Seeking Refuge: How Australian Plants use Soil for Shelter
The soil is a place of security for many animals who use it to hide from predators or wait out bad conditions. What’s often not contemplated, however, is how the ground acts as shelter for plants. When danger comes, plants can’t just pull up their roots and saunter into the nearest hole to shield themselves, so they use other means. And, because of our many fire-dominated ecosystems in Australia, some of these tactics are quite interesting.
Ecologists sometimes divide plants of fire-prone areas into two different life strategies: the seeders which are ‘fire sensitive’ and replaced through seed, and the ‘re-sprouters’ that stockpile energy in safe parts of the plants to avoid death.
There are many ways re-sprouters shelter underground. A lot of Australian grasses have regenerative buds at or below the soil surface for post-fire regeneration, including species of the ‘hummock grasslands’ that cover a large amount of the Australian interior. But a more unusual example is the lignotuber, a sub-terranean storage organ that’s essentially a swollen root crown. Originally coined to describe those of some eucalypt species, lignotubers are also present in many Banksia species, and in other plants of the world’s ‘Mediterranean’ climates. A woody, swollen root crown, lignotubers hold supplies of starch and water, and are dotted with dormant buds that spring into action when what’s above is destroyed.
The quintessential example of the lignotuber is the mallee eucalypts, of which there are over 100 species.”
Mallees are ostensibly a small tree or shrub, but instead of producing a single vertical trunk that branches out above ground, a single plant has many separate ‘trunks’ emerging from a single lignotuber, which can be just visible above the soil, like the tip of an iceberg. One study of New South Wales mallee species found lignotubers weighing up to 610kg and possessing up to 13,3000 buds per tuber. Once burnt to the ground, a tiny fraction of these buds spring into action to replace what was above.
Australia is unusual in that we have a whole ecosystem built around the lignotuber. Mallee woodlands and shrublands are major vegetation types that – before British colonisation – covered a vast amount of semi-arid southern Australia. Much of this was cleared for farming, but not without a fight.
The mallee lignotuber was so successful at regenerating from below ground that it proved to be some of the most difficult territory to destroy to make way for European-style farming.”
Common practice saw the vegetation felled, then the regrowth crushed with a roller, then the area burned, sowed with wheat, harvested and then burned once again. And repeated if necessary! This finally managed to kill the plant, but still didn’t do anything to remove what was below ground. A special type of plough had to be invented, and while successful, the ‘stump jump plough’ still didn’t actually remove lignotubers, but only had a mechanism to dodge the obstacles when it encountered them.
Re-sprouters tend to invest in a long life for the individual plant, often with the assistance of soil to protect a large root-system. On the other hand, seeders grow quicker and produce a greater volume of offspring through many flowers and floral rewards (nectar and pollen) but tend to die from fire. This strategy can also employ the soil for protection – but for their seeds instead.
In 1859, the same year as publishing On the Origin of Species, Charles Darwin found time to dig up three tablespoons of mud from a lake and observe it, studiously removing anything that germinated. A total of 537 live plants grew from this tiny sample over six months. The experiment eventually resulted in the concept of the ‘soil seed bank’, re-conceiving soil as a place for plants to maintain population consistency over time, in the face of fluctuating conditions.
Out in the open, seeds can be eaten, burned or desiccated. Often it is the seeds that get below the soil that do better, as soil can provide less risky conditions until the right time to germinate comes along.”
How do seeds get themselves underground, if they can’t move? Many are simply blown into cracks or covered by wind-blown dirt, but a more active strategy that’s common in Australia is called myrmecochory, where plants recruit ants to do the heavy lifting. Myrmecochorous plants – which include many of Australia’s wattles (Acacia spp.) – produce a high-fat appendage attached to their seeds, for which the ants schlep the whole thing back to their nest, eat the delicious reward, and discard the seed in one of the nest’s underground trash piles. Here they are safe from seed predators and the peak temperatures of wildfires, and also have a more nutrient rich spot to germinate away from the parent plant. During a fire, many of these seeds sense smoke compounds and heat, stimulating germination. So keep an eye out for wattle seedlings germinating from ant nests after fires!
Myrmecochory is very common in the Australian flora, especially in dry ecosystems. Though wattles are the most well documented group that practices ant dispersal, about 1500 different Australian species within 87 distinct genera use this method to some degree.
So, the soil can be employed as a fire blanket by both seeders and re-sprouters. The reasons that the fire regularity can be critical for maintaining diversity in many Australian ecosystems are linked to these different strategies. Too infrequent fires and the seeder’s short life cycle will run its course and fade away, eventually relegated to the seed bank for too long. But if fires occur too often then seeders don’t get a chance to set seed and re-sprouters can deplete their storage and die. And different fire temperatures also promote certain plants over others. This is why the burning practices of First Nations peoples maintain biodiversity, by producing a mosaic of different phases of regrowth and establishment. Increased understanding of these and other strategies by non-indigenous Australians may give us a better grasp on how to be good custodians of this burnt continent.