How buttongrass in Australia is naturally flammable enough to ignite in a drizzle

(writing in progress)

Through evolutionary time, fire has controlled the structure of vegetation, determining whether a landscape is covered in grasses or trees.

Indeed, under climates capable of supporting large plants, the main factor maintaining vegetation in a treeless state is wildfire - including fires set by humans under hunter-gatherer or traditional pastoral economies.

Lightning, the original spark of such fire, has been superseded: humans have been igniting their environment for thousands of years, making wildfires on most landmasses mainly anthropogenic.

However, for fire to be effective in suppressing the woody plants in most terrestrial ecosystems, an annual dry season remains necessary. It stands to reason that vegetation cannot burn effectively unless desiccated to some degree, explaining why the rainiest climates – with precipitation in all months of the year – tend to retain wooded vegetation instead of the treeless grasslands typical of drier climates2. 2 Communities dominated by plants with pronounced adaptations to fire are all characterised by a season warm and dry enough to cause dormancy, if not death, of the photosynthetic organs.

Contrary to popular belief, plants deploy fire rather than being victims of fire – with the adaptive benefits of this botanical engineering of fire applying particularly to nutrient-poor soils, regardless of climate. In the course of evolution, plants adapted to soils rich enough in phosphorus and certain trace elements to support herbivores tend to evolve into fire-retarders. By contrast, plants adapted to poor soils tend to promote fire rather than herbivory, thus facilitating the recycling of nutrients essential for all green growth (https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-185X.2007.00017.x and https://d1wqtxts1xzle7.cloudfront.net/48420595/j.1469-185x.2007.00017.x20160829-21403-dqy1kr-with-cover-page-v2.pdf?Expires=1654850200&Signature=gh8GENzLja9oOltvC~9tXthv73apmptAmjppOYz9aNZfVcI3-srExwU8tGJ3Fqy2BSUMGeYrzCyJhIolG3gF7BzoaJdIHYijY-Fr2SmH4vnBjA1rxNhHtPEQSHn0n4LlgV2S8oQ3TeR~U4uLUMcEGwIyV6ps~799A46f0UKJjEVkwQIhR-AB2x~yovNIk4JVfgX3U8dXR9F37cdFoQ3nvU7WQwoGXuGKIjfnOmqe8VICN0DNgQXxfMLqjOkfTQOXeWTJN7t8rX1~Lpigb4agJc-EMYFi0Hrkw0gOkAcX6CdkLbbSZ3It8rePvCGRR8wcLEmN00J~ado3QE~IJJHjBA__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA).

These processes are powerful enough to override seasonal desiccation. As one mechanism, species on poor soils have tended to evolve particularly combustible foliage, rich enough in flammable substances - particularly hydrocarbons - to ignite even in a green state. Indeed, extreme poverty of the soil has resulted in the evolution of plants capable of burning in cool, foggy and rainy climates lacking a dry season.

And conversely, nutrient-rich soils under a warm dry climate – in which wildfire would be expected – can lead to the evolution of succulent vegetation that excludes fire. This is epitomised by South African spekboomveld, a type of thicket dominated by the tall shrub Portulacaria afra https://www.inaturalist.org/taxa/326086-Portulacaria-afra, which has turgid leaves so lacking in flammability that this vegetation remains permanently free of wildfire despite occurring in a semi-arid climate with warm summers.

The epitome of such 'pyrophilic' vegetation is moorland, dominated by buttongrass (Gymnoschoenus sphaerocephalus https://www.inaturalist.org/taxa/85478-Gymnoschoenus-sphaerocephalus), on peaty soils in Tasmania. This is a species of sedge (Cyperaceae).

Buttongrass moorland (https://nre.tas.gov.au/conservation/conservation-on-private-land/bush-information-management/identify-your-bush-type/bush-that-is-treeless/buttongrass-moorland) remains naturally treeless under climates with up to two meters mean annual rainfall and no dry season.

The natural environment in Tasmania includes human effects, because Homo sapiens has been present on this Scotland-size island for at least 40 thousand years.

The dominant hummock sedge is inherently flammable enough to burn even during a drizzle.

Testimony to the continual sogginess of this environment is that – even on slight slopes – the role of detritivores has been taken over by burrowing crustaceans instead of the expected earthworms. These are crayfishes (class Malacostraca: order Decapoda: family Parastacidae: genus Engaeus (https://www.inaturalist.org/observations?taxon_id=88162), most species of which occur only in buttongrass moorland or related vegetation types.

Being razed frequently, yet at the same time being kept wet enough to be a permanent habitat of amphibious crustaceans, buttongrass moorland sorts cause from effect in the origin of vegetation. What determines whether nutrients are recycled by combustion, herbivory, or detritivory is soil fertility rather than climate.

In buttongrass moorland, indigenous people have been ecosystem engineers for 50 thousand years. However, buttongrass itself emerges as an ecosystem co-engineer in this situation.

Although the role played by humans is more obvious and more mechanistic, the role played by buttongrass itself arguably as profound in the chain of cause and effect.capable of promoting both saturation and combustion – an apparently paradoxical combination – as part of its strategy of adaptation to extremely poor soils.

Since buttongrass presumably evolved before fifty thousand years ago, it must have been an ecological engineer in its own right at that time?

(writing in progress)

Posted on June 10, 2022 07:02 AM by milewski