Bistability of mangrove forests and competition with freshwater plants
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
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