A possible seed dispersal mechanism for Banksia integrifolia in urban areas
Okay... I gotta say this is possibly my most speculative, unhinged post to date. But here we go:
I was in the beachy suburbs of Melbourne, where I stumbled upon a Banksia integrifolia aka Coastal Banksia dropping its flowers into the gutter, as such:
In the photo above you can see the parent tree (the source of these flowers) and the nearby street drain. On the parent tree you can also make out the cones (i.e the woody fruit of the Banksia, that I will discuss in detail later).
Numerous councils in Melbourne have planted Banksia integrifolia on nature strips, particularly those councils surrounding Port Phillip Bay. It is a preferred street tree due to its hardiness, ability to survive in sandy, nutrient poor soils and provide decent shade.
The fallen plant material was likely from birds messing about on the tree and inadvertently breaking the flowers off the parent plant. This is a very common occurrence as they love drinking the nectar in the flowers. The large yellow flower you see here (ranging from 6-12cm long) isn't actually one flower, rather made up of many smaller flowers - an inflorescence, so I will use this term from now on.
What fascinated me however, was that it appears a street drain is intercepting the inflorescences of Banksia integrifolia.
This set off a series of thoughts, and raised possibilities that I had never considered before.
In theory
So this is my theory:
The urban populations of Banksia integrifolia, especially those planted as street trees on sealed roads, are able to use rainfall and the stormwater drainage system to carry (disperse) its seeds, into nearby creeks, rivers or directly out to sea. I further suggest this dispersal is mediated by individual cones carrying the seeds. At some point on this journey, the cones become beached or otherwise washed ashore, and could, under suitable conditions, germinate, giving rise to a Banksia integrifolia seedling.
It should be noted, I haven't actually tested this. I guess you could say this is a theoretical piece. Ecology is a very applied science for the most part, so theories usually have to be tested in the field (empirically validated) to carry any sort of weight/credibility. This is based on no empirical evidence.
Back to the observation
As mention prior, the woody fruits are called "cones". This is true amongst all Banksia species. To better understand, we need to see these cones in detail.
I haven't got a good image of the cones yet, so this was taken from the wikimedia page of Banksia integrifolia, credit to Geoff Derrin. Also notice how precariously that cone is attached to that twig of a branch.
So how do these cones form? The woody fruit takes 8-12 months from flowering or more specifically the full bloom, anthesis, to develop. The inflorescence start out looking fairly neat, when the individual flowers are not open, however by the end of the flowering period, the inflorescence all look pretty rugged and messy. In my street photo above you can see the inflorescences are at different stages, some look neat, others look messy.
Over the next year, the whole inflorescence will turn into a hard woody cone. Fertilised flowers may develop into a follicle (a seed pod that surrounds the seed). Not all follicles fully develop, but the ones that do turn into this clam-shell shape that house the seed within. Once the fruit is fully ripe, the follicles open up and release the seed.
I acknowledge that in my street photo above it appears only infloresences are entering the drain - and not the cones. But I do see fallen cones on the ground often. So I think it is likely for cones to also fall into an urban drainage system. The cylindrical shape also means the cones are able to roll somewhat. I don't think this is particularly relevant or central to my theory, but I just wanted to put it out there.
Banksia integrifolia is a non-bradysporous species - this is possibly the worst bit of botany jargon in existence. All this means is that it releases seed from the cone readily and spontaneously upon ripening. Unlike other Banksia species (the bradysporous species) that have long term retention of seeds in the fruit and require fire to crack open these follicles. There is also evidence to suggest the chemical compounds in smoke may further enhance seedling fitness in these Banksias. In fact to test this, they separated smoke from fire! In contradiction to the popular saying 'where there's smoke there's fire' - by treating Banksia seeds with smoke infused water - a very cool scientific method indeed.
Anyway, I am going off on a tangent. By Banksia integrifolia being non-bradysporous this avoids a potential problem that would occur in other Banksia species, where you require fire for seed dispersal, and by extension, germination.
It must be stated, my theory would need ideal conditions....
The perfect storm
All it takes is a heavy rainfall event to float all these cones down the drain, through the drainage system, and out to sea. Whilst some cones/seeds may move entirely through a series of hardscapes (concreted pipes, culverts etc.) to reach the sea, others may be directed from drains into nearby creeks and rivers. These cones will spend more time floating in rivers/tributaries, before being taken out to sea.
I believe a large summer storm makes this dispersal method most feasible. Not only does a storm provide ample water to sweep up cones into drains. The large volume of water will result in faster flowing water and reduce time in the drainage system. The less time in the water the better. A storm is also vital for the next stage.
After reaching the sea, the next stage is for the cones/seeds to be washed ashore, carried past the intertidal zone and onto the primary sand dunes. You may wonder how can it get past the intertidal zone? Well a storm surge is the answer, or in my opinion at least the most plausible method.
I mean maybe a dog-walker on the beach picks up a washed ashore banksia cone, and then throws it into the dunes, but the dog decides, "you know what, I will not retrieve this cone. I have my own autonomy", and is not interested. Now all of a sudden the cone is in the ideal place to release its seeds and germinate. There's really a multitude of low-probability methods to cross the intertidal zone and reach the foredune.
A Coastal Banksia Banksia integrifolia on the foredunes of Port Phillip Bay:
Finally, once the seed arrives at the foredune, we now enter the easy part. Well maybe. The Coastal Banksia, hinted by its name of course, is very well adapted to life on the beach. Once established it can certainly thrive in sandy soils, withstand high winds, sea spray, and in general all the hostilities of a beach location. Propagation from seed is known to be reliable under nursery conditions. I do reserve hesitation as the recruitment of Coastal Banksia has proven challenging even under normal circumstances as highlighted by this paper.
Potential problems
A main problem is environmental. There are a litany of ways in which a cone can become stuck in the drainage system. There are litter traps on some drains. I have also noticed some drains have steel bars that serve the dual purpose of preventing people entering these culverts/drains, but also block large debris. The blockage of large debris may cause subsequent blockage of smaller debris. In both these examples, the stuck cones/seeds will likely become saturated, absorbing all the water and eventually sink. Rendering any chance of germination futile.
There's also just vegetation, rocks and the standard obstacles present in creeks.
In fact, this is why I believe my theory is more likely to occur in highly urbanised areas near beaches. In places where the drainage system can take the cones directly out to sea, bypassing all the river and creeks that are riddled with obstacles.
There is another conundrum. I am unsure exactly what is more likely; 1) the seed falls out of the cone prior to entering the drainage system or 2) the cones carry the seeds through the drainage system.
In my first scenario, the seeds may float. However maybe only initially. I imagine it wouldn't take long for one seed to be submerged, absorb all the water and drown. Then again seeds may navigate urban drains, creeks and rivers easier in comparison to the larger, clunkier cones.
In the second scenario, the cones are essentially boats with seed cargo. I know the cones float for a long time. They are after all, bits of wood. Further the cones can protect the seeds, from the harsh external world whilst in transit, thus this increases chances that the seed is viable for germination.
Designing an experiment
Collect a bunch of banksia cones with seeds inside.
Mark them all in some way.
2.1 The most inexpensive way would be painting numbers on each of the cones.
2.2 A more high tech option, would be to print out QR codes on some waterproof material, attach to each cone, then if anyone else discovers the cone, they can scan the QR code, and help provide data (e.g. date, time, location) on a form that is linked to the QR code.
2.3 The most expensive way would be to attach some sort of GPS unit onto each cone, then track them all remotely.Drop them at various street drains. I would maximise chances by waiting until there is a forecasted heavy downpour. A big storm, ideally with lots of rain in the coming days.
Let the rain fall and the drainage system do its thing.
I think you would need to do daily walks on the nearby beach for a couple weeks (months?) to confirm if any of these cones have successfully navigated the drainage system and emerged out the other side.
Record results. Obviously if you find a marked Banksia integrifolia cone on the beach you know there is potential for this theory.
The follow up experiment, or second part of this experiment would be to test germination rates of Banksia integrafolia on foredunes. You may be able to mimic these conditions by leaving cones/seeds in saltwater for a period of time, then transplanting them onto a beach.
Conclusion
There are sort of two parts to this theory; 1) The seed dispersal and 2) the germination. Both parts of this theory are equally important and rely on the other part.
It is nice to think of all these planted Banksia integrifolia street trees being a seed bank, from which new seedlings may emerge on the beaches.
Assessing the likelihood
I will not say that its impossible. Given enough time, the likelihood that this occurs increases. In fact I am bold enough to say that there is a distinctly non-zero chance of this occurring.
These seedlings are the result of a series of unlikely (yet entirely possible) events. These events (and the circumstances leading to these events) I believe have a probability that get higher and higher with each passing day. Maybe right now dispersal via urban drainge systems resulting in germination has never occurred. But we are closer to p=1 than p=0, if that somehow makes sense. It's a non-zero chance. It's the law of truly large numbers.
Thanks for reading.