Friday, 19 September 2014

When is a mangrove not a mangrove?

When it is a mangrove associate. Mangrove associates are plants that grow along or just below highest astronomical tide level. The term mangrove associates is used more widely used but is sort of a non-category that fails to respectfully identify this interesting group of plants.  Others have also struggled with this issue and was seems to be lacking is a good simple name for these species.  In particular, there is no good name for species which can grow in saline mud together with exclusive mangrove species.  Supra-littoral forest is the place where these plants occur. Let me call these species opportunistic mangroves.  The term mangrove associates covers a broad range of species, but is mixed grab bag of species as it includes species from all types of transition zone, including rocky mangrove coasts, sandy levees and beach ridges and within freshwater backswamps.  Let me define opportunistic mangroves as species that will take opportunities to grow below the high tide line on mangrove soils in places that experience fully saline conditions for at least one event each year.  The opportunities to grow in the mangrove edge are common but never abundant and opportunistic mangroves are always limited to small stands or scattered individuals.

Opportunistic mangroves are salinity tolerant trees and shrubs which can grow in the supra-littoral zone, which is defined as the zone above normal high tide level. The roots of these species are covered by seawater on a few occasions every year so they have to be halophytes (salt tolerant species).  To be exposed to raw seawater, the opportunistic mangroves have to be located in places with a dry season and with a large tidal range.  They also need to be in places where fresh groundwater seepage does not fill or flush the ground, protecting the trees from exposure to salt.  Often these places are very dry during prolonged dry conditions.  A final criterion would be that the trunk of the plant is located in a place with an unambiguous mangrove understorey, usually bare ground with crab holes or bare sand that is washed over by tides.

For comparison, some rivers which have freshwater baseflow all year round have freshwater tidal zones and river bank and freshwater swamp plants often dominate these places. These places are about as close as you can get to Pandora in the movie Avitar. There are several specialised mangroves species that live in tidal freshwaters but most of the plants live there are not referred to as mangroves as they can also be found in lowland rainforests other non-saline wetland habitats.  Which of these mangrove transition zones is an incubator for new mangrove species?

freshwater mangroves
A tidal freshwater system near Cooktown, complete with beasts that squat in the mud and want to kill you.
Opportunistic mangroves are not restricted to growing near tidal waterways. Some can even grow on elevated inland areas as paradoxically the same adaptations that work in swamps make plants tolerant to swings between seasonal drought and waterlogging. Indeed one of the main minor supra-littoral mangroves around Cairns, Acacia oraria is common on the western flank of the Great Dividing Range.  The label opportunistic mangrove describes a capacity to grow on tidally affected land and is a functional category. Weeds are now defined as plants growing where they are not wanted. Calling a species a weed is discouraged, as a given species can be a weed in one context and a desirable plant in another. The term opportunistic mangrove should be similar, it describes plants than can grow in tidally affected areas with heavy soils and not just plants that have to grow there.

The definition of opportunistic mangroves solves a issue for me as the separation between mangrove and non-mangrove vegetation can be indistinct in areas with very flat terrain and places with moderate to high rainfall. Some species are classed as mangroves, yet other species with similar appearances and very similar habitat preferences are not. The concept of opportunistic mangrove lets me sidestep this issue.

In the Barron River Delta, near Cairns, some hectare-sized areas with opportunistic mangroves can be found on flat expanses just below highest astronomical tide level. These places have vegetation that is intermediate between mangroves and littoral rainforest. The largest supra-littoral areas are only a few tens of metres wide but may more than one hundred of metres long as they follow the tide line.  Unlike the landward zone of the mangrove swamp which has larger than normal mangrove trees, the trees in the supra-littoral mangrove area are usually short and shrubby.  Some supra-littoral areas are isolated low rises located deep in the mangroves which have no terrestrial habitat at all.  Only seasonal rainfall stops these areas from being salt pans. 

Saline littoral rainforest
A mangrove with a light trunk surrounded by poor littoral rainforest. 
Mimusops elengi, Acacia oraria
A stilt mangrove growing around a rainforest tree and an acacia (trunk at the back).
In my naval gazing about how mangroves evolved, these places seem to be one of the potential sites of crossover from a terrestrial to intertidal existence. Last week, I examined beach ridges and found that most of the species on beach ridges shrivel when exposed to seawater. Opportunistic mangroves seem to have much greater salt tolerance. Indeed the few millimetres of sea level rise that is now occurring has already greatly increased the exposure of many of these species to regular tides. Even without sea level rise, the land surface in these places is falling fast. As leaf-litter is swept away by tides and eaten by crabs, the ground is unprotected and tropical deluges spatter the surface and carry away the loosened material. Tonnes of material are lost every year, particularly in areas with overland flow. Imagine an extensive, slowly falling ground surface covered with opportunistic mangroves, would that not be an ideal environment for mangrove evolution? Each new generation of trees would encounter an environment that is slightly more marine than before. My horticultural experience tells me that the worst growing conditions of all is salty, dry and shady, with intense root competition from surrounding trees so this may not be the case. I have seen that combination around saltwater swimming pools, under stands of coconut palms and in the landward zones of mangrove swamps. The clear understorey in most of the transitional areas suggest that horticultural combination of death often applies to this zone and this may be why healthy existing opportunistic mangroves do not leave many seedlings.

Seedlings of mangroves, ixora (I. timoriense) and mock orange (Atractocarpus fitzalanii) were common in this thirsty contested zone.
Another factor may prevent trees from passing through the supra-littoral portal into the mangrove zone. One of the current theories of evolution, is known as punctuated equilibrium. This theory notes that species stay the same for long time scales and then new species rapidly evolve. A key part of this theory is the idea that small reproductively isolated populations are required, as large populations that are spread over wide areas are subject to evolutionary pressures the pull in opposing directions. A tree might be selected for seed dormancy in one part of its range and for seeds that germinate and take root as soon as they touch the ground in another. The result of these opposing pressures is that the species as a whole does not change. With some species of opportunistic mangroves, the bulk of the population lies in the terrestrial environment and this limits their ability to adapt to mangrove life. In some cases, the mangrove fringe often has small populations of plants that are very far from their terrestrial kin (Myoporum montanum, Bauhinia binatum, and Cathormion umbellatum to name a few. Perhaps genetic research will find the legendary reproductively isolated small population that is undergoing rapid change and the mangrove fringe would be one of the first places I would look.

Mangrove edge, landward zone
A paperbark and a stilt mangrove are as close as old mates, surely they experience similar conditions.
Mangrove ecotone
A tuckeroo (front), probably Brisbane's most common street tree grows in the same saline muddy environment as a giant sized white mangrove (back).

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