Aveneu Park, Starling, Australia

THE Quisthoudt et al. found that temperature

THE PROBLEM
IN SUNDERBAN

The
physiological response of an organism to increasing temperature follows a
sigmoid curve, in which an initial rapid rise in functional processes (e.g.,
respiration, growth rate) slows, plateaus, and then declines as a critical
lethal threshold is reached and then exceeded. Mangrove plants and animals
presumably respond so, but the critical temperatures at which functionality
plateaus and organisms begin to die are uncertain. Rates of leaf photosynthesis
for most species peak at temperatures at or below 30 °C, and leaf CO2
assimilation rates of many species decline, either sharply or gradually, as
temperature increases from 33 to 35 °C. Photosynthesis in exposed leaves is
often depressed due to photo inhibition; mid-day declines of assimilation have
been observed ensuring survival for the photochemical machinery.

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What has
been the response of mangroves in the field to the ongoing rise in temperature?
Temperature increases alone are likely to result in faster growth,
reproduction, photosynthesis, and respiration, changes in community
composition, diversity, and an expansion of latitudinal limits. Field data
indicate that mangroves are indeed currently expanding into higher latitudes in
North America, New Zealand, Australia, southern Africa, and southern China.
This global expansion poleward is most likely in response to the global rise in
sea surface temperatures.

 

As these
changes are occurring in the subtropics and tropics, mangrove expansion may
also be coupled to changes in precipitation. In an analysis of mangrove
latitudinal changes, Quisthoudt et al. found that temperature alone does not
delimit the latitudinal range of Rhizophora and Avicenna due partly to large
regional differences in monthly temperature change, for instance, warmest month
temperatures are higher at the latitudinal limits in the northern, than in the
southern, hemisphere. While mangrove expansion and salt marsh contraction are
consistent with the poleward increase in temperature and the reduction in the
frequency of extreme cold events, other variables such as changes in
precipitation cannot be ruled out as co-factors.

 

The
expansion of mangroves at the expense of salt marshes suggests that a number of
complex ecological interactions are operating during the transition. Proffitt
and Travis propose that this migration may be facilitated by increasing
propagule abundance from greater reproductive rates and greater genetic
variation caused by outcrossing. From field surveys conducted along the
Atlantic and Gulf coasts of Florida, they found that reproductive frequencies
varied significantly, but increased with latitude and more strongly along the
Gulf coast, with a concomitant increase in outcrossing. The migration of
mangroves is self-re-enforcing; more colonizers lead to more propagules and
outcrossing leads to enhanced genetic variation, thus perpetuating and
promoting adaptation to a new environment.

 

What effect
has the rise in temperature had and/or will have on mangrove-associated fauna?
No studies have yet demonstrated a change in mangrove fauna associated with
global warming, but the results from a few studies of macro- and megafauna from
adjacent habitats have implications for mangrove organisms. An experimental
study has shown that juvenile mullet (Liza vaigiensis) and crescent terapon
(Terapon jarbua) frequenting tropical seagrass beds can be acclimated to higher
water temperatures, approaching the critical limits for marine vertebrates.
Other organisms such as tropical gastropods may respond actively by seeking
cooler sites to survive when temperatures exceed 33 °C. However, tropical
organisms are closer to their upper thermal thresholds than boreal and
temperate organisms, and are thus more vulnerable to rising temperature.

 

Mangrove
responses to increasing or decreasing precipitation are more straightforward,
but such changes are likely to co-occur with rises in sea level, temperature,
and atmospheric CO2 concentration. Compared to arid-zone stands, mangrove
forests in the wet tropics have greater biomass and productivity, consist of
less dense but taller trees, and tend to inhabit finer sediment deposits, but
there are no clear species richness or diversity patterns between high and low
precipitation areas; low species richness may be attributable to high
variability in annual rainfall. But mangroves clearly thrive in wet
environments where they can likely deal less stressfully with lower salinity
and more available fresh water.

ANNALYSIS
AND PREDICTION

What then are we to predict
about the global future of mangroves in the face of climate change? There have
been a number of general and local prognostications, especially in regard to
sea level rise, but there have been few attempts at global prediction. There
has been only one sophisticated attempt to forecast mangrove distributions
under climate change. Using several mangrove databases for 30 species across 8
genera, Record et al used the BIOMOD model to make predictions of mangrove
species and community distributions under a range of sea level rise and global
climate scenarios up to the year 2080. The model runs came up with two clear
predictions: some species will continue migrating poleward but experience a decline
in available space; and Central America and the Caribbean will lose more
species than other parts of the world. The latter prediction is in agreement
with the work of Polidoro et al in which extinction risk of threatened species
was assessed and the main geographical area of concern was found to be the
Atlantic and Pacific coasts of Central America.

The recent
climatological forecasts by the Intergovernmental Panel on Climate Change
(IPCC) for until the end of this century predict that globally sea surface
temperatures will rise by 1–3 °C, oceanic pH will decline by 0.07–0.31,
and mean atmospheric CO2concentrations will increase to 441 ppm
(from 391 ppm in 2011). Regional differences (Table 1) will occur for some parameters such as sea level,
which will continue to rise globally at an average rate between 1.8 and
2.4 mm year?1; precipitation will increase and decrease in
some regions such that arid areas will become more arid and the wet tropics
will become wetter; and salinity will change in tandem with changes in
precipitation. Considering these climatic predictions and the known and likely
responses of mangroves to changes in temperature, salinity, sea level rise,
etc., I offer some predictions:

·        
Prediction 1 (red
lines): Mangrove forests along arid coasts will decline as salinities increase,
freshwater becomes most scarce, and critical temperature thresholds are reached
more frequently (e.g., NW Australia, Pakistan, Arabian Peninsula, both Mexico
coasts).

·        
Prediction 2
(orange lines): Mangrove forests will decline as sediment yield declines,
salinity increases, and sea level rises in tropical river deltas subject to
subsidence intervals (e.g., the Sundarbans; the Mekong, Zaire, Fly Rivers).

·        
Prediction 3
(purple lines): Mangrove forests will decline as sea level rises and there is
little or no upland space to colonize (e.g., low islands of Oceania, many
Caribbean islands).

·        
Prediction 4
(blue lines): Mangroves forests will continue to expand their latitudinal range
as temperature and atmospheric CO2 concentrations increase (New
Zealand, USA, Australia, and China).

 

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