Seed Dispersal

Skyrail Nature Diary: December 2012

December is the official start of summer with rain often turning up around Christmas. There should also be more fruiting during the month as the last gasps of flowering go through their motions. This could, of course, be turned completely on its head should El Nino appear over New Year. El Nino is a high pressure system that pushes the monsoon further north so it's life giving rain misses our region completely. It’s worth noting that what we consider a drought is what is considered as normal rain further south.

In November, Rose Silky Oak flowered in small numbers along Skyrail. The flowers are dark pink and are borne in the forks of the tree. They will hopefully be followed by green fruits in shortly. The fruits look like two woody domes pressed against each other and are filled with large pleated seeds that are dispersed by the wind. The leaves of Rose Silky Oak are large and leathery. While young, the leaves are strongly lobed. Growing older, the leaves lose their lobes and point upwards to avoid receiving the full brunt of the harsh sun in the canopy. These beautiful trees were once harvested for their heavy timber which was, amongst other things, ideal for doors.

Another beautiful tree flowering is the Blush Silky Oak. The flowers are borne on small spikes hanging down under the leaves. They mostly look like small strips of  paper and are easily overlooked. The fruits are zucchini shaped capsules full of papery seeds. The leaves are lobed when young and long and smooth and pointing upwards when mature. At all ages the leaves are covered with dark brown glossy hairs that stand out.

The Hard Milkwoods started flowering last month and will most likely still be flowering this month. Towards the end of December we should be seeing the first fruits. The flowers are tiny and cream and are borne in large round clusters on top of the trees. The fruits are large double needles resembling pine needles that are filled to the brim with tiny hairy seeds that are dispersed by the wind. The leaves are soft and arranged around the twigs in swirls of threes and fours. Hard Milkwoods have copious quantities of white, sticky sap that the Djabugandjii used to stun fish in local creeks at the end of the dry season. These trees grow up to 30 metres.

Last month the easily spotted vine Silky Milkpod flowered in small numbers at Skyrail. The small flowers are borne on small round clusters along the length of the vine. The fruits are sausage shaped woody capsules full of hairy “silky” seeds. The leaves are large and glossy. Like the Hard Milkwood, Sliky Milkpod has copious amounts of sticky white sap.

In most biology documentaries where plants and forest ecology are portrayed it’s usually flower pollination that receives most of the attention.  And yet pollination is absolutely useless if the resulting seeds are spread out and about. If all seeds germinate next to the mother plant research shows that the young are often weeded out with various toxins released into the immediate environment. Plants cannot move which means they need help to spread their offspring out and about. There are three basic methods of dispersal; by animals, by wind or by water.

Spreading seeds with the help of animals is by far the most common method, accounting for about 90% of all seed dispersals in the wild. There are many ways of getting animals to help disperse your seeds. The most well known is by supplying the animals with food in the form of fruit flesh. Big fruits are eaten by cassowaries and voided some distance away from the feeding place. The seeds are designed to tolerate the digestive juices of the cassowary’s gut and have the advantage of being fertilised as well as being dispersed. Smaller fruits have slight modifications to this. Mistletoes make berries that attract birds. Normally being voided would be enough but the mistletoe wants to be deposited in the tree tops. To achieve this the seeds have sticky threads that cling to the bird’s tail feathers when voided. This forces the bird to rub its rump on a branch to rid itself of the annoying seeds which is exactly what the mistletoe wants. Wattles on the other hand are dispersed mainly by ants. The seeds are too large to be swallowed so each seed has a ribbon of thin flesh attached to it. The ants find this fruit flesh irresistible and cart the wattle seeds to the nest where they are fertilised by the ants’ waste.

Seeds are interestingly not always designed to be eaten. When birds and mammals appeared on the scene several millions of years ago the plants suddenly had a much cheaper method of transport, burrs! Burrs are designed to get stuck on a bird’s or mammal’s feathers or fur when they pass by. Unintentionally the carriers take with them the freeloaders to a hopefully better place before they drop off at some stage. Common examples in Cairns that aren’t introduced weeds are the Macarangas. All of these extremely common trees have dull fruits covered in soft spines that are ideal in getting stuck onto bird feathers.

Spreading seeds with the help of wind is the second most common way of dispersing seeds, accounting for almost 10% of all cases. The seeds of these plants have various ways of soaring in the wind away from the parent plant. A very common adaptation is the presence of tiny hairs on tiny, light seeds. Examples are numerous; hoyas, rose butternuts, milky pines and silky milkpods. Alternatively the seeds can be so tiny that they literally float on the wind. The same applies to plants that have spores instead of seeds. Good examples are orchids and ferns. An unusual adaptation to wind dispersal is the use of large wings on often large seeds, giving the seed the appearance of a helicopter on its way down to the forest floor. These seeds often have spiny surfaces that are possibly meant to discourage animals eating them. Examples are red tulip oak, maples and helicopter tree.

Spreading seeds with the help of water is very rare in Australia due to the paucity of permanent water here. The ancient Idiotfruit is believed to be dispersed by Olive Creek in Daintree but this may be due to the fact that its original animal disperser is now extinct. Either way, the plant can’t spread very far using this method. There is ample evidence that the Idiotfruit was once widespread throughout the Wet tropic but was decimated by loggers due to its moderately good timber. The timber was marketed as Ribbonwood.

By Tore Lien Linde