Wednesday, 21 September 2016

Australia's Spurred Mangroves (Ceriops sp.)

Spurred mangroves probably account for at least one third of all mangroves in northern Australia.  They form extensive pure stands on higher and drier parts of the tidal range yet most people have probably never ever seen them as there are few places where these mangroves can be seen without entering the mangrove forest.  The name 'spurred mangroves' is from the buttress roots at the base of the trunk which foresters refer to as spurs. 

Spurred mangrove forests are highly visual - click to enlarge
In Australia, there are three species of spurred mangroves, which even experienced botanists can struggle to distinguish.  It seems crazy that nobody has written a clear guide to identifying these species based on bark and leaves so this post will provide one.  I will also provide some insights into how plants are classified which import for understanding botany.  Identifying species correctly is the first stage in understanding ecosystem ecology and that is why it is important to know one species from another.

The laws of ecology act in a way that forces very similar species apart so there should be observable differences between species.  If the species are too similar they will compete directly with each other so it pays for both species to evolve differences that reduce the level of competition.  Botanists are usually incredibly good at finding these small differences.

However finding differences is only the first part of the puzzle.  Identifying plants is based on comparing specimens of plants to official reference specimens, so even if you are very good at distinguishing species in the forest, unless you have a means of comparing the plants you have observed with reference specimens, it is hard to be certain what they are.  Photo in books usually do show the parts that are needed to make a rigorous identification clearly.  Books also tend to show only perfect specimens so identifying plants that are in a different stage of the life cycle or which have been damaged by drought or flood can be difficult.  Professional botanists have to be able to identify plants in any condition and from spent flowers as well as new flowers.  Photos on the net which have been uploaded by professional botanists often provide good detail and cover the full range of what a plant can look like.  Written descriptions of the differences between species also help point out what to look for as the key features may not be obvious.    

Below is a list of differences that I have found between the species in the Barron River delta.  Before stating that a feature is a reliable means for identification, I need to study the plant across its entire range.  As I cannot do this, this list will need to be confirmed by you for mangroves in your area. Sometimes the features that I have associated with a species also appear to occur on a few individuals of another species (on very close inspection, these features are still slightly different), so I recommend using more than one feature for identification.  Also, the environment sometimes influences which features are displayed.  Near Port Douglas, there is a Ceriops forest where the forest floor is thick with pneumatophores but only 50 km away in the Barron River delta, the same species in a similar landscape position produces very few or no pneumatophores.

Spurred mangroves belong to the genus Ceriops and are also called yellow-leaved mangroves or yellow mangroves as the most common species has yellow-green leaves.  They belong to the family Rhizophoraceae which includes stilt mangroves (Rhizophora species).  To save space, the word Ceriops is abbreviated to the letter C.  Ceriops australis used to be classified as C. tagal var. australis and C. tagal var. tagal.  Var is short for variety.  However on investigation, it was found that the varieties were reproductively isolated (can't cross pollinate to produce fertile progeny), and were therefore separate species.  As the two species were once regarded as varieties of one species, this has made a mess of distribution maps which show C. tagal occurring as far south as Brisbane.  When a species is split up, records cannot be automatically updated as each record or specimen has to be re-examined to work out which of the revised species it should to be assigned to.  It takes a long time for everyone to catch up with the current classification and often some people will use the old name and others then new.  In a similar way C. pseudodecandra used to be C. decandra, however someone showed that the mangroves in South East Asia are different from the ones in Northern Australia so the Australian species was renamed to C. pseudodecandra.

Queensland Distribution
C. australis – north of Brisbane;
C. pseudodecandra – north of Ingham;
C. tagal – north of Mackay.

Primary Identification Features
Hypocotyls and fruit are one of the most reliable means for distinguishing the species and should be used whenever there is any doubt.  A hypocotyl is the mangrove dropper which falls out of the fruit on maturity.  It is actually a large but partially developed seedling.

Ceriops fruit with very young hypcotyls.
left-C. pseudodecandra, right C. australis  below C. tagal
Mature hypocotyls are much larger and may have collars at the top (C. tagal)
Fruit (brown objects)
C. australis – a compact heart-shaped fruit under a star formed from yellow sepals;
C. pseudodecandra – a brown fruit with sepals attached part way down the fruit;
C. tagal – a long windsock-shaped fruit, with sepals ringing the flat top of the fruit.

Hypocotyl (green objects - ones shown here have just emerged and are very small)
C. australis - a smooth dropper up to approximately 10-15cm long when mature;
C. pseudodecandra - high ridges on dropper even when small.
C. tagal - almost smooth when small, but ridged when larger, approx 20-25 cm long; 

C. pseudodecandra hypocotyls point up, whereas the other species point out from the stem or down
Flowers
Sepals, which are the covers that protect the flowers before they open and which look like thick point petals, are the most obvious feature of Ceriops flowers.  The petals of all species are white when fresh.

C. australis - dense clusters of flowers are reddish brown, sepals have little taper except at the tip;
C. pseudodecandra - small clusters of greenish flowers that are held close to stem;
C. tagal - slightly less dense clusters than C. australis and flowers are slightly larger.  Sepals are reddish with green tips and taper uniformly from base to tip.

Ceriops australis flowers - magnified
Ceriops pseudodecandra flowers
Ceriops tagal flowers
C. australis (left), C. tagal (right)
In botany, colour is the least reliable feature so always look for shape and numbers of flower parts before using colour.  

In the Barron River delta, C. tagal flowered in 2016 before C. australis flowered in general and there are also hints that flowering peaks at different times of day for each species.  Differences in flower timing might help to reproductively isolate these species.  Imagine if they could cross pollinate but infertile offspring were produced.  The cost of supporting large hypocotyls is quite high in comparison to producing seeds so there would be considerable evolutionary pressure for each species to avoid cross pollination.

Secondary Identification Features
Habitat
C. australis – makes pure stands in dry, saline areas, also occur in mixed stands;
C. pseudodecandra – in the upper part of the tidal range, mainly in non-hypersaline landward zone;
C. tagal – occurs with other mangrove species both in the landward zone and in the transition zone between C. australis stands and stilt mangrove forest. 

C. australis often forms large single species stands with open understoreys
C. tagal tends to occur in mixed species swamps (with Xylocarpus and Avicennia here).
Both C. tagal (pictured) and C. australis sometimes occur as old giants
C. pseudodecandra stands are uncommon and consist of large multi-stemmed bushes
Bark – smaller trunks 
C. australis – bark on small branches of low trees is often very smooth and salmon-coloured;
C. pseudodecandra – bark is dark and finely fissured, very similar to C. tagal but darker
C. tagal – bark on small branches is finely fissured and light brown with an oak-grain pattern. 

C. australis usually has smooth pinkish bark with fine horizontal striations
C. tagal tends to have light brown bark with fine fissures that in an 'oak-grain' pattern
Sometimes C. tagal has smooth bark but lacks the horizontal striations and has vertical brush lines
C. pseudodecandra usually has grey bark rectangular cracking
Bark – medium trunks
Medium sized trees usually have numerous lenticels.  Lenticels are the small patches of flaky bark that allow the trunk to breath.

C. australis – lenticels common and without persistent flakes of bark above and below point of eruption;
C. pseudodecandra – bark develops into tessellating grey squares approximately 1 cm in size;
C. tagal -  a flange of bark persists mainly above a row of erupted lenticels, bit like an awning.

C. australis bark remains smooth on medium sized trunks but gains white lichens and moss
C.tagal is often covered with white and orange lichens
Some large C. tagal and possibly C. australis are covered with so many lenticels, that their bark looks black
Bark – larger trunks
C. australis and C.tagal both develop rough bark covered with black squares.  It is really had to tell the species of large trees by bark alone.  C. australis seems to get more clusters of coral like roots dangling from the elbows of granddaddy trees.  C. pseudodecandra does not get that big.

Old C.tagal and C. australis can develop rough bark which can be dark and irregular or covered with light blocks
C. pseudodecandra does not get large but clumps expand into rings with time
Buttresses and pneumatophores
C. australis – very rarely has pneumatophores, tops of spur roots are about as wide as a ladies finger;
C. pseudodecandra – buttress roots are relatively small and I have not observed pneumatophores;
C. tagal – usually has some pneumatophores but these are hard to identify as it often occurs with species that can produce similar pneumatophores.  

C. australis often has smooth bark on the spur roots and the spurs are narrow
C. tagal has darker bark on the spur roots which are broader when seen from above
C. tagal sometimes has lots of serpent-like breathing roots but usually has very few
C. pseudodecandra has poorly developed spurs but the bark is similar to C. tagal.
Most C. pseudodecandra clumps have dead timbers poking up in the centre
Basal bark
The bark of of the spur roots or base of trunk is the easiest way I have found for distinguishing Ceriops species.

C. australis - spur roots are usually covered with smooth bark in larger individuals and in light brown loose flaky bark in advanced seedlings/small saplings
C. pseudodecandra - bark is non-descript but shoots usually rise from tree bases of living trunks.
C. tagal - tops of spur roots are usually two fingers wide and the bark on the tops of the roots has often split to form a roof that shades the sides of the roots.  Rough dark chocolate-coloured flakes of bark often cover the trunk between the spurs and on the faces of the spurs.

The bark on top of C. tagal spurs spreads out like a roof over the spur
Shaggy flakes of chocolate brown bark often cover the faces of the spur roots
C. australis has either smooth bark on the spurs or the bark spits at the top, rather than forming a roof
If bark is flaky, it is pinkish-brown as seen above.
Saplings of C. australis (left) and C. tagal (right)
Some C. australis can be flaky around the base but the 'roof ' is not well formed and the colour is lighter
C. pseudodecandra often has twigs risking up from the bottom.
The dark material at the bottom is mud thrown up from rain spatter.
Seedlings and leaves

In this guide, I have not covered any of the similar mangroves from other genera that share habitat with Ceriops.  With all Ceriops, the new shoot is like a small flattened spear tip, so check for this feature first. 

C. australis – seedlings occur in large numbers around the margins of stands, leaves tend to be held erect in bright areas and partially erect when seedlings are in the understorey.  Only in dark area are leave horizontal.  Even small seedlings had flowers in September.  Leaf stalks are green and leaf blades taper down to the leaf stalk.

C. pseudodecandra – leaves are far narrower and longer than other species and veins are hard to see in upper surface which has a mat finish.  New leaves are dark red and leaf stalks are a very dark purple-red.  Seedlings are quite hard to find and possibly occur in only small numbers;

C. tagal – seedlings are common in shady parts of the landward zone and have large horizontal almost round (elliptical leaves) and lateral veins create a texture in the upper leaf surface that can be seen in leaves are held an angle that reflects the surface shine.  Leaf stalks are a dark rusty colour. 

C. australis shade leaves have green stalks and the base of the leaf blade is tapered
It there is any direct sunlight during the day, the leaves are raised into an erect posture
C. pseudodecandra has long leaves which are purplish when new
C. pseudodecandra leaves are on long stalks and leaves are not tightly clustered to avoid self-shading
Red new leaves of C. pseudodecandra
C. tagal seedling leaves are roundly elliptical and have brown stalks
C. pseudodecandra seedling leaf (left) and C. tagal seedling leaf (right)
Ecology

Ecology is often dominated by factors that cannot be seen so my understanding which comes from viewing Ceriops in a wide variety of landscape is preliminary only.  Crab predation on hypocotyls is an example of a hidden factor that controls the distribution of many mangrove species.

C. australis seems to be a hypersaline area and bright light specialist.  It raises its leaves into the erect position even in partial shade.  This may explain why seedlings of this species are usually clustered around the edges of salt pans and not in the understorey of the mature stands.  This is not how forests normally regenerate (gap dynamics) and is worth of more study.


C. tagal is a typical mangrove competing for prime habitat with many other species.  It has shade tolerant seedlings and is possibly competitive due to not investing heavily in pneumatophores in the way that its competitors do.  As a result, it is usually restricted to the more freely draining areas.

C. pseudodecandra usually belongs to the shady middle storey of tall mangrove forests in the landward zone.  It is a shade tolerant multi-stemmed woody shrub which resembles a larger version of the river mangrove (Aegiceras corniculatum).  I only found very few seedlings and I suspect that this species lives a very long slow life and only very occasionally reproduces successfully.  It can take hundreds of years for a clump to form a ring, yet for this species rings are common.

Final comments
For years I have been stomping around in the swamp and not correctly identifying these species and so have a number of other botanists, so the time is right to prepare a guide.  C. pseudodendra is a candidate for the ugliest mangrove is existence with its scrappy form and collection of dead trunks jutting out of the mud like punji sticks, so I probably though it was a malform example of another species and kept on going.  With C. australis and C. tagal, it was only when I found a stand of one species next to a stand of the other that I suddenly realised how different they really are.

Referenences



Reevaluating the taxonomic status of Ceriops australis (Rhizophoraceae) based on morphological and molecular evidence in Botanical Studies (2009) 50: 89-100



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