Critically Endangered Littoral Rainforests

Littoral rainforests are rainforests that grow beside the sea, usually on sand ridges or coral cays. In Australia, rainforests that grow on headlands have also been included under the littoral rainforest category for the purposes of environmental regulation.  Over the years, much native vegetation has been cleared from picturesque coastal locations and littoral rainforests on the east coast of Australia are now classified as critically endangered under the Commonwealth Environment Protection and Biodiversity Conservation Act and are strongly protected. 

Wangetti Beach littoral rainforest

Between Cairns and Port Douglas are some of the largest remaining areas of littoral rainforest and in this post we will explore these forests.  The most noticeable thing about these forests is that they are very green places full of ferns and lilies.  In fact some of the littoral rainforests near Cairns look more like temperate oak forests than Australian forests.  When I was watching the movie Christmas Dragon which is set in medieval times somewhere in Europe (actually Provo, Utah), the look and feel of some of the forests was almost identical to the forest of Wangetti Beach shown in this post.  Botanists have noticed these strange similarities and have studied them to reveal some of the secrets of forest ecology.  Plant physiognomy is the study of this phenomenon.

Forest scene from Christmas Dragon

Forest scene from Wangetti Beach with scrub fowl nest in distance

In most littoral rainforests, tall trees are the exception and the rainforests composed of low twisted trees or even dense thickets.  Beach scrub is an old name for this type of vegetation.  Sand has little water holding capacity so during droughts there is almost no soil moisture left for the trees.  Only trees that can survive for months with very little water do well and even they can suffer greatly.  During severe dry seasons many plants will have canopies filled with leaves that have died but which remained attached.  In a nutrient poor environment trees try to hold on to their leaves and during droughts leaves often die slowly from tip to base.  Littoral rainforests are home to dry rainforest species, many of which can also be found in dry rocky areas such as on the sides of rocky headlands.

The largest tree in the forest was this Mimusops elengi (Spanish cherry, Red Coondoo)

Base of this large mimusops tree is almost a metre thick

Littoral rainforest trees can be very slow growing.  At East Point in Mackay, littoral rainforest trees were scattered across a wide sandy plain.  Using the earliest aerial photo I have ever seen (1949) and an aerial photo from 50 years later, I discovered that almost all the trees present in 1949 were still present and that no new trees had become established.  Most of these trees were only 2 or 3 m high and were wind pruned and spreading.  In a garden, the same type of tree (Cupaniopsis anacardioides) would need less than 10 years to grow to this size.  

Littoral rainforests can be quick to colonise new beach ridges that form near river mouths, such as at Yorkeys Knob.  However the species present in the newly vegetated area are different and tend to be softer, faster growing species such as beach hibiscus.  Areas that fail to be colonised by rainforest trees tend to remain treeless forever.  Most littoral rainforests outside the wettest parts of Queensland have natural open spaces within.  When there is not enough rainfall or fertility to support continuous rainforest, the rainforest organises itself into patches which accumulate resources and open areas which provide resources, mainly groundwater. 

A patch of new forest beside a desolate space on the foredune

The bare patches can persist even in old forest - this space has moss on the ground

Fruit eating birds make up most of the wildlife in littoral rainforests as the forest patches are too small and spread out to support rainforest specialists.  Many migratory species fly along the coast and spend time in the rainforests including fly-catchers and cuckoos.  Some birds of rainforest edges such as black butcherbirds and scrub fowl live permanently in the littoral rainforests but can move between patches.  Orange-footed scrub fowl (Megapodius reinwardt) make enormous mound nests on the forest floor that can be more than 3 m high. 

3 m high nest mound of an orange-footed scrub fowl

In wind exposed areas, the forest canopy is often wind pruned into a wedge of vegetation that points toward the sea.  The wind also cuts paths or tunnels through the littoral rainforest.  Salt spray is as much of a factor in wind pruning as is fast moving air.

A wedge of littoral rainforest allow the forest to creep into the wind

A herb flat has fingers extending the forest on the hind dune due to wind tunneling

If the wind gets into the forest it can clears out foliage between the canopy and the ground

Threatening processes is the name applied to events that destroy or degrade ecosystems.  For littoral rainforests, the main threatening process was local councils clearing vegetation to provide recreation areas.  Forty years ago it was common to clear tracks through coastal vegetation by bulldozer.  Vegetation has failed to reclaim these disturbed areas even without any traffic or wind tunneling, so littoral rainforests basically do not recover from disturbance. Perhaps man made gaps persist indefinitely for the same reason that natural gaps persist.

However now that clearing is banned, weeds are becoming the main issue.  Unfortunately one of the most destructive plants is the iconic coconut palm.  The palms interfere with the wind flows that shape littoral rainforests.  Beneath coconut palms the soil is so full of palm roots that digging a hole with a spade is very hard and the palms consume all the soil resources.  Coconuts also acts like umbrellas preventing rain from falling on and washing the foliage of samplings and shrubs below so their foliage is burned by salt from salt spray carried in by the wind from breaking waves.  Finally, plants below coconut palms are subject to falling fronds.  Within ten to twenty years of the arrival of coconut palms, the littoral rainforest can be replaced with a messy thicket of coconuts.  Captain Cook, never found coconut palms in Australia and after his stay in Tahiti, he knew what they were and considered them worth stopping for to add to the menu.  Coconuts are an introduced plant that has multiplied during the last twenty years to become a serious pest of littoral rainforests.  

Understorey plants being buried by coconut trash

Beach Recovery at Yorkeys Knob

In the 1950's the sea was threatening to eat the houses on esplanade at Yorkeys Knob.  The original cause of this crisis took place in 1927, when the Barron River had carved a new channel through the cane fields to connect Thomatis Creek which flowed to the Barron with Richters Creek that went to the sea.  This new route was seven kilometres shorter than the old course of the Barron River.  Some years later, the mouth of the Barron also moved north by nearly 2 km after breaking through the beach during a flood.  Rivers pump sand out into the sea, yet only when the near shore waters near river mouths become almost choked with sand, do waves to return some of this sand to the beach.  It has taken nearly eighty years for the flow of sand back to the beach to be restored and beach to grow to its maximum width.

Changes in Yorkeys Knob Beach 1952-2015 (click here to enlarge)

Sand mining from the bed of the Barron River continued until the 1990's and intercepted both the 20 000 tonnes of sand coming down the river each year and consumed an additional 70 000 tonnes per annum of previously deposited bed load.  This consumption of the sand supply had a terrible effect on the beaches particularly Machans Beach and Holloways Beach, which lie between the old mouth of the Barron River and the new mouth at Richters Creek.  The fight to protect Holloways Beach and Machans Beach are covered in previous posts (see Coastal Protection in the subject index).  In contrast, Yorkeys Knob Beach is positioned to receive the high volume of sand that takes the shortcut to the new mouth and as soon as sand mining stopped began to grow quickly.

The growing width of Yorkeys Knob Beach provides more than a happy story of how a beach was saved, it is a chance to answer a swag of questions about how coastal features form as they have literally been forming in front our photographic eyes.  These questions are important to land management as well as being scientifically interesting.  Developers like to claim that dense coastal vegetation is just regrowth and that they should be able to remove some of it to create views.  Dense vegetation developing where previously there was open sea could affect the public by blocking cooling sea breezes and possibly by allowing increased mosquito activity.  Conversely, a wider beach with dense vegetation provides a much better buffer against storms.  Scientifically interesting aspects include the development of landscape features such as chenier ridges form and how and why some areas become impoverished grassland yet metres away a ferny rainforest is created.  Topics of scientific interest will be covered in future posts.

In overview, it appears that between 2002 and 2008, the beach got wider by approximately 5 metres  per year.  From 2008 to 2016, the position of the beach has been stable as sand is now able to escape around the rocks at the northern end of the beach.  Recently the beach has been getting higher rather than wider, with the foredune growing about one metre in height.  As the beach grew, a series of small ridges and swales were created that are now stabilised by dense grass and herb cover.  In places trees colonised, mostly in lines that were probably created by high tides washing seeds up onto the foredune.  Many of these seedlings have grown to become trees that are approaching maturity.  In places the dense wall of regenerating trees is lifting off the ground and an open understorey is developing where is breezy and open but shady.

2010 (left) and similar 2016 views (right).  The foredune has become much higher and has continuous vegetation

Another important observation is the resurgence of native plants.  In my first intensive survey of this area, much of the vegetation on the low dunes was composed of introduced species.  There was para grass, guinea grass, Tridax daisy, Hyptis, Singapore daisy, Chinese violets and Mossman River grass, which has nasty burrs that penetrate our skin.  Most of theses exotics have waned and been replaced by a vigorous sward of native grasses and herbs.  Healthy natural vegetation looks better than weed infested areas.  Native plants seem to fit together, each providing a different visual texture and each occupying a defined area.  In contrast exotic species tend to run rampant and form smothering tangled masses which 'lack natural design'.  The exotics are still present but have only a minor presence.  The photos below show how they were.

Top: Tridax daisy and Mossman River Grass (the nasty one)| Bottom: Singapore daisy and Hyptis

For me, seeing these changes is not a matter of good memory.  Since approximately 2004, I have been photographing a very wide range of subjects using geotagged photos.  I am possibly one of the leading practitioners in the world when it comes to using photos to track ecological change, yet after more than fifteen years of development, I am still working hard to make a system that makes it easy monitor the environment with photos.  If anybody thinks that they can do time series research without having developed or acquired powerful tools for this purpose, they will have a very hard time matching photos in future.  In another post I will describe how to use photos for monitoring.  Most of the hard work is done by a database application that I have been developing for many years.  The information in this post comes mainly from my personal photo collection, aerial imagery from Google Earth (thanks Google) and some really old aerial imagery that I have scanned in.  To make the beach fit better on the page. the aerial photography has been rotated.

Coverage of geotagged photos - you can never have enough

Did Mangroves Evolve from Beach Vegetation?

Mangroves are not the only trees with floating seeds that are dispersed by sea.  Plants growing in tropical sandy foredunes also have floating seeds and fruit.  The most famous of these plants is the coconut.  Despite growing side by side with mangroves, there seems to be a strong ecological barrier that prevents plants from the foredune from evolving into mangroves.  Others have noted this barrier but there is only vague speculation on what the nature of this barrier is.  In this post, I will investigate the barrier between mangroves and beach vegetation.

Strand vegetation near Cairns with taller dark green littoral rainforest behind

I refer to vegetation dominated by plants with seeds that are dispersed by sea as strand vegetation.  Wikipedia defines strand vegetation as ‘a plant community of flowering plants that form along the shore in loose sand just above the high tide line.’  It goes on to describe the key features of this habitat.  “Many plants that grow in this area are endemic to the strand. The community has low species diversity because so few plants can tolerate the harsh conditions of high winds, battering salt spray, and extreme high temperatures in the summer. Plants must also be adapted to sandy saline soils, with extremely low nutrient loads, and low water holding capacity.”  Although this definition was crafted for North American strands, the term strand vegetation is also used in the same way from Africa to Asia.  In Australia, the term foredune vegetation is often used, however I tend to associate this term with the high energy surf beaches and not the relatively calm tropical coasts.  There are several distinct vegetation communities that occur beside the high tide line, so vegetation community names should be used with care.  For example, strand forests often occur beside littoral rainforests but littoral rainforest trees are almost all bird dispersed and littoral rainforests create and grow on soils rich in humus and depend on efficient nutrient recycling.

Nature provided a natural experiment which helped me to see the barrier between mangroves and beach vegetation.  Some recent weather events effectively raised the normal high tide level by twenty or thirty centimetres and the strand vegetation was subject inundation with sea water.   Cyclone Dylan came to visit in January 2014, however Cairns being on the northern side of the system had offshore winds and the sea was quite calm save for a low swell.  The low atmospheric pressure associated with cyclones can lift the surface of the sea by approximately one metre.   In the Cairns area, the cumulative effect of the lower air pressure and high spring tides was to raise the sea by approximately 30 cm above normal (~3.6-3.8 m AHD) and the lower parts of the foredune were inundated.

High sea levels associated with Cyclone Dylan and swells gently swashing over the foredune

Some low lying coastal parks and some freshwater swamps were also inundated.  In the parks, fig trees dropped their leaves from the shock of salt water.  Freshwater swamps usually occur in basins such as old billabongs and swales.  As the seawater has trouble draining back out of the basins, many of the freshwater swamps were badly or permanently damaged with large paperbark trees and groundcover vegetation being killed.  Many of these swamps will change into mangrove swamps with time, especially as erosion of the barrier between the swamp and sea is often reduced by these events.

A park tree (Ficus lyrata) dropped its leaves due to a short period of saltwater inundation.

Seawater gets into some freshwater swamps but can't get out and kills all the vegetation.

A freshwater swamp turning into a mangrove swamp, Redden Island.

It turns out that even though strand plants live by the sea, most of them are also sensitive to saltwater inundation.  Many species drop their leaves.  In mangroves, salt is accumulated in old leaves before they are dropped.  I am not sure that this is the case with other species.

A beach almond dropping leaves after saltwater inundation.

Beach creepers were complete defoliated.

Cyclones are not the only reason plants suddenly find themselves exposed to seawater.  Sometimes erosion removes the land from around the trees.  Other times, plants become established on low lying ground and grow for months before encountering very high tides and rough seas from trade winds that put swash across the beach.  

Beach Cabbage (Scaevola taccada) with signs of salinity stress

Crinum lily damaged by exposure to seawater

Severe wilting of Cordia subcordata seeding on a sand bar in a mangrove swamp.

However not all strand trees are impacted as severely by salinity.  Some species hardly seem to notice.  Beach casuarinas are a bit special.  They drip concentrated saltwater from their leaves and are perhaps the only sandy soil specialist to easily cope with high loads of salt.  Most other species which cope with seawater inundation can also survive on riverbanks with dense silty soils and clays so I think there is a link between soil type preference and reaction to salinity.  Beach hibiscus (H. tiliaceus) , Portia trees (Thespesia populena) and Pongamia (Milletia pinnata) keep on growing until the sea physically washes them away. In one small section of swamp, I counted more than twenty living Portia trees suspended by mangrove roots.  They can almost teach mangroves tricks about living in salt water.

Beach casuarina growing at approximately neap high tide level.

A Pongamia tree survives where other trees have perished.

A leaning Portia tree perched in the mangroves well below spring high tide level.

Portia trees continue to grow and fruit despite exposure to the sea. 

In summary, salinity has dire effects on most vegetation.  However some species seem to cope easily with high levels of salinity and have seeds dispersed by sea, yet are not mangroves.  My conclusion is that most species that are specialised for growing in clean sand are likely to have adaptations that seem to make them susceptible to salinity. Mangroves are unlikely to have evolved from strand species.  A few of the species present in strand vegetation also grow around the terrestrial margins of salt flats or adjacent to the landward zones of mangrove swamps where soils are heavy silts and clays.  These species handle seawater inundation better.  I suspect that mangroves evolved in river deltas, rather that from beach species that crossed over the high tide line.

A Clean New Land

It is a surreal experience to go to a place that you know well and to find that it is entirely different.  Ellie Point has never been attractive.  It is a stark, shadeless place with an incredible sandfly population and appears to be the final resting place of any rubbish that the good folks of nearby Cairns throw into the sea.  It it a place that I wished could be better and maybe I have gotten my wish.  I found my self standing on clean new land where only three years ago, I was motoring my boat.

A new beach facing the City of Cairns

Ellie Point with a new beach - it would be more than 400 m long and would extend 100 m into the sea

In my life time, the coast line has only ever seemed to retreat, but now I am beginning to see this trend reversed.  The long period of beach erosion was caused in part by human appetite for sand.  For many years, sand was dredged from the nearby Barron River as it was transported toward the sea.  More than 20 000 tonnes per year were taken.  As a result, the Cairns northern beaches were starved of sand supply and began to erode.  Seawalls are still being built to protect houses from disappearing into the sea where the beaches have been all but lost.  Even as I write, I can hear machinery crunching on massive rocks to build a new seawall for Machans Beach on the northern side of the Barron River.  Sand was also mined from the sand flats at Ellie Point.  This was unfortunate as it delayed the development of sand flats around the recently moved Barron River mouth.  The new mouth emptied into deeper water and sand was carried by the flow of the river into deeper water where waves could not bring it back to the beach.  For these reasons sand extraction from rivers was banned (~1990) and sand mining at Ellie Point ceased.  Sand mining is now only allowed in so called in-active sand deposits which are old beach ridges or alluvial deposits which are located mainly under sugarcane fields.

View from the new beach back to the previous shoreline across a shallow sandy basin

Geology maps tell me that the oldest beach ridges are about 5 km inland and that they are about 5000 years old.  This suggests that the coastline should be prograding at an average rate of about 1 m per year.  In recent years, the beach at Yorkeys Knob has been prograding even faster as it receives sand from both the Barron River and Richters Creek.  Only this year have there been clear suggestions that beaches are again growing within the delta.  Shallow waters over sandflats now extend out hundreds of metres from the beach and waves can been seen refracting around the higher sandbars, resulting in sand being moved toward the beach.  However within the delta, prevailing winds and waves can also strip sand from one area and deposit it in another so prograding beaches are often matched with nearby areas of coastal erosion.

Just to the north the coastline is regressing and trees are falling into the sea.

There is another reason why sand is not mined at Ellie Point.  It contains acid sulfate materials.  Behind the new beach is a backwater that has filled with mangrove detritus.  When this material is buried, the organic matter feeds bacteria which combine sulfates from seawater with iron from seawater or from the mangrove detritus to create iron sulfide.  On exposure to air, the sulfides oxidise and become sulfuric acid which can eat concrete and steel and burn living organisms.  Whilst the process of acid sulfate soil formation is well known, as far as I can tell, it has never been visually documented.  Hopefully, I can fill this gap.  Another post describes the acid and hydrogen sulfide being produced by organic matter that was buried in the old Barron River mouth 40 years ago.

In the basin behind the new beach is a vast deposit of mangrove detritus

Where it has been buried by sand, black rivulets issue from the ground