Ecosystems (2007) 10: 648–660 DOI: 10.1007/s10021-007-9050-y
Competition between Hardwood Hammocks and Mangroves Leonel da Silveira Lobo Sternberg,1,* Su Yean Teh,2 Sharon M. L. Ewe,3 Fernando Miralles-Wilhelm,4 and Donald L. DeAngelis1,5 1
Department of Biology, University of Miami, Coral Gables, Florida 33124, USA; 2Universiti Sains Malaysia, Penang, Malaysia; Southeast Environmental Research Center, Florida International University, Miami, Florida 33199, USA; 4Department of Civil and Environmental Engineering, Florida International University, Miami, Florida 33174, USA; 5Florida Integrated Science Center, U.S. Geological Survey, Florida, USA
3
ABSTRACT The boundaries between mangroves and freshwater hammocks in coastal ecotones of South Florida are sharp. Further, previous studies indicate that there is a discontinuity in plant predawn water potentials, with woody plants either showing predawn water potentials reflecting exposure to saline water or exposure to freshwater. This abrupt concurrent change in community type and plant water status suggests that there might be feedback dynamics between vegetation and salinity. A model examining the salinity of the aerated zone of soil overlying a saline body of water, known as the vadose layer, as a function of precipitation, evaporation and plant water uptake is presented here. The model predicts that mixtures of saline and freshwater vegetative
species represent unstable states. Depending on the initial vegetation composition, subsequent vegetative change will lead either to patches of mangrove coverage having a high salinity vadose zone or to freshwater hammock coverage having a low salinity vadose zone. Complete or nearly complete coverage by either freshwater or saltwater vegetation represents two stable steady-state points. This model can explain many of the previous observations of vegetation patterns in coastal South Florida as well as observations on the dynamics of vegetation shifts caused by sea level rise and climate change.
INTRODUCTION
systems characterized as a stable steady-state. Our understanding of these types of ecosystem changes is in part due to a body of evidence indicating that terrestrial vegetation does not always react passively to climate. Ecosystems can have a complex interaction with climate, where climate–vegetation feedback loops can influence vegetation structure and function (Charney 1975; Brovkin and others 1998; Rodriguez-Iturbe and others 1999; Sternberg 2001; Oyama and Nobre 2003). In addition to a regional scale of feedback between vegetation and climate, small-scale microsite effects can be important in this type of dynamic as well (Scheffer and others 2005). Examples of unstable species or functional group assemblages having the potential of changing to
Key words: vadose layer; sea level rise; mangroves; hammocks; steady-state; fragmentation.
Recent analysis of ecosystem dynamics has shown the possibility of a sudden, catastrophic change from one ecosystem type to another (Scheffer and others 2001). Certain species or functional group assemblages can be considered as stable states, whereas other assemblages, even those characterized as steady-states, are easily perturbed from their stable state. Slight perturbations from an apparent stable state will lead to changes towards one of the eco-
Received 26 March 2005; accepted 11 April 2007; published online 22 May 2007. *Corresponding author; e-mail:
[email protected]
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Competition between Hardwood Hammocks and Mangroves stable assemblages have been observed in lake ecosystems (Scheffer and others 1997; Carpenter and others 1999), a tropical forest/savanna ecosystem (Sternberg 2001; Oyama and Nobre 2003), a desert/ scrub ecosystem (Charney 1975; Brovkin and others 1998) and several other ecosystem types (see references in Scheffer and others 2001). Southern Florida coastal tropical hardwood hammocks and mangrove ecosystems may be yet another example of stable state ecosystems, whereas an ecosystem characterized by a mixture of components from mangrove and coastal hardwood hammocks is rarely seen and may represent an unstable state. Tropical hardwood hammocks occupy slightly elevated and rarely tidally inundated sites along coastal Florida (Snyder and others 1990). They are closed broad-leaved forests containing several species of evergreen and semi-evergreen tropical tree species intolerant of salinity. Mangroves in South Florida are found in low-wave impact habitats and are dominated by three species of salt tolerant trees: Rhizophora mangle, Avicennia germinans, and Laguncularia racemosa (Pool and others 1977; Odum and McIvor 1990). Mangrove ecosystems can be as productive as tropical forests (for a tabular summary on the productivity of these two types of forests see Clark and others 2001, Sherman and others 2003). All three mangrove species have a high range of salinity tolerance and the ability to grow in freshwater (Sternberg and Swart 1987; Odum and Mclvor 1990). Three major mechanisms are utilized by mangroves to adapt to the high salinities encountered in the rhizosphere—ultrafiltration, salt secretion and salt accumulation (Tomlinson 1994). These mechanisms are not mutually exclusive and all mangrove species exhibit all three of these adaptations to some extent. L. racemosa, for example, can exclude salts, secrete it from younger leaves and accumulate it in older leaves. We propose that the dynamics between these two ecosystem types are guided by the previously mentioned interaction between vegetation and microsites based on at least three observations. First, the boundaries between hammocks and mangroves are sharp (Snyder and others 1990) and mixtures of hammock and mangrove species cohabitating have rarely been observed. Segregation of plant communities in the Florida Keys seems to be primarily related to small differences in elevation above sea level, but several other factors, including salinity, seem to interact with elevation and determine the presence of hammocks or mangroves (Ross and others 1992). Second, there does not seem to be a gradual change in plant water status as determined by measurements of predawn
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water potential, an indicator of the osmotic environment present in the rhizosphere, in a transect from mangroves to hardwood hammocks (Sternberg and others 1991). Third, long-term changes in coastal ecosystems in Florida seem to be triggered by a combination of drought and storms (Williams and others 2003) suggesting that systems stay in the vicinity of one attractor until a large enough disturbance drives them to another. A model of the interaction between mangroves and coastal hardwood hammocks is presented here. Pivotal to this model is the observation that moderate levels of salinity have two effects on coastal hammock glycophytes: a long-term toxicity effect, which can lead to death, and a short-term osmotic effect which decreases transpiration (Kozlowski 1997; Munns 2002). Such effects are observed in mangroves only at very high salinities. There may be other types of coastal freshwater and saline ecosystems showing the same pattern, which would make them good candidates for the abovedescribed vegetation–microsite interaction. For example, an abrupt change from coastal salt marsh and pinelands (Pinus elliottii) ecosystems is observed in more northerly latitudes of the southeastern and Gulf Coast of the USA.
Model of Competition between Hardwood Hammock and Mangrove Vegetation Mangroves can grow in high salinity environments as well as in freshwater (Sternberg and Swart 1987; Odum and McIvor 1990). They cannot compete, however, with the freshwater vegetation in areas occupied by hammock species and are excluded from such areas. These two vegetation types obtain water from the vadose layer (an aerated zone of soil above the permanent water table), which in coastal southern Florida often has a lower salinity than ocean water and overlies a saturated zone of saline water (Figure 1). If enough water is withdrawn from the vadose layer by plant water uptake or evaporation, saline water will infiltrate by capillary action into the vadose layer and increase soil salinity. On the other hand, if precipitation exceeds evaporation plus the withdrawal of water by plant water uptake, then salinity in the vadose layer percolates towards the underlying saline water layer and salinity decreases (Swain and others 2003). High salinity that develops during Florida‘s dry season is considered to be the major determinant of vegetation distribution in Florida for this model. To better understand this process, first let us assume a particular average salinity of the vadose
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Salinity
Salinity
HAMMOCK
MANGROVE
Figure 1. Coastal hardwood hammock species transpiration and water uptake (T) as a function of salinity of the vadose layer compared with mangrove species. The vadose layer, often having lower salinity water, overlays a body of saline ocean water. P, E and I are precipitation, evaporation and infiltration, respectively.
P
E T VADOSE ZONE
I
SALINE WATER TABLE
layer during the dry season, which is not sufficiently high to decrease the complete domination of hammock species in an area. During the dry season, as freshwater hammock species continue to transpire and as they consume the water in the vadose layer, saline water will infiltrate and increase the salinity of the vadose layer. Because freshwater plant transpiration is sensitive to salinity (Munns 2002), transpiration decreases, which in turn prevents further infiltration of the underlying saline water. In this way the salinity of the vadose layer is stabilized to values that are not lethal to freshwater plants (Figure 1). Second, consider the alternate state where mangroves dominate an area. As mangroves absorb the water in the vadose layer, saline water will infiltrate as previously, but unlike the hammock species, mangroves will continue to transpire and continue to increase the salinity of the vadose layer to levels which cannot be tolerated by freshwater hammock species. If the dynamics of this system conforms to that of other ecosystems that can undergo regime switches (Scheffer and others 2001), then there exists an equilibrium state at which both vegetation types are present, where the salinity of the vadose layer is at an intermediate level of lethality for freshwater hammock species. However, that state is unstable, and an incremental increase in the area of freshwater plants with the concomitant decrease in mangrove area would have an effect of decreasing the overall transpira-
tion of the system, leading to a decrease in saline water infiltration and subsequently a decrease in salinity which would lead to a further increase in hammock area, until complete hammock coverage is reached. Conversely, a small incremental decrease in hammock area would lead to an increase in mangrove area, leading to increased transpiration, infiltration of saline water into the vadose layer and subsequently an increase in salinity, leading to a further decrease in hammock area until mangroves dominate the system. This concept can be used to attempt to explain observations at the landscape level. Think of the landscape as a grid of microsites, or spatial cells. Each cell can be occupied by either a mangrove or a hammock individual. Each cell is exposed to precipitation, evaporation, tidal deposition of saline water (depending on elevation in the landscape) and on a transpiration rate that depends on the presence of the vegetation type in the cell (Figure 1). The vadose layer of the cell is also influenced by the lateral water uptake by plants in the neighboring cells. Our hypothesis is that an initial model landscape, subjected to these abiotic factors, and starting from some initial random distribution of mangroves and hammocks across the landscape, will self-organize into a pattern similar to those observed in nature having strong aggregation into areas of either solid hammock or mangrove vegetation (vegetation
Competition between Hardwood Hammocks and Mangroves clumping) as well as abrupt changes of vegetation along gradual clines in microtopography. This selforganization has been previously reported in other systems (Klausmeier 1999; Rietkerk and others 2004). Our conceptual model considers an initial landscape having microsites that support either freshwater hammock or mangrove species occupying a proportion of this area. There is continuous feedback between salinity and vegetation type until a stable steady state is achieved. The halophytic mangroves tend to reinforce high salinity in the vadose zone, whereas the glycophytic hardwood hammock vegetation tends to limit salinity. The specific equations and assumptions for the behavior of mangrove and hardwood hammock vegetation in a single spatial grid cell, or microsite, are described in the next section.
Model Equations The salinity in a given spatial cell at a height z above the water table is determined by the difference between the precipitation, P, which brings in fresh water to the top of the vadose zone, evaporation, E, and plant uptake of water, RTOTAL. This difference is called the infiltration rate, INF; I NF ¼ E þ RTOTAL Pðmm=dayÞ;
ð1Þ
and the dynamics of salinity in the vadose zone are given by the equations
qz
dSV ¼ INF Swt dt
for I NF > 0
ð2Þ
although this seldom occurs because of high precipitation during the wet season. Conversely, negative values occur when precipitation exceeds evaporation and transpiration demands; then water percolates downward into the underlying ground water table. The formulation of the above equations assumes that the water content of the vadose zone is constant. A reasonable assumption, because the proximity of the vadose zone to the water table, allows for capillary rise of the water from the underlying water table. Evaporation is assumed to be minimal and can be neglected because both mangroves and hammocks form a dense canopy that inhibits evaporation. Moreira and others (1997) and Harwood and others (1999) observed that in forests transpiration dominates as the vapor generator compared to evaporation. RTOTAL depends on the transpiration and gross productivity of each vegetation type in the spatial cell and its four neighboring cells. The maximum possible water uptake rate by freshwater hammocks is 2.6 mm/day. This value is based on previous studies indicating that transpiration in tropical forests lies within this range (Cabral and others 1996). We use the same value of maximum transpiration for mangroves as hammocks at 0 parts per thousand (ppt) salinity because mangrove productivity and transpiration are comparable to tropical forests (Becker and others 1997; Clark and others 2001; Sherman and others 2003). Uptake of water as a function of salinity by the hardwood hammock R1(Sv) and mangrove species R2(Sv) are given by the respective empirical relations (Figure 1):
R1 ðSv Þ ¼ 2:6 qz
dSV ¼ INF SV dt
for I NF < 0
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2:73Sv 3 þ Sv
ð4aÞ
ð3Þ
where q is the porosity, and SV and Swt are the salinities of the pore water in the vadose zone and of the underlying saline water table, respectively. Positive values of infiltration occur when precipitation is less than the water demanded by evaporation and transpiration; then water from the underlying saline ground water infiltrates upward into the vadose zone. Note that when water leaves a cell through evaporation or transpiration no salt leaves the cell; therefore, the loss of salt from that cell is not included in equation (2). Because salt is not lost during net upward movement of water, SV can potentially build up to greater than Swt,
60 Sv R2 ðSv Þ ¼ 3:9 90 Sv
ð4bÞ
in which hammocks reduce their transpiration by ½ when the salinity of the pore water is 2.7 ppt and mangrove transpiration is reduced by ½ when the salinity of the pore water is 45 ppt. In a mixed stand the uptake of water is a linear combination of the above uptake rates, each weighted by the percentage of that vegetation type. The roots of plants in a given spatial cell are assumed to extend into adjacent cells and to extract water from the vadose zones of those cells. This
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results in RTOTAL being a function of uptake from roots originating in the focal cell and from roots originating in the four surrounding cells: iþ1;j RTOTAL ¼ 0.20 Ri;j 1 or 2 þ 0.20 R1 or 2 i;jþ1 þ 0.20 R þ 0.20 Ri1;j 1 or 2 1 or 2 i;j1 þ 0.20 R1 or 2 ,
ð5Þ
where R is the water uptake by either mangrove or hammock roots, or both, in the central cell (Ri,j) or the roots which originated in the adjacent cells (Ri±1,j, Ri,j±1). We performed certain simplifying assumptions in this model. Death and recruitment occur at the same rate in each established vegetation patch. There is no limitation in seeds or propagules, that is, if a patch of soil is available either mangroves or hammocks will establish in this empty patch. Further, based on empirical evidence, hammock vegetation is assumed to be able to take over a cell if the 2 year running average salinity is less than 5 ppt, whereas mangrove vegetation will take over if the running average salinity is greater than or equal to 5 ppt. Because mangrove vegetation reinforces high salinity and hammock vegetation reinforces low salinity, any particular isolated spatial cell will stay in a particular vegetation type indefinitely, barring any major external forcing that changes vadose zone salinity. However, the vegetation of the surrounding cells, through lateral water uptake, can affect the salinity of a cell. This lateral water uptake occurs because the extension of roots in mangrove and other tropical trees can occur beyond their central stem (Sternberg and others 2002, 2004). Therefore, over a whole landscape of many spatial cells, there will be a tendency towards self-organizing to eliminate individual cells of one vegetation type that are surrounded by a vegetation of the opposite type. The model was first developed with the assumption that the water table is infinite and has a constant salinity, that is, any percolation of water from the vadose zone into the water table will not affect the water table salinity (constant water table salinity, or CWTS version). In a second version of the model we increased the realism of the hydrology by adding a hydrological component in which the vadose zone overlies a water lens having a fixed gradient in height (H), in millimeters, showing a salinity gradient from 30 ppt near the sea to 10 ppt inland. The water table in turn overlies saline
ocean water having a salinity of 30 ppt (So). In this model, hereafter called the variable water table salinity (VWTS) model, the salinity of a water table cell underlying the vadose zone can be affected by: percolation of water from the vadose zone, infiltration of the underlying ocean water and movement of excess water from the landward cell directly adjacent to the cell. We used the VWTS model to examine the seasonal response of the vadose zone and the water table in relation to changes in precipitation. Salinity change of the water table after the addition of water from the vadose zone is given by:
H
dSi;j i;j i;j i;j wt ¼ a Si1;j wt Swt INF Sv Swt dt i;j for INF 0, then the underlying ocean water infiltrates upward to the water table and the change in salinity of the water table is given by:
H
dSi;j i;j i1;j i;j wt ¼ INF So Si;j S þ a S wt wt wt dt i;j for INF 0
ð7Þ
In either of the above cases the level of the water table is fixed to H by the flow of excess water towards the sea or replenishment of water from the underlying ocean water.
Model Simulations Model simulations were performed on landscapes created on a 100 · 100 grid of cells, each cell assumed to be 1 · 1 m2, by an algorithm that produces natural-looking topographies based on a transect by Ross and others (1992), with a net elevation gradient along one axis (Figure 2). For all simulations precipitation and effects of tides were prescribed on a daily basis. Means and standard deviations of daily precipitation (NOAA, National Weather Services Forecast Office, Florida, USA) and daily tidal height (NOAA, Tide and Current Historic data base, Key West Station, Florida, USA)
Competition between Hardwood Hammocks and Mangroves
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Figure 2. Typical complex topography used in the model simulation. Vertical and horizontal axes are in meters, with the vertical axis representing distance from the ocean. Isoclines represent elevation relative to sea level in millimeter.
for each month were derived from 162 and 5 years of empirical data, respectively. Daily precipitation and tidal height values were made to vary stochastically by summing the monthly average with the product of the respective monthly standard deviation and a normal random number. The random number values were truncated at zero. The effect of tides on the salinity of spatial cells was calculated as follows. On each day a single high tide was assumed. The height of the tide above the surface of each spatial cell was generated as a function of the mean and a randomly generated variation within the observed limits of tidal flux of the empirical data. The quantity of salt contained in the volume of water above the cell, assumed to have a salinity of 30 ppt, was allowed to mix homogeneously with the vadose zone below. Initially, each spatial cell was assigned randomly as either mangrove or hammock vegetation. The simulation was run for the initial distribution of vegetation on daily time steps. On each day, the flux of salinity into and out of the cell was computed, and the resultant salinity gain or loss was calculated. Salt concentration was assumed to be homogeneously distributed within the cell‘s vadose zone. In this simulation, the CWTS model in which the underlying water table was assumed to have a constant salinity and not be affected by infiltration of water from the vadose zone was used. After an initial simulation period of 2 years, the running average of salinity in each spatial cell was computed. This running average was used to determine whether the vegetation in any given cell would change. If the 2 year running average of the vadose
zone salinity (Sv) was ‡5 ppt in a cell, mangroves are either maintained in the cell or replace hammocks. On the other hand if Sv was
