during dry‐down conditions; (2) deconsolidation of excessively waterlogged peat, followed by transport; (3) compaction of aerenchyma tissue in wetland plant roots, and possibly collapse of root channels; and (4) acceleration of soil mineralization due to the addition of labile carbon (dying roots), oxygen (decreased flooding), nutrients (eutrophication), or sulfate (saltwater intrusion). Scientists and land managers should focus efforts on monitoring vegetation health across the coastal landscape as an indicator for peat collapse vulnerability and move toward codifying the term “peat collapse” in the scientific literature. Once clarified, the contribution of peat collapse to coastal wetland loss can be evaluated.

Highlights

Coastal wetlands are highly vulnerable to sea level rise, with estimates of 22–30% global areal loss by 2100 (Nicholls et al 1999, IPCC 2007)
Based on our current understanding, coastal wetland peat collapse is a specific type of shallow subsidence unique to highly organic soils in which a loss of soil strength and structural integrity contributes to a decline in elevation, over the course of a few months to a few years, below the lower limit for emergent plant growth and natural recovery
We suggest the prognostic role of vegetation stress and/or death in coastal wetland peat collapse on the basis of the consistent reference to it as the proximate driver in published case studies (Table 1), and the logic that the existence of healthy vegetation coverage means the maintenance of a strong root network to support the soil matrix and maintain its’ strength, effectively preventing the initiation of a loss in strength implied by the term “collapse.” Due to the heterogeneity of vegetation and micro-habitats within coastal wetlands, we expect peat collapse will initially manifest as a “patchy” deterioration of a wetland, rather than the loss of a large area all at once

Summary

INTRODUCTION

Coastal wetlands are highly vulnerable to sea level rise, with estimates of 22–30% global areal loss by 2100 (Nicholls et al 1999, IPCC 2007). If soil structure is compromised, allowing the soil to consolidate, soil bulk density should increase at the bottom of the collapse zone as previously existing pore space is replaced by the overlying solid material This increase in surface or shallow subsurface bulk density is commonly found in collapsed inland peat domes (Franzen 2006, Kool et al 2006) and in slumping permafrost (Abbott and Jones 2015), but results from coastal wetlands are inconsistent. Others have found no relationship between elevation loss and bulk density, instead suggesting the soil material may have been washed away following the collapse, or rapidly oxidized (Delaune et al 1994, Day et al 2011) We propose that this inconsistency may be a result of hydrology, such that sites undergoing regular dry-down (e.g., due to seasonality, drought, or a large tidal range) are likely to experience pore space compression leading to greater bulk densities.

SET-MH

Findings

CONCLUSIONS