Saturday, 7 May 2016

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. 

Cardwell lilies and oak leaf fern
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
Ferny dry rain forest
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.

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

Megapodius reinwardt nest, Wangetti Beach
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.  

Cocos nucifera
Understorey plants being buried by coconut trash

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