Elsevier

Agricultural and Forest Meteorology

Volume 213, November 2015, Pages 283-290
Agricultural and Forest Meteorology

Bistability of mangrove forests and competition with freshwater plants

https://doi.org/10.1016/j.agrformet.2014.10.004Get rights and content

Highlights

Abstract

Halophytic communities such as mangrove forests and buttonwood hammocks tend to border freshwater plant communities as sharp ecotones. Most studies attribute this purely to underlying physical templates, such as groundwater salinity gradients caused by tidal flux and topography. However, a few recent studies hypothesize that self-reinforcing feedback between vegetation and vadose zone salinity are also involved and create a bistable situation in which either halophytic dominated habitat or freshwater plant communities may dominate as alternative stable states. Here, we revisit the bistability hypothesis and demonstrate the mechanisms that result in bistability. We demonstrate with remote sensing imagery the sharp boundaries between freshwater hardwood hammock communities in southern Florida and halophytic communities such as buttonwood hammocks and mangroves. We further document from the literature how transpiration of mangroves and freshwater plants respond differently to vadose zone salinity, thus altering the salinity through feedback. Using mathematical models, we show how the self-reinforcing feedback, together with physical template, controls the ecotones between halophytic and freshwater communities. Regions of bistability along environmental gradients of salinity have the potential for large-scale vegetation shifts following pulse disturbances such as hurricane tidal surges in Florida, or tsunamis in other regions. The size of the region of bistability can be large for low-lying coastal habitat due to the saline water table, which extends inland due to salinity intrusion. We suggest coupling ecological and hydrologic processes as a framework for future studies.

Introduction

In tropical and subtropical regions, assemblages of mangrove forests parallel the coastline and riverbanks, transitioning sharply to salt-intolerant plant species, such as hardwood hammocks or freshwater marsh, farther inland. Explanations for the sharpness of the boundary between these two vegetation types have centered on abiotic environmental attributes, such as elevation, salinity, and tidal flooding, as well as biotic processes such as mangrove propagule dispersal and interspecific competition with freshwater plants (Ball, 1980, Davis et al., 2005, Lugo, 1997, Mckee, 1995, Youssef and Saenger, 1999). A widely accepted perspective is that of realized niche differentiation through a combination of abiotic limitation and competition; i.e., freshwater plants cannot survive outside of their physiological salt tolerance range, while mangrove can grow in freshwater as well as saltwater, but do not occur in strictly freshwater environments due to superior competition from freshwater plant species (Krauss et al., 2008, McKee, 2011, Medina et al., 2010, Odum and McIvor, 1990, Sternberg and Swart, 1987). This niche differentiation between halophytic and glycophytic species has been tested in transplant experiments. Transplanted salt marsh species from the intertidal zone to freshwater habitats perform well when competing plants are removed, but are suppressed by competition if freshwater plants are present (Bertness and Ellison, 1987, Cui et al., 2011, Grace and Wetzel, 1981).

An implication of the niche differentiation hypothesis is that fitness of mangrove and freshwater plants might be similar over some intermediate range of salinity levels. In this case, one would expect a gradual replacement of mangrove vegetation with freshwater plants as underlying environmental conditions gradually change. Yet there are sharp ecotones between halophytic forests (mangroves and buttonwood hammock) and freshwater communities, despite extremely gradual changes in topography in some regions, such as coastal areas of southern Florida (Giri et al., 2011, Ross et al., 1992, Saha et al., 2011) and East Africa (Di Nitto et al., 2014). One possible explanation for the sharp ecotones is that the environmental gradient of salinity, determined purely by abiotic factors such as tidal flux, is also sharp, separating salinity tolerant mangroves from the salinity intolerant plants. But this explanation fails to account for the boundaries located at upper intertidal zone, which are seldom inundated by tides; e.g., fringe mangrove forest (Pool et al., 1977).

Sternberg et al. (2007) hypothesized that mangrove forests compete with hardwood hammocks as alternative stable states of either pure mangrove forests or pure salt-intolerant hammock species, a case of the general phenomenon of bistability in other systems (Beisner et al., 2003, Holling, 1973, May, 1977, Scheffer et al., 2001). According to the bistability theory, a mixture of the two alternative vegetation types is rarely observed, and an initially mixed system will move toward complete dominance of one or the other type. According to the hypothesis of Sternberg et al. (2007), both mangroves and freshwater plants obtain their water from the vadose zone; that is, the unsaturated soil layer. In coastal areas, this vadose zone is underlain by highly brackish ground water, so that evapotranspiration, by depleting water in this zone during the dry season, can lead to infiltration by more saline ground water (Fass et al., 2007, Passioura et al., 1992, van Duijn et al., 1997). Although freshwater plants tend to decrease their evapotranspiration when vadose zone salinities begin to increase, thus limiting salinization of the vadose zone, mangroves can continue to transpire at relatively high salinities (Ewe and Sternberg, 2005, Sternberg et al., 2007). Each vegetation type thus tends to promote local salinity conditions that favor itself in competition. This hypothesis of boundary formation through positive feedbacks has been supported through simulation models in which the interactions of vegetation types with each other and with local salinity conditions are simulated (Jiang et al., 2012a, Teh et al., 2008).

Until recently, few data have been available to test the bistability hypothesis. Here we link available data on two spatial scales, remote sensing and vegetation physiology, to provide further evidence that the mangrove ecotone pattern at landscape level emerges from lower-level physiological traits. We use remote sensing imagery to analyze spatial patterns of mangrove forests and hardwood hammocks in southern Florida. We also document what is known about the transpiration regime of mangrove in response to soil salinity. A mathematical model (Jiang et al., 2012b) is then applied to elucidate the bistability dynamics. In the model, environmental factors such as tidal flux, precipitation, evaporation, and soil properties etc., form a physical template that influences the competition between mangrove forests and freshwater plants, especially at the larger spatial scale. Ignoring or downgrading the contribution of physical template would overestimate the role of positive feedback. By including the positive feedback along with the physical template, we provide a framework toward more predictive large-scale vegetation changes.

Section snippets

Interspersion and juxtaposition of mangroves and hammocks

Various landscape metrics have been used to assess spatial relationships between different vegetation types in heterogeneous wetlands and other environments (Fernandes et al., 2011, Guzy et al., 2013, Shoyama and Braimoh, 2011, Stapanian et al., 2013). To evaluate horizontal interspersion and juxtaposition of hardwood hammocks and mangroves we employed Fragstats version 4.2 software, which is commonly used to analyze spatial patterns within categorical vegetation and land cover maps (Mcgarigal

Mechanisms of bistability

Fundamental to the bistability hypothesis (Jiang et al., 2012a, Sternberg et al., 2007, Teh et al., 2008) is the assumption that during the dry season mangroves will continue to transpire despite the high salinity of the vadose zone, causing capillary rise of saline water from water table and a consequent increase in the salinity of the vadose zone. Freshwater plants during the dry season, on the other hand, will decrease or cease transpiration, thus maintaining a low salinity of the vadose

Implication for large-scale vegetation changes

Much of what has been documented concerning the spatial and temporal shifts of mangroves along coastal habitats can be explained by gradual environmental changes (Berger et al., 2008, Chen and Twilley, 1998, Doyle et al., 2003). Sea level rise and anthropogenic decreases in freshwater flow cause salinity intrusion and a landward shift of the mangrove bands. For example, Doyle et al. (2003) used computer simulations to project possible inland migration of mangroves along the southern Florida

Concluding remarks and future direction

We demonstrated here how interactions between mangroves, freshwater plants and local soil conditions could result in bistability along an environmental gradient of water table salinity. Sharp ecotones are usually indicators of positive feedbacks that cause bistability between differing vegetation types, such as forest-grassland, forest-mire, Alpine treelines, etc. (Agnew et al., 1993, Wiegand et al., 2006). The mangrove forest—salt marsh transitions are also suggested to result from positive

Acknowledgements

We appreciate two reviewers for insightful comments on this manuscript. JJ was supported as Postdoctoral Fellow at the National Institute for Mathematical and Biological Synthesis (NSF Award #DBI-1300426) with additional support from The University of Tennessee, Knoxville. DOF and LSLS were supported by the NASA Water SCAPES (Science of Coupled Aquatic Processes in Ecosystems from Space) Grant NNX08BA43A. DLD was partially supported by the FISCHS Project (Future Impacts of Sea Level Rise on

References (94)

  • M.A. Stapanian et al.

    Disturbance metrics predict a wetland vegetation index of biotic integrity

    Ecol. Indic.

    (2013)
  • T. Takemura et al.

    Physiological and biochemical responses to salt stress in the mangrove, Bruguiera gymnorrhiza

    Aquat. Bot.

    (2000)
  • S.Y. Teh et al.

    A simulation model for projecting changes in salinity concentrations and species dominance in the coastal margin habitats of the Everglades

    Ecol. Modell.

    (2008)
  • J.B. Wilson et al.

    Positive-feedback switches in plant communities

    Adv. Ecol. Res.

    (1992)
  • Y. Ye et al.

    Effects of salinity on germination, seedling growth and physiology of three salt-secreting mangrove species

    Aquat. Bot.

    (2005)
  • T. Youssef

    Stomatal, biochemical and morphological factors limiting photosynthetic gas exchange in the mangrove associate Hibiscus tiliaceus under saline and environment

    Aquat. Bot.

    (2007)
  • T. Youssef et al.

    Mangrove zonation in Mobbs Bay, Australia

    Estuarine Coastal Shelf Sci.

    (1999)
  • L. Achenbach et al.

    Differences in salinity tolerance of genetically distinct Phragmites australis clones

    AoB Plants

    (2013)
  • A.D.Q. Agnew et al.

    A vegetation switch as the cause of a forest/mire ecotone in New Zealand

    J. Veg. Sci.

    (1993)
  • M.C. Ball

    Patterns of secondary succession in a mangrove forest of Southern Florida

    Oecologia

    (1980)
  • M.C. Ball

    Salinity tolerance in the mangroves Aegiceras corniculatum and Avicennia marina. 1. Water-use in relation to growth, carbon partitioning, and salt balance

    Aust. J. Plant Physiol.

    (1988)
  • M.C. Ball et al.

    Growth and water use of the mangroves Rhizophora apiculata and R-stylosa in response to salinity and humidity under ambient and elevated concentrations of atmospheric CO2

    Plant Cell Environ.

    (1997)
  • M.C. Ball et al.

    Plant responses to salinity under elevated atmospheric concentrations of CO2

    Aust. J. Bot.

    (1992)
  • P. Becker et al.

    Sap flow rates of mangrove trees are not unusually low

    Trees-Struct. Funct.

    (1997)
  • B.E. Beisner et al.

    Alternative stable states in ecology

    Front. Ecol. Environ.

    (2003)
  • M.D. Bertness et al.

    Determinants of pattern in a New England salt marsh plant community

    Ecol. Monogr.

    (1987)
  • R.G. Chen et al.

    A gap dynamic model of mangrove forest development along gradients of soil salinity and nutrient resources

    J. Ecol.

    (1998)
  • D. Childers et al.

    Responses of sawgrass and spikerush to variation in hydrologic drivers and salinity in Southern Everglades marshes

    Hydrobiologia

    (2006)
  • B. Clough

    Primary productivity and growth of mangrove forests

    Coastal Estuarine Stud.

    (1992)
  • B.F. Clough et al.

    Changes in gas-exchange characteristics and water-use efficiency of mangroves in response to salinity and vapor–pressure deficit

    Oecologia

    (1989)
  • N.C. Coops et al.

    Assessing changes in forest fragmentation following infestation using time series Landsat imagery

    Forest Ecol. Manag.

    (2010)
  • B. Cui et al.

    Determinants of annual–perennial plant zonation across a salt–fresh marsh interface: a multistage assessment

    Oecologia

    (2011)
  • P. D’Odorico et al.

    Vegetation–microclimate feedbacks in woodland–grassland ecotones

    Global Ecol. Biogeogr.

    (2013)
  • S.M. Davis et al.

    A conceptual model of ecological interactions in the mangrove estuaries of the Florida Everglades

    Wetlands

    (2005)
  • D. Di Nitto et al.

    Mangroves facing climate change: landward migration potential in response to projected scenarios of sea level rise

    Biogeosciences

    (2014)
  • T.W. Doyle et al.

    Modeling mangrove forest migration along the southwest coast of Florida under climate change

  • S. Ewe et al.

    Growth and gas exchange responses of Brazilian pepper (Schinus terebinthifolius) and native South Florida species to salinity

    Trees-Struct. Funct.

    (2005)
  • S.M.L. Ewe et al.

    Seasonal plant water uptake patterns in the saline southeast Everglades ecotone

    Oecologia

    (2007)
  • T. Fass et al.

    Development of saline ground water through transpiration of sea water

    Ground Water

    (2007)
  • S. Fedorov

    GetData Graph Digitizer Version 2.26

    (2013)
  • C. Giri et al.

    Status and distribution of mangrove forests of the world using earth observation satellite data

    Global Ecol. Biogeogr.

    (2011)
  • J.B. Grace et al.

    Habitat partitioning and competitive displacement in Cattails (Typha): experimental field studies

    Am. Nat.

    (1981)
  • P.M. Hasegawa et al.

    Plant cellular and molecular responses to high salinity

    Annu. Rev. Plant Biol.

    (2000)
  • C.S. Holling

    Resilience and stability of ecological systems

    Annu. Rev. Ecol. Syst.

    (1973)
  • J. Jiang et al.

    Spatial pattern formation of coastal vegetation in response to external gradients and positive feedbacks affecting soil porewater salinity: a model study

    Landscape Ecol.

    (2012)
  • J. Jiang et al.

    Analysis and simulation of propagule dispersal and salinity intrusion from storm surge on the movement of a marsh–mangrove ecotone in South Florida

    Estuaries Coasts

    (2014)
  • J. Jiang et al.

    Towards a theory of ecotone resilience: coastal vegetation on a salinity gradient

    Theor. Popul. Biol.

    (2012)
  • Cited by (0)

    View full text