Toward a mechanistic understanding of “peat collapse” and its potential contribution to coastal wetland loss

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.

Increasing Rates of Carbon Burial in Southwest Florida Coastal Wetlands

“…There is much concern about the potential for SLR to cause degradation and loss of wetland SOM (Hopkinson et al, ; Steinmuller et al, ), including coastal peat collapse (Chambers et al, ) in the coming century. However, these data and our analysis suggest that OC burial in coastal wetlands may be more responsive to SLR than previously thought, including periodic assistance from large storms.…”

Frequency of extreme freeze events controls the distribution and structure of black mangroves (Avicennia germinans) near their northern range limit in coastal Louisiana

“…Although the resistance and resilience of A. germinans to freezing has likely increased in the last 30 years (Osland et al, 2015; Osland, Hartmann, et al, 2019), a major freeze event like those that occurred in the 1980s would have the potential to damage mangroves across the entire Louisiana coast (e.g., see red in Figure 2a for the 1989 Christmas freeze). Freeze events that result in massive mangrove mortality also have the potential to result in peat collapse, erosion and conversion of coastal wetlands to open water if post‐disturbance natural regeneration does not occur quickly (Cahoon et al., 2003; Chambers, Steinmuller, & Breithaupt, 2019; Osland, Feher, Anderson, et al, 2020; Ward, Friess, Day, & MacKenzie, 2016). For example, on the Timbalier and Isle Dernières barrier island arcs, mangrove dieback during extreme freeze events in the 1980s are thought to have contributed to more rapid rates of erosion and shoreline change (McBride & Byrnes, 1997; Penland et al., 2003).…”

Soil Organic Carbon Development and Turnover in Natural and Disturbed Salt Marsh Environments

“…Decades following emergent grass dieback and submergence led to pond surface soils with different characteristics than the adjacent marsh. Collapse of marsh grass roots and water infilling of pore spaces contributed to higher water content, lower bulk densities, and initial subsidence (Figure 1, Chambers et al, 2019). Pond surface soils characterized with F 14 C < 1 and δ 13 C values similar to marsh soils at the same elevation (1.17 m; Figures 3b and 3f) are consistent with limited accumulation of recent production from submerged plant and algal communities and imply that ponds do not trap exported detritus (Figure 3; Spivak et al, 2017).…”