storms and monsoons. Viviparous seeds may possess adequate food reserves due to their extended development
while still on the parent tree and are capable of relatively
rapid establishment, but there is a trade-off in that such
comparatively large seedlings attract a number of predators – to the extent that seed predation can play an important role in determining recruitment success and species
composition and community structure of forests.
Forest structure and dynamics
Patterns of recruitment and of the forest structure that
eventually develops are the end result of complex interactions among propagule survivability, environmental
factors, including climate, and phenology (Feller et al.,
2010). The interplay between extreme trait plasticity and
specialized adaptations characterizes mangroves and their
environment. Mangrove traits are highly plastic in relation
to salinity, nutrient availability, and other environmental
drivers such as temperature, light, sea-level rise, and the
extent of tidal inundation (Krauss et al., 2008). Such plasticity of plant traits can result in forests of variable species
composition, age, and community structure.
The apparent zonation of one or a few species across the
intertidal seascape has long been regarded as
a conspicuous feature of mangroves (Figure 2). Zonation
is an oversimplification as any classification is subjective;
some forests conform quite well to such categorization,
while others defy any classification. Many drivers have
been suggested as the causative agent for these tidal gradients, including geomorphological controls; physiological
adaptation to physical gradients, especially salinity; tidal
sorting of propagules; differential predation on propagules; and interspecific competition (Smith, 1992; Crase
et al., 2013). Some or many of these factors can drive
intertidal zonation and can vary within a coastal region
and even within an estuary. Environmental gradients and
species responses are thus very complex. No one factor
regulates zonation where it occurs. It has even been
suggested that the process is random with the first species
and successive ones present eventually determining community composition (Ellison et al., 2000).
The establishment of seedlings through to mature
stages is a complex process for forested ecosystems, with
terrestrial forests showing peak structural complexity in
middle age with a slow decline toward senescence. Disturbance and recovery in terrestrial forests play a key role in
maintaining forest diversity and community structure.
Present theory indicates that when species die, they are
replaced by fast-growing species that are poor competitors, leading over time to eventual replacement of these
pioneers by a succession of superior competitors – mostly
to monopolize light – until an equilibrium is achieved in
climax and postclimax sequences (Odum, 1981). Mangrove forests, in contrast, appear to have species and communities with more pioneer-stage than mature-stage
characteristics, including light-demanding seedlings,
competition for light, dispersal by tides rather than by
biota, long propagule dormancy and viability, dependence
on seed reserves, continuous production of numerous
propagules, early reproductive age, uniform crown shape,
prolonged flowering period, poor species richness, no or
little canopy stratification, few climbers, and few epiphytes (Smith, 1992).
Changes in forest structure and composition occur
within a milieu of natural disturbance to equilibrium or
steady-state conditions, as all ecosystems are subject to
a variety of disturbances that are a driving force in facilitating adaptive change. The timescale in which mangroves
recover from disturbance depends in part on the intensity,
duration, and scale of the disturbance. Recovery may follow classic large gap-phase dynamics whereby enhanced
recruitment rates are matched by greater mortality following gap formation as the forest returns to a closed canopy
state; the primary factor limiting recruitment in gaps is
light availability.
Trends in recovery are not stochastic, but the early
sequences of species replacement are greatly determined
by the species present at initial recovery (Souza et al.,
2007). With increasing forest age, tree densities decline
but individual trees become larger due to self-thinning;
with fewer but larger trees, aboveground biomass
increases with forest age. Long-term changes in mangrove
forest structure have rarely been examined, but a few studies suggest that mangroves are a mosaic of patches of different stand ages if there is a high frequency of gaps
(Berger et al., 2006). If gaps are absent or few, there are
still transitory variations in what are otherwise zonal or
monospecific forests; intermediate disturbances are
unlikely to culminate in a classic climax or postclimax
community (Lugo, 1980). As stated by Alongi (2008,
p. 5), “stand composition and structure in mangrove forests are the new result of a complex interplay of physiological tolerances and competitive interactions leading to
a mosaic of interrupted or arrested successional sequences
in response to physical/chemical gradients and to changes
in geomorphology.” That is, if a forest remains relatively
undisturbed for long time periods relative to individual
life spans or if a primary forest is being established,
mangroves can undergo a series of successional stages
similar to those that are undergone in terrestrial forests.
However, in most coastal regions, intertidal areas are frequently disturbed by natural (typhoons, seasonal monsoons) or anthropogenic (wood harvesting, pollution)
forces, so mangroves are often a patchwork of interrupted
successional stages, as are most ecosystems undergoing
ecological succession under stress (Odum, 1981).
Mangrove forests, partly for these reasons, have low
plant diversity, have a relatively simple architecture, and
rarely have a significant understory, but they do have
a variety of features that help make them resilient to disturbance. These characteristics include (1) a large reservoir of
belowground nutrient pools that serve to replenish nutrient
losses; (2) rapid rates of plant-microbial-soil cycling of
carbon and nutrients that facilitate retention of these elements; (3) complex and highly efficient biotic controls,
MANGROVES
397
while still on the parent tree and are capable of relatively
rapid establishment, but there is a trade-off in that such
comparatively large seedlings attract a number of predators – to the extent that seed predation can play an important role in determining recruitment success and species
composition and community structure of forests.
Forest structure and dynamics
Patterns of recruitment and of the forest structure that
eventually develops are the end result of complex interactions among propagule survivability, environmental
factors, including climate, and phenology (Feller et al.,
2010). The interplay between extreme trait plasticity and
specialized adaptations characterizes mangroves and their
environment. Mangrove traits are highly plastic in relation
to salinity, nutrient availability, and other environmental
drivers such as temperature, light, sea-level rise, and the
extent of tidal inundation (Krauss et al., 2008). Such plasticity of plant traits can result in forests of variable species
composition, age, and community structure.
The apparent zonation of one or a few species across the
intertidal seascape has long been regarded as
a conspicuous feature of mangroves (Figure 2). Zonation
is an oversimplification as any classification is subjective;
some forests conform quite well to such categorization,
while others defy any classification. Many drivers have
been suggested as the causative agent for these tidal gradients, including geomorphological controls; physiological
adaptation to physical gradients, especially salinity; tidal
sorting of propagules; differential predation on propagules; and interspecific competition (Smith, 1992; Crase
et al., 2013). Some or many of these factors can drive
intertidal zonation and can vary within a coastal region
and even within an estuary. Environmental gradients and
species responses are thus very complex. No one factor
regulates zonation where it occurs. It has even been
suggested that the process is random with the first species
and successive ones present eventually determining community composition (Ellison et al., 2000).
The establishment of seedlings through to mature
stages is a complex process for forested ecosystems, with
terrestrial forests showing peak structural complexity in
middle age with a slow decline toward senescence. Disturbance and recovery in terrestrial forests play a key role in
maintaining forest diversity and community structure.
Present theory indicates that when species die, they are
replaced by fast-growing species that are poor competitors, leading over time to eventual replacement of these
pioneers by a succession of superior competitors – mostly
to monopolize light – until an equilibrium is achieved in
climax and postclimax sequences (Odum, 1981). Mangrove forests, in contrast, appear to have species and communities with more pioneer-stage than mature-stage
characteristics, including light-demanding seedlings,
competition for light, dispersal by tides rather than by
biota, long propagule dormancy and viability, dependence
on seed reserves, continuous production of numerous
propagules, early reproductive age, uniform crown shape,
prolonged flowering period, poor species richness, no or
little canopy stratification, few climbers, and few epiphytes (Smith, 1992).
Changes in forest structure and composition occur
within a milieu of natural disturbance to equilibrium or
steady-state conditions, as all ecosystems are subject to
a variety of disturbances that are a driving force in facilitating adaptive change. The timescale in which mangroves
recover from disturbance depends in part on the intensity,
duration, and scale of the disturbance. Recovery may follow classic large gap-phase dynamics whereby enhanced
recruitment rates are matched by greater mortality following gap formation as the forest returns to a closed canopy
state; the primary factor limiting recruitment in gaps is
light availability.
Trends in recovery are not stochastic, but the early
sequences of species replacement are greatly determined
by the species present at initial recovery (Souza et al.,
2007). With increasing forest age, tree densities decline
but individual trees become larger due to self-thinning;
with fewer but larger trees, aboveground biomass
increases with forest age. Long-term changes in mangrove
forest structure have rarely been examined, but a few studies suggest that mangroves are a mosaic of patches of different stand ages if there is a high frequency of gaps
(Berger et al., 2006). If gaps are absent or few, there are
still transitory variations in what are otherwise zonal or
monospecific forests; intermediate disturbances are
unlikely to culminate in a classic climax or postclimax
community (Lugo, 1980). As stated by Alongi (2008,
p. 5), “stand composition and structure in mangrove forests are the new result of a complex interplay of physiological tolerances and competitive interactions leading to
a mosaic of interrupted or arrested successional sequences
in response to physical/chemical gradients and to changes
in geomorphology.” That is, if a forest remains relatively
undisturbed for long time periods relative to individual
life spans or if a primary forest is being established,
mangroves can undergo a series of successional stages
similar to those that are undergone in terrestrial forests.
However, in most coastal regions, intertidal areas are frequently disturbed by natural (typhoons, seasonal monsoons) or anthropogenic (wood harvesting, pollution)
forces, so mangroves are often a patchwork of interrupted
successional stages, as are most ecosystems undergoing
ecological succession under stress (Odum, 1981).
Mangrove forests, partly for these reasons, have low
plant diversity, have a relatively simple architecture, and
rarely have a significant understory, but they do have
a variety of features that help make them resilient to disturbance. These characteristics include (1) a large reservoir of
belowground nutrient pools that serve to replenish nutrient
losses; (2) rapid rates of plant-microbial-soil cycling of
carbon and nutrients that facilitate retention of these elements; (3) complex and highly efficient biotic controls,
MANGROVES
397