Plants are Jerks: The Kamikaze of the Outback

Plants have a pretty good reputation. Sure, there are few bad apples that can make us itchy or sneezy, and, sure, Socrates probably could've done without hemlock, but that's water under the bridge at this point. These days we're mostly concerned with eating them or sending their reproductive organs to our loved ones. And that, frankly, is a shame. Because the plant world is full of cutthroat competition, and, since plants don't really have a moral code, the world has more than a couple of species that gleefully punch below the belt. This series will explore the life histories of some of those plants. Today's topic is fire ecology done differently. 

The continent of Australia is, with the exception of a few coastal areas, an arid place. As with dry regions the world over, wildfires are common. Coping with this hazard can be difficult for organisms as flammable as plants, and so a few different classes of adaptations have arisen to cope. These strategies include:

  1. Dig in. When a wildfire breaks out, the soil even a few inches below the surface can still be livable, so all a plant needs to do is keep an emergency stash of nutrients in a storage organ such as a tuber (potato) or a bulb. After the fire ends, these resources can be used to start anew.
  2. Fireproof yourself. Large plants, such as trees, can't really take advantage of storing resources belowground. Instead, they focus on shielding the living tissue in the trunk with a thick layer of spongy bark known as cork, the same stuff used for stoppering wine bottles.
  3. Put your hope in future generations. Some plants just accept the fact that they're going to burn, and so they put extra effort in protecting their seeds, using a strategy called serotiny. For example, the serotinous cones of the jack pine are sealed with a layer of wax that melts open when heated. The scales then lift away, releasing the seeds to colonize the fertile ashes.

The fourth option, of course, is to be a jerk, a strategy employed by Australia's Eucalyptus globulus. When not opening your airways or hosting kookaburras, these Eucalyptus trees hang out in the fire-prone outback. Like the jack pine, Eucalyptus has developed serotinous seed pods. However, the Eucalyptus's convoluted evolutionary path has decided to parody this otherwise sensible strategy. Unlike most serotinous species, the eucalyptus keeps its seed pods high up in its canopy. This is an unusual move because the typical brushfire stays low to the ground, where it consumes dead wood and leaf litter. This behavior is good for most plants, because low brushfires burn colder and shorter; however, the Eucalyptus is much less enthused, because it needs heat to reach its canopy to reproduce.  

Eucalyptus globulus, commonly known as blue gum, shown here pretending to mind its own business. Source

Eucalyptus globulus, commonly known as blue gum, shown here pretending to mind its own business. Source

What then, is the solution to this conundrum? The cunning Eucalyptus employs several strategies: first, it sheds its bark, building a nice, attractive tinder pile at the base of the tree. Next, it fills its leaves with highly flammable oil. Then it waits. When an ordinary brushfire comes along, it meets a Eucalyptus tree and whoomphs into a cataclysmic inferno. The fire hits one canopy and begins hopping from tree to tree, taking the whole forest down in a dark carnival of destruction . The Eucalyptus is happy, since its seeds are free to colonize the ashes, but the rest of the forest is less fortunate. The fire burns hot enough to take out both cork-insulated trees and plants that hide nutrients underground, so the eucalyptus is the last one left standing.

An older E. globulus, having started to shed leaves and bark, waits for an errant flame. Source.

An older E. globulus, having started to shed leaves and bark, waits for an errant flame. Source.

Despite its pyromaniacal lunacy, Eucalyptus globulus has become an incredibly popular timber tree, and several plantations were established in California in the early 20th century. Problematically, the tree has decided it likes the States a little too much and has escaped into the wild, where it is considered invasive and, unsurprisingly, a fire hazard.

In an ironic coda, the world's tallest Eucalyptus trees are found not in its native Australia, but neighbor New Zealand, where the climate is too wet for it to burn itself down. Photo by the author.

In an ironic coda, the world's tallest Eucalyptus trees are found not in its native Australia, but neighbor New Zealand, where the climate is too wet for it to burn itself down. Photo by the author.