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

Abstract

Ecotones represent locations where vegetation change is likely to occur as a result of climate and other environmental changes. Using a model of an ecotone vulnerable to such future changes, we estimated the resilience of the ecotone to disturbances. The specific ecotone is that between two different vegetation types, salinity-tolerant and salinity-intolerant, along a gradient in groundwater salinity. In the case studied, each vegetation type, through soil feedback loops, promoted local soil salinity levels that favor itself in competition with the other type. Bifurcation analysis was used to study the system of equations for the two vegetation types and soil salinity. Alternative stable equilibria, one for salinity-tolerant and one for salinity intolerant vegetation, were shown to exist over a region of the groundwater salinity gradient, bounded by two bifurcation points. This region was shown to depend sensitively on parameters such as the rate of upward infiltration of salinity from groundwater into the soil due to evaporation. We showed also that increasing diffusion rates of vegetation can lead to shrinkage of the range between the two bifurcation points. Sharp ecotones are typical of salt-tolerant vegetation (mangroves) near the coastline and salt-intolerant vegetation inland, even though the underlying elevation and groundwater salinity change very gradually. A disturbance such as an input of salinity to the soil from a storm surge could upset this stable boundary, leading to a regime shift of salinity-tolerant vegetation inland. We showed, however, that, for our model as least, a simple pulse disturbance would not be sufficient; the salinity would have to be held at a high level, as a ‘press’, for some time. The approach used here should be generalizable to study the resilience of a variety of ecotones to disturbances.

Introduction

Ecological resilience and regime shifts have been major topics in the study of ecosystem dynamics during the past two decades (Briske et al., 2010, Scheffer et al., 2001). These concepts are particularly important to the study of ecotones, or zones over which there is a rapid transition between adjacent types of vegetation. Ecotones are zones where changes in vegetation type are likely to occur when environmental conditions change. Ecotones usually occur along externally imposed environmental gradients, that is, changes in edaphic characteristics or climatic variables, such as temperature or precipitation. In some cases, these environmental gradients are strong enough that they determine a boundary that rigidly separates the vegetation types, such as that occurring at the shore of a lake. In other cases the gradients are weak enough, or the vegetation types are plastic enough in their tolerances, that there is a large potential range of overlap between the vegetation types. In such cases, the ecotone might occur as a gradual change from dominance of one vegetation type to the other. However, situations also exist where even relatively weak environmental gradients are characterized by ecotones so sharp that they almost resemble edges. These can result from what has been termed a positive feedback ‘switch’, in which each vegetation type alters the local environment through positive feedback in a way that favors itself (Lloyd et al., 2000, Wilson and Agnew, 1992), such that it excludes the other type.

When different vegetation types can each potentially occupy the same sites along some region of a gradient, and also exhibit this switch behavior, then alternative stable states, or bistability, can exist in this region. Alternative stable states and their mechanisms have been identified for some ecotones that have been studied in detail. Among these are grass–tree ecotones (e.g., Accatino et al. (2010), Boughton et al. (2006), Sternberg (2001), Vilà et al. (2001)), alpine tree lines (e.g., Bekker (2005), Malanson (1997), Nishimura and Kohyama (2002), Wiegand et al. (2006)), tropical alpine treelines (Bader et al., 2008, Martin et al., 2010) rush–mangrove ecotones (Walker et al., 2003), Sphagnum bog–vascular plant ecotones (Ehrenfeld et al., 2005, Hotes et al., 2010), forest–mire ecotones (Agnew et al., 1993), ecotones between different vegetation successional stages in a calcareous dune slack (Adema and Grootjans, 2003), sclerophyllous shrub–forest ecotones (Odion et al., 2010), and ecotones around allelopathic plants (Gentle and Duggin, 1997). These systems all involve a switch mechanism (sensu (Wilson and Agnew, 1992)) of some sort, in which vegetation types alter the environment to favor themselves and exclude other types.

Some of the mechanisms that maintain ecotones act continuously in time, but others are episodic. Fire is an episodic mechanism that favors pyrogenic vegetation such as grass over forest in such ecotones as Mediterranean Basin woodlands (Vilà et al., 2001), higher elevation pines over lower elevation cloud forest in tropical mountains (Martin et al., 2010), and sclerophyllous shrub vegetation over forest in mountains such as the Klamath Mountains of California (Odion et al., 2010). In the absence of fire, the non-pyrogenic vegetation is superior to the pyrogenic vegetation is superior (e.g., forest shades out grass), so the ecotone could move shift in favor of the former during intervals between fires, but occasional fire events will burn back the young growth of non-pyrogenic vegetation, setting back the ecotone. The forest–mire ecotone is an example of continuous mechanism of enforcement of a sharp ecotone. Tree seedlings cannot establish in the mire, but they can establish on fallen tree boles, maintaining the boundary (Agnew et al., 1993). In both of these examples, the vegetation types show this switch mechanism.

In all of the examples there are zones along the environmental gradient where the fundamental conditions are such that either of two alternative vegetation types could exist. The ecotone merely represents the current position of the sharp boundary within this area of overlap, influenced by the positive feedbacks of each vegetation type favoring itself and excluding the other. Because this overlap area is a zone of bistability, it is possible that either changing environmental conditions or disturbances could lead to regime shifts, that is, sudden, spatially extensive changes in favor of one of the vegetation types that shifts the position of the ecotone. The reason that regime shifts can be sudden and extensive stems from the positive feedbacks that maintain the ecotone between the two vegetation types. Because of these self-reinforcing positive feedbacks, the ecotone resists change until the change in the environment is great enough to overcome the feedbacks. That is termed ‘resilience’. But once the resilience of the self-reinforcing feedbacks is overcome, feedbacks operate to promote change to the alternative vegetation type. Two types of environmental change can trigger regime shift (e.g., Beisner et al., 2003, Briske et al., 2008). One is gradual change in some environmental variable that eventually reaches a threshold past which the shift occurs (e.g., Carpenter et al. (1999), Folke et al. (2004), Petersen et al. (2008), Scheffer et al. (2001)). The second type of change is a large disturbance that pushes the system beyond the threshold, such that it cannot return to the original vegetation state, but moves to the alternative state (e.g., May (1977), Stringham et al. (2003)). If the disturbance is not so large that it pushes the system outside its domain of ecological resilience, the ecosystem can return to its original state following a disturbance (Holling, 1973).

Areas on which gradients are slight may be vulnerable to regime shifts covering large areas, The Everglades in southern Florida, which is very flat in elevation, is one such place, and thus is of special concern with respect to the potential effects of climate change. Regime shifts from one stable state to an alternative stable state due to disturbance have been hypothesized to be possible in both the coastal margin and freshwater marshes of the Everglades (D’Odorico et al., 2011, Larsen and Harvey, 2010, Sternberg et al., 2007).

Because ecotones are places along which regime shifts are most likely to occur, ways of estimating the resilience of these ecotones are needed (Briske et al., 2008). This paper examines an ecotone in the coastal Everglades that has been the object of recent study (Sternberg et al., 2007, Teh et al., 2008), the case of competition between salt-tolerant (halophytic) mangroves and salt-intolerant (glycophytic) hardwood hammock or freshwater marsh vegetation types that can coexist in coastal areas, such as the southern coast of Florida. We use this as a specific case of an ecotone that is vulnerable to a regime shift. We analyze a model of this system and estimate the resilience of the system against the most likely cause of regime shift, a storm surge. Although the model is applied to the specific case of an ecotone between halophytic and glycophytic vegetation, it is generic in nature.

Empirical research shows that mangrove and hardwood hammock vegetation types are spatially separated by sharp ecotones, such that salt-tolerant mangroves line the coastal areas, and salt-intolerant species, hardwood hammocks or freshwater marsh, occupy slightly higher elevations where salinity is lower (Ross et al., 1992). The soil salinity level decreases sharply across the boundary from salt-tolerant to salt-intolerant vegetation. The differences in elevation may be so slight that it is not clear why the sharp ecotones exist precisely where they do. This led Sternberg et al. (2007) to propose that feedback effects of the two vegetation types on local soil salinity maintain the sharp ecotone, which has been studied through simulation modeling by Jiang et al. (2012). The ecohydrology of the salt-tolerant vegetation (mangroves) promotes high local soil salinity by maintaining high transpiration even when soil salinity is high, while the salt-intolerant vegetation tends to promote low levels of local soil salinity, by decreasing transpiration when soil salinity is high (Lin and Sternberg, 1992, Passioura et al., 1992, Volkmar et al., 1998). But it has also been suggested that a sufficiently large pulse of salinity, due to a storm surge, could cause a regime shift, moving the location of the ecotone inland from the coast (Teh et al., 2008).

Our objective was to build a model of an appropriate degree of complexity to not only capture the mechanisms in the mangrove–hammock ecotone, but to also allow analysis. Models built by theoretical ecologists to describe regime shifts can, for convenience, be classified into three general categories of increasing complexity: (1) systems with a single variable (e.g., a species population) with multiple equilibria (May, 1977), (2) systems with two or more variables (e.g., competing species populations) interacting through positive feedback loops (Accatino et al., 2010, Churkina and Svirezhev, 1995, Genkai-Kato and Carpenter, 2005, Gilad et al., 2007), but still analytically tractable, and (3) large network simulation models, which can only be studied numerically (Shannon et al., 2004). Systems of two competing vegetation types, each of which tends to create a local environment (e.g., abiotic conditions) favorable to itself, can often be described with models that fall into the second category, that is, they are simple enough that some mathematical analysis is possible. Such systems can be described fairly simply in terms of feedback loops between each vegetation type and its local environment. We take this approach to consider the case in which the one vegetation type can create changes in the local environmental conditions that inhibit the other vegetation type, while the other vegetation type is a better competitor in the absence of those high levels of the inhibitor. Together, these mechanisms maintain a stable spatial boundary or ecotone between the vegetation types. We hypothesize, however, that a sufficiently strong external disturbance, by influencing the inhibitor concentration over parts of the spatial domain of the competing species, might cause a regime shift involving the two vegetation types, in which one vegetation type expands in space at the expense of the other. The inhibitor in our case is salinity and storm surges are such disturbances, as they can push large volumes of sea water far inland, carrying salinity far up the usual gradient from marine to freshwater conditions (Krauss et al., 2009).

Theoretical studies have shown that spatial heterogeneity may weaken the tendency for large-scale catastrophic regime shifts in ecosystems if local environmental characteristics vary along a smooth gradient (van Nes and Scheffer, 2005). This is a situation that applies to our case of coastal vegetation, as groundwater salinity, which plays a role in soil salinity dynamics, decreases gradually as the distance inland from the coastline increases. Here, we first examine the dynamics of two competing coastal plant species, along with the inhibitor, salinity, which is explicitly considered as a variable. Second, we extend the model to the case in which there is a slight gradient in an environmental condition, specifically groundwater salinity along one dimension, in order to investigate the effect of the gradient on potential large-scale regime shifts. We calculate the resilience of the ecotone, that is, the characteristics of the disturbance needed to cause a large spatial shift in the ecotone.