Hurricane wrack generates landscape‐level heterogeneity in coastal pine savanna

Abstract

Wrack (vegetation debris) deposited by storm surges of major hurricanes along the northern Gulf of Mexico produces depressant effects that vary from partial to complete mortality of groundcover vegetation in coastal savannas. As wrack decomposes or is relocated by a subsequent hurricane, patches are opened to colonization. We postulated that patterns of wrack deposition and removal, coupled with differential responses by savanna plant species should produce alternate states of groundcover vegetation. We explored extreme effects of wrack deposited by Hurricane Katrina (2005) in savannas dominated by slash pine Pinus elliottii and cordgrass Spartina patens and located above mean high tide at the Grand Bay National Estuarine Research Reserve, Mississippi, USA. In 2008, we established plots in adjacent areas with and without wrack deposits. Almost no groundcover plant species occurred in wrack deposits compared to adjacent groundcover without wrack. We simulated redistribution of wrack during a new storm surge by removing wrack from replicated plots and depositing it in plots without wrack, recording plant species in treatment and control plots before, then one month and one year after manipulations. One year later, about half the species present before wrack addition (especially dominant graminoids) grew back through redistributed wrack, suggesting that some species were resistant to burial of limited amounts of wrack. Wrack removal resulted in germination and establishment of numerous herbaceous plant species not in undisturbed groundcover, doubling total aboveground numbers of species in the pine savanna and shifting groundcover communities to alternate states not present prior to Katrina. Removal of wrack opens space colonized by resilient species, including those transported in wrack and those surviving intervals between disturbances belowground. Wrack dynamics (deposition and removal) generated alternate states that resulted from resistance‐ and resilience‐driven changes in different patches of groundcover in coastal savannas.

Effects of a tropical cyclone on salt marsh insect communities and post‐cyclone reassembly processes

“…Low resistance and high resilience might characterize foliar insect communities in coastal marshes that experience frequent low‐intensity tropical cyclones. Hurricanes and tropical storms cross Louisiana frequently; over the last century, coastal marshes in this region have experienced strong winds and storm surges on average every three years (Keim and Muller , Doyle , Platt et al ). Thus, plants and insects indigenous to coastal marshes likely have become adapted for recurrent cyclones, but in different ways (Connell , Platt and Connell ).…”

“…To explore the generality of our low resistance/high resilience hypothesis, information is needed on insect populations in salt marshes of different sizes exposed to cyclones that differ in intensity and degree of damage to the vegetation. Along exposed coastlines or in estuaries, as well as inland less‐saline marshes, especially during intense cyclones, vegetation may be more disrupted due to substrate deposition and erosion (Turner et al ), damage and uprooting of exposed vegetation in high winds (Howes et al ), and removal of dead culms that are subsequently deposited as large patches of wrack (Platt et al ), especially during more intense cyclones. Such large‐scale, intense disruptions of the vegetation might be expected to produce larger effects on the resident insect communities than those we observed.…”

“…These storms directly damage and influence survival of individuals within populations, as well as initiate post‐cyclone responses, often in altered environments (Whitmore , , Batista and Platt , Turton and Stork , Lin et al 2012, Turton , Chi et al , Ibanez et al ). As a result, tropical cyclones impact local species distributions, species interactions, community structure, and ecosystem functions (Vandermeer et al , Schoener et al , Turner et al , Spiller and Schoener , Willig et al , Sanchez‐Galvan et al , Platt et al ). Further, recurrent tropical cyclones produce consistent compositional and structural patterns regionally and worldwide, despite large taxonomic and evolutionary differences among organisms in the different regions experiencing these recurrent disturbances (Quigley and Platt , Keppel et al , Ibanez et al ).…”

Wrack and ruin: Legacy hydrologic effects of hurricane-deposited wrack on hardwood-hammock coastal islands

“…One longlasting impact is the large transfer of wrack (saltmarsh organic litter (Reidenbaugh and Banta 1980)) onto isolated salt-intolerant, hardwood forests situated within marshes (hereafter 'coastal hammocks'). The effect of these deep (0.2 m to >2 m thick (Roman et al 1994, Guntenspergen et al 1995, Bush et al 1996, Platt et al 2015), persistent (often lasting years (Roman et al 1994, Guntenspergen et al 1995, Bush et al 1996, Platt et al 2015) wrack layers on precipitation inputs to the soils of upland ecosystems has not yet been investigated, but we hypothesize that they are large because even order-of-magnitude thinner organic litter layers heavily influence ecosystem water budgets Savenije 2011, Coenders-Gerrits et al 2020). Other ecological effects of wrack deposition, like the smothering of vegetation and altered habitat use by related fauna, have been previously described (Bertness and Ellison 1987, Roman et al 1994, Guntenspergen et al 1995, Bush et al 1996, Pennings and Richards 1998, Stalter et al 2005, Stalter et al 2006, Platt et al 2015.…”

Microtopographic structures facilitate plant recruitment across a saltmarsh tidal gradient

“…Coastal wetlands are suffering severe and long‐term global degradation (Lotze et al, ) due to land conversion and climate change. Unexpected events, such as reclamation activity (Garbutt & Wolters, ), spring drought (He, Silliman, Liu, & Cui, ), or storms (Platt, Joseph, & Ellair, ), mean that even rapidly colonizing halophytic species will suffer deterioration in saltmarshes, which leaves the area bare. In some cases they might self‐recover; however, in many other cases they might lose their self‐recover ability and remain bare or deteriorated due to some unknown feedback loops (Suding, Gross, & Houseman, ).…”