Mass tree mortality leads to mangrove peat collapse at Bay Islands, Honduras after Hurricane Mitch

Abstract

Summary 1We measured sediment elevation and accretion dynamics in mangrove forests on the islands of Guanaja and Roatan, Honduras, impacted by Hurricane Mitch in 1998 to determine if collapse of underlying peat was occurring as a result of mass tree mortality. Little is known about the balance between production and decomposition of soil organic matter in the maintenance of sediment elevation of mangrove forests with biogenic soils. 2 Sediment elevation change measured with the rod surface elevation table from 18 months to 33 months after the storm differed significantly among low, medium and high wind impact sites. Mangrove forests suffering minimal to partial mortality gained elevation at a rate (5 mm year − 1 ) greater than vertical accretion (2 mm year − 1 ) measured from artificial soil marker horizons, suggesting that root production contributed to sediment elevation. Basin forests that suffered mass tree mortality experienced peat collapse of about 11 mm year − 1 as a result of decomposition of dead root material and sediment compaction. Low soil shear strength and lack of root growth accompanied elevation decreases. 3 Model simulations using the Relative Elevation Model indicate that peat collapse in the high impact basin mangrove forest would be 37 mm year − 1 for the 2 years immediately after the storm, as root material decomposed. In the absence of renewed root growth, the model predicts that peat collapse will continue for at least 8 more years at a rate (7 mm year − 1 ) similar to that measured (11 mm year − 1 ). 4 Mass tree mortality caused rapid elevation loss. Few trees survived and recovery of the high impact forest will thus depend primarily on seedling recruitment. Because seedling establishment is controlled in large part by sediment elevation in relation to tide height, continued peat collapse could further impair recovery rates.

A global standard for monitoring coastal wetland vulnerability to accelerated sea-level rise

“…The RSET method has been useful not only for documenting trends in surface elevation change in specific wetlands 40 , but also for comparing rates of elevation change among hydrogeomorphic zones within a site (for example, a delta 41 ), differentiating between sites dominated by surface processes 40 versus subsurface processes 14 , documenting high variability in sediment deposition from singular storm events 44 , capturing rapid peat collapse from small-scale (for example, lightning strikes) and large-scale (for example, hurricanes) disturbances 45,46 , measuring the effects of elevated atmospheric CO 2 concentrations on surface elevation change 47 , and disentangling complex interactions among herbivory, biomass production and surface elevation change 48 . RSET data have also contributed to more applied research assessing the impacts of management practices on site-specific vulnerability to SLR, such as prescribed burning 49 , sedimentation from point-source coastal management interventions 50 , water diversion 51 , fence construction to enhance sediment deposition 52 and thin-layer deposition of dredged sediment 53 .…”

Turning the Tide: How Blue Carbon and Payments for Ecosystem Services (PES) Might Help Save Mangrove Forests

“…Cyclones in the Bay of Bengal show a similar average 29-year periodicity (Singh et al 2000). Hurricanes and cyclones can certainly cause large scale destruction of mangrove forests; Cahoon et al (2003) cite papers showing that ''powerful storms have caused mass mortality of at least 10 Caribbean mangrove forests during the past 50 years.'' However, there is evidence that mangroves are more resistant and resilient compared with other forest types when exposed to the same storms.…”

Coastal Systems and Low-Lying Areas

“…Changes in storm intensity can also affect vegetated coastal wetlands. Cahoon et al (2003) analysed the elevation responses from a variety of hurricane-influenced coastal settings and found that a storm can simultaneously influence both surface and subsurface soil processes, but with much variability. Saltmarshes (halophytic grasses, sedges, rushes and succulents) are common features of temperate depositional coastlines.…”

“…Nevertheless, it seems highly likely that similar principles are in operation and that the sedimentary response of the shoreline is a function of both the availability of sediment (Walsh and Nittrouer, 2004) and the ability of the organic production by mangroves themselves to fill accommodation space provided by sea-level rise (Simas et al, 2001). Mangroves are able to produce root material that builds up the substrate beneath them (Middleton and McKee, 2001;Jennerjahn and Ittekkot, 2002), but collapse of peat occurs rapidly in the absence of new root growth, as observed after Hurricane Mitch (Cahoon et al, 2003) and after lightning strikes (Sherman et al, 2000). Groundwater levels play an important role in the elevation of mangrove soils by processes affecting soil shrink and swell.…”

“…A global assessment of mangrove accretion rates by Saenger (2002) indicates that vertical accretion is variable but commonly approaches 5 mm/yr. However, many mangrove shorelines are subsiding and thus experiencing a more rapid relative sea-level rise (Cahoon et al, 2003).…”