Bird-beak hakea epitomises plants dedicated to combustion

Bird-beak hakea (Hakea orthorrhyncha, http://anpsa.org.au/h-ort.html and https://en.wikipedia.org/wiki/Hakea_orthorrhyncha and https://florabase.dpaw.wa.gov.au/browse/profile/2192 and https://www.bgpa.wa.gov.au/about-us/conservation/plant-of-the-month/2001-july-2015) has the 'perfect resume' as an example of adaptation to an ecological syndrome which is seen at its most extreme in Western Australia.

It is one of those plants that 'says it all' in its combination of the various adaptive features of organisms to a particular regime in the natural environment.

Australia - and particularly Western Australia - has the poorest soils in the world: extensive deep sands exhausted of most nutrients by eons of weathering and leaching on a flat landscape.

On such soils, plants are not worth eating, which means that they tend to be consumed and recycled by combustion instead of digestion.

Plants well-suited to such environments, with their periodic wildfires, include multi-stemmed shrubs. They have foliage which is flammable even when green, and the means to regenerate new foliage rapidly from the ashes.

In the case of bird-beak hakea this means a woody burl just below ground-level, which survives even if all the stems of the plant are killed by the heat. This lignotuber (https://en.wikipedia.org/wiki/Lignotuber) produces new shoots without having to start again from seed.

And a crucial point to understand about plants is that - if appropriately adapted and living in sunny climates - they can make plenty of carbohydrate even on poor soils. This is partly because the enzymes of photosynthesis depend on metals, particularly magnesium and iron, which remain sufficient even where the core nutrients (particularly phosphorus and zinc) have become vanishingly scarce.

Carbohydrate is what sugar, plant fibre and wood are made of, depending on the degree of polymerisation. Because carbohydrate is the one product that nutrient-poor plants are affluent in, they use it in various ways to offset all the other disadvantages in their environments.

This is why bird-beak hakea has recruited birds to transport its pollen. Instead of settling for bees, it has the nectar to attract far larger, more energetic pollinators. Red is a hue invisible to bees but conspicuous to birds (https://apps.des.qld.gov.au/species-search/details/?id=1482#!lightbox-uid-0), and it signifies a font of sugar as well as hinting at the flames that propagate the plant in the longer term.

So bird-beak hakea is both 'pyrophilic' (loving fire) and 'ornithophilic' (loving honeyaters https://en.wikipedia.org/wiki/Honeyeater and other pollinating birds).

Bird-beak hakea uses its carbohydrates to fortify its leaves with lignin, making them stiff, spinescent and nearly as flammable as cardboard. And it also converts its roots into a dense, cardboard-like mat, protected from fire by being just below the sand (see https://en.wikipedia.org/wiki/Cluster_root). This mat absorbs, before they are lost, any nutrients that land in the form of dust and ash, thus providing the means for regrowth.

Furthermore, this species uses its carbohydrates in a remarkable way to protect its seeds. The seed-capsule is fortified into a lump of wood, 2 cm by 4 cm, which remains sealed and alive for years until the next fire arrives.

This makes bird-beak hakea both 'bradysporous' (storing the seed on the plant instead of in the ground, https://en.wikipedia.org/wiki/Bradyspory) and woody-fruited (protecting the seed from parrots and other seed-eating animals).

Various flammable plants on other continents show one or two of the above features, but none beyond Australia combines them all in one species. This is largely because nutrient-poverty is not as extensive and extreme, and wildfire does not replace herbivory as thoroughly, on other continents.

For example, in the Mediterranean Basin there are several types of shrub which possess lignotubers (https://www.researchgate.net/publication/229476710_Resprouting_of_the_Mediterranean-type_shrub_Erica_australis_with_modified_lignotuber_content and https://en.wikipedia.org/wiki/Erica_Australis and https://jgpausas.blogs.uv.es/?s=Phillyrea). Others have evergreen, spinescent leaves (https://en.wikipedia.org/wiki/Juniper). But none of these plants is ornithophilous or bradysporous, and all lack cluster roots.

In South Africa there are proteas, belonging to the same family as bird-beak hakea, that have lignotubers, cluster roots, bird-pollinated flowers, and even bradyspory (e.g. https://en.wikipedia.org/wiki/Protea_cynaroides). But they lack leaf-spinescence and woody fruits.

In North America there are 'closed-cone pines (see https://en.wikipedia.org/wiki/Pinus_serotina and https://en.wikipedia.org/wiki/Pinus_contorta) with what amount to woody fruits. However, these lack all the other features of the syndrome, including a shrubby growth-form.

Plants such as bird-beak hakea (http://www.northqueenslandplants.com/Australian%20Plant%20Families%20N-S/Proteaceae/Hakea/Hakea%20orthorrhyncha%20.html and https://domusnursery.com.au/plants/plant.cshtml?plant_code=hakort and https://www.australianseed.com/shop/item/hakea-orthorrhyncha and https://mucheatreefarm.com.au/product/hakea-orthorrhyncha/ and https://davesgarden.com/guides/pf/showimage/413766/#b and https://triggplants.com.au/product/hakea-orthorrhyncha-in-50mm-forestry-tube/ and http://www.consultaplantas.com/index.php/en/plants-from-d-to-l/2218-hakea-orthorrhyncha-or-bird-beak-hakea-care-and-growing) nowhere dominate the vegetation, even on the coastal sandplains of southwestern Australia (https://www.alamy.com/biodiversity-of-flora-in-heath-kwongan-habitat-wheatbelt-frank-hann-national-park-november-2013-image342310831.html).

But the fact that any such species exists is testimony to the ecological peculiarity of Australia among the continents. In its own way, bird-beak hakea is as odd, by global standards, as the kangaroos which exert a minimal effect in its habitat (https://www.rswa.org.au/publications/Journal/80(2)/80(2)wann.pdf).

Publicado por milewski milewski, 13 de outubro de 2021, 08:36 PM

Comentários

The Geraldton sandplains (https://en.wikipedia.org/wiki/Geraldton_Sandplains and https://www.researchgate.net/figure/The-South-west-Floristic-Region-SWAFR-by-interim-biogeographic-regionalisation-of_fig1_304467412), home to bird-beak hakea, occupy an area the size of Belgium under a mediterranean-type climate, at about the same latitude as southern Morocco.

Publicado por milewski 14 dias antes (Sinalizar)

I would argue that leaf spinescence shows that this needs to project its leaves from herbivory. Unlike many other sclerophyllous species from the Cape (and Australia) that dont even bother: nothing other than fire would bother eating them, so why the need for any defences?
But Australia is also unique with its large parrots. Other parts of the world dont need to worry about such destructive predators, so why produce large cones to protect a mere two seeds? But seeds are loaded with precious nutrients, and do need protection, hence the heavy predation, which in the absence of parrots suffices with smaller cones. But dont neglect myrmecochory as a strategy: using ants to take seeds rapidly underground safe from rodent predation is equally effective (or even using Dung Beetles as a Capereed does). Curiously, two sections of Leucadendron actually feed rodents, and rely on mast fruiting and caching to get their seeds buried: a suicidal strategy? Now would it not be impressive if a Hakea actually fed parrots to ensure its survival?

There is one other feature in which Birdbeak Hakea fails the epitome of thriving "hydroponically on pure glass" and that is that it releases its seeds. The Plateseed Conebush Leucadendron platyspermum does not even bother doing that: the seeds germinate in the cones and push themselves out of the cones before gliding to the soil to establish, thus remaining safe in the cones between the fire (summer) and germination (autumn) seasons, rather than being exposed on the ground for several weeks or months to the vagiaries of predators, heat and other fates.

Still, I have to concede (reluctantly) that Australia takes growing on glass to a level unseen - or perhaps merely hinted at - in the Cape.

Publicado por tonyrebelo 11 dias antes (Sinalizar)

@tonyrebelo Many thanks for this valuable comment.

Publicado por milewski 10 dias antes (Sinalizar)

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