When I Google mangrove evolution all I find are a few pithy generic statements and no real information. I think that if all the mangroves we currently have were to disappear, other plants would race to fill the ecological vacuum. Next time you are in the mangroves, look for non-mangrove trees that are dipping their toes in saltwater. It is surprising how many there are and how many species can actually survive in the edges of mangroves swamps, where they are surrounded on all sides by mangroves and are growing in mud. The question is why are these species not completing the transition? That is a very big question so lets investigate a small part of it, how mangrove stilt roots may have developed.
The one scientific paper on mangrove evolution that I could get for free (most are locked up behind science journal paywalls) was about the biogeography of mangrove evolution and it said that almost all the mangroves present today evolved in the Tethys Sea which was sort of where Arabia and India are today. The earliest species were Acrostichum, the mangrove fern and Nypa, the mangrove palm. These species like lower salinities so that is a clue. The main mangrove species (Avicennia and Rhizophora) were around 50-70 million years ago so forests just like those lining the worlds tropical oceans today would have seen dinosaurs. Perhaps the Tethys Sea was like the sea between Australia and the island of New Guinea is today. The information from the paper tells me what evolved and where but does not tell me how the plants evolved. If I knew how mangroves evolved, then perhaps I would be able to understand the barrier that is preventing more mangroves from evolving.
Recently I have been seeing stilt roots on some species of mangrove that normally do not have them. These mangroves usually have pneumatophores which are breathing roots that protrude from the mud. If the trees have a choice of stilt roots or pneumatophores, why do they choose pneumatophores? Fortunately it is possible to collect evidence on conditions that favour stilt root development.
In a swale, which is a shallow valley between sand ridges, I found several white mangroves (Avicennia marina var. eucalyptifolia) with stilt roots instead of the usually pencil roots. This is very strange as Avicennia is the champion producer of pneumatophores. The stilt roots are not as well developed as those of stilt mangroves (Rhizophora) but are clearly stilt roots none-the-less.
|Avicennia tree with stilt roots instead of pencil roots (pneumatophores)|
The swale traps a pool of freshwater in the wet season and saltwater in the dry season. I happened across the swale on one of the few days when it was completely dry. During the wet season, the swale appears to be a window lake, which means that the freshwater table is above ground level. Several large paperbarks live or lived among the mangroves. However as sea level rises by a few millimetres each year and the drainage line to the sea expands due to tidal and storm water flows, the ingress of seawater is increasing and the paperbarks are declining.
|Mangroves growing around a paperbark tree (Melaleuca leucadendra)|
Two days later a full moon brought in one of the highest tides of the year and flooded the swale and covered the entire stilt root system of the Avicennia trees. It seems that in the centre of the swale the water is just too deep for pneumatophores. Avicennia on the margins of the swale grow thickets of pneumatophores.
|Mangrove swale with standing water at full depth|
|Stilt roots and a few pneumatophores when pool is full|
Avicennia can also grow adventitious roots (roots growing from the trunk above ground level) in more exposed situations, such as river mouths, where regular tides are experienced. In these areas, however the adventitious roots rarely become stilt roots.
|Adventitious roots on tree a creek mouth|
Another mangrove species that can also form stilts instead of pneumatophores is the black mangrove (Lumnitizera racemosa). Normally, black mangroves have very few pneumatophores unless they are in areas with freshwater seepage, then they grow strange root loops. In one special place, near Pormpuraaw in the Gulf of Carpentaria, I found a forest where black mangroves have stilt roots instead. The Gulf has only one high tide per day, whereas the east coast of Australia has two high tides each day. There is much less tidal exchange of water and the headwaters of rivers probably remain brackish longer than their east coast equivalents. Upstream of the tidal creek is a vast flat swampy plain that slowly drains into the mangroves for several months a year, creating freshwater tides for part of the year. It is these special conditions which create this strange forest.
|Lumnitzera mangroves in headwaters of a creek which has strong seasonal freshwater flows|
|Lumnitzera on stilts with super long Avicennia pneumatophores|
Brackish water swamps with still pools of water do not foster stilt roots, in these places pneumatophore development goes crazy. In a back swamp close to the black mangroves is a brackish water swamp with pools of water surrounded by mangroves and reeds. The mangrove pneumatophores are exceptionally long and dense.
|Avicennia around a brackish pool with Baumea reeds|
|The Avicennia pneumatophores are as dense as the reeds|
So my concluding conjecture is that stilts seem develop more in areas with seasonal freshwater flooding or tides. In fact this is just the type of place that the forest mangrove grows. The forest mangrove (Carallia brachiata) is the only non-mangrove member of the stilt mangrove family in Australia. Carallia has a propensity to grow masses of adventitious roots even when it is in rainforest on wet foot hills, so perhaps the stilt mangrove families propensity for this type of root structure combined with an opportunity is what lead to stilt mangroves.