Plant–plant interactions in a subtropical mangrove‐to‐marsh transition zone: effects of environmental drivers

Abstract

Abbreviations AvgeM = Avicennia germinans monoculture pot; AvgeS = Avicennia germinans single mangrove species mesocosm pot; AvLa = Avicennia germinans and Laguncularia racemosa mixed mangrove species mesocosm pot; LaraM = Laguncularia racemosa monoculture pot; LaraS = Laguncularia racemosa single mangrove species mesocosm pot; Meso = Mesocosm pot without mangrove species added. Nomenclature Integrated Taxonomic Information System (ITIS) AbstractQuestions: Does the presence of herbaceous vegetation affect the establishment success of mangrove tree species in the transition zone between subtropical coastal mangrove forests and marshes? How do plant-plant interactions in this transition zone respond to variation in two primary coastal environmental drivers?Location: Subtropical coastal region of the southern United States. Methods:We conducted a greenhouse study to better understand how abiotic factors affect plant species interactions in the mangrove-to-marsh transition zone, or ecotone. We manipulated salinity (fresh, brackish or salt water) and hydrologic conditions (continuously saturated or 20-cm tidal range) to simulate ecotonal environments. Propagules of the mangroves Avicennia germinans and Laguncularia racemosa were introduced to mesocosms containing an established marsh community. Both mangrove species were also introduced to containers lacking other vegetation. We monitored mangrove establishment success and survival over 22 mo. Mangrove growth was measured as stem height and above-ground biomass. Stem height, stem density and above-ground biomass of the dominant marsh species were documented.Results: Establishment success of A. germinans was reduced under saturated saltwater conditions, but establishment of L. racemosa was not affected by experimental treatments. There was complete mortality of A. germinans in mesocosms under freshwater conditions, and very low survival of L. racemosa. In contrast, survival of both species in monoculture under freshwater conditions exceeded 62%. The marsh species Distichlis spicata and Eleocharis cellulosa suppressed growth of both mangroves throughout the experiment, whereas the mangroves did not affect herbaceous species growth. The magnitude of growth suppression by marsh species varied with environmental conditions; suppression was often higher in saturated compared to tidal conditions, and higher in fresh and salt water compared to brackish water. Conclusions:Our results indicate that herbaceous marsh species can suppress mangrove early seedling growth. Depending on species composition and density, marsh plants can slow mangrove landward migration under predicted climate change scenarios as salinity in freshwater and oligohaline wetlands increases with rising sea levels. Change in the relative coverage of mangrove forests and marshes will depend on both the ability of marsh species to migrate further inland as mangroves advance, and the ability of shoreline mangroves to adjust to rising sea level through accretionary processes.

Jump‐starting coastal wetland restoration: a comparison of marsh and mangrove foundation species

“…Our findings show that in 18 months, Avicennia expanded relatively little and had very few new recruits, while Spartina had rapid expansion as well as some recruitment. Avicennia 's slow expansion and lack of natural recruitment rate in this site may be attributed to slower growth capacity (Alleman & Hester ), difficulty competing with surrounding marsh at early life stages (Patterson et al ; McKee & Rooth ; Pickens ; Howard et al ), a lack of asexual reproduction (Baldwin et al ), and only one reproductive period prior to the end of the experiment. Mangrove propagules during this single reproductive period were often found desiccated or rotten as much of the site remained bare after the first 6–8 months (Yando, pers.…”

Modelling spatial biodiversity in the world’s largest mangrove ecosystem—The Bangladesh Sundarbans: A baseline for conservation

“…Biotic interactions (e.g., competition or facilitation) between plants can influence species composition at a local scale (Howard et al, ). Competitive exclusion of weak competitors in stressed mangrove habitats may lead to species‐poor mangrove communities dominated by a single or few opportunistic species (Saenger, ).…”

Review of the ecosystem service implications of mangrove encroachment into salt marshes

“…Near the poleward limits of mangroves in North America, declines in the occurrence of extreme winter temperature events have been identified as a primary driver of mangrove survival and expansion in recent decades (Cavanaugh et al., ; Stevens, Fox, & Montague, ). Biotic interactions between mangrove and marsh species, plus positive biological feedbacks (such as increased resilience to frost damage with mangrove age), may also influence the distribution and rate of mangrove expansion and salt marsh loss, beyond what would be predicted from temperature changes alone (Guo, Zhang, Lan, & Pennings, ; Howard et al., ; Osland et al., ; Peterson & Bell, ). The drivers of within‐range mangrove expansion are less clear and may include rises in relative sea level (whereby mangroves migrate upslope into salt marshes) (Kelleway et al., ; Raabe, Roy, & McIvor, ; Rogers, Battley, et al., , Rogers, Wilton, & Saintilan, ); changing rainfall patterns (Eslami‐Andargoli, Dale, Sipe, & Chaseling, ); increasing atmospheric CO 2 concentration and soil nutrients (McKee, Rogers, & Saintilan, ; McKee & Rooth, ); sedimentation and geomorphic change associated with catchment land use (Swales et al., ); changes in tidal influence and freshwater inflow (Howard et al., ), among other factors (Saintilan & Williams, ).…”