The puzzle of Hakea as an indigenous woody weed in currantbush mixed shrubland in Western Australia

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Most naturalists would not regard proteas (Proteaceae, https://www.inaturalist.org/observations?taxon_id=64517) as weedy plants. The family is generally associated with infertile soils, ancient land surfaces, archaic lineages, ecologically fragile floras, and species threatened with extinction.

It is true that a few shrubby spp. of proteas have become weedy, after being introduced from Australia to New Zealand, Portugal, and South Africa (https://www.inaturalist.org/taxa/321137-Hakea-sericea and https://www.cabi.org/isc/datasheet/27302#tosummaryOfInvasiveness and file:///C:/Users/Antoni%20Milewski/Downloads/minids_HKASE.pdf and https://www.arc.agric.za/arc-ppri/Pages/Silky-hakea-Hakea-sericea.aspx and https://keyserver.lucidcentral.org/weeds/data/media/Html/hakea_sericea.htm and https://onlinelibrary.wiley.com/doi/epdf/10.1111/epp.12527 and https://inaturalist.nz/taxa/321136-Hakea-gibbosa and https://www.marlborough.govt.nz/repository/libraries/id:1w1mps0ir17q9sgxanf9/hierarchy/Documents/Environment/Biosecurity/RPMP%20Individual%20Programmes%20List/Willow_leaved_hakea.pdf).

However, these tend to invade in situations where regimes of wildfire suit the life history strategy and dispersal mechanisms of a pyrophytic (https://en.wikipedia.org/wiki/Pyrophyte) proteaceous genus, namely Hakea (https://www.weedbusters.org.nz/what-are-weeds/weed-list/willow-leaved-hakea/ and https://www.weedbusters.org.nz/what-are-weeds/weed-list/downy-hakea/).

It surprised me, therefore, to read of a case, in Western Australia, where an indigenous species of Hakea has become weedy, in a situation characterised by a combination of:

  • semi-arid climate,
  • location partly in the tropics,
  • utilisation by domestic livestock, to the point of degradation of the vegetation,
  • freedom from wildfire, and
  • natural abundance of a nodal-spinescent indigenous shrub.

My source is Burnside et al. (1995) Reading the Rangeland: a guide to the arid shrublands of Western Australia. Western Australian Dept of Agriculture (https://researchlibrary.agric.wa.gov.au/cgi/viewcontent.cgi?referer=&httpsredir=1&article=1001&context=pubns). In particular, please see page 117.
 
The town of Carnarvon (https://en.wikipedia.org/wiki/Carnarvon,_Western_Australia) lies

The economy of Carnarvon is based mainly on irrigated orchards, on soils naturally enriched by the alluvia of the Gascoyne River (https://www.agric.wa.gov.au/rangelands/report-gascoyne-river-catchment-following-2010%E2%80%9311-flood-western-australia and https://en.wikipedia.org/wiki/Gascoyne_River).
 
Around Carnarvon (but nowhere else in Australia as far as I know) there is a vegetation type called currantbush mixed shrubland. It occupies >4000 square km of the less sandy, non-saline soils, on the land systems flanking erodible floodplains in the westward-draining system.
 
Burnside et al. (1995) state:
“In the natural state these shrublands support an unusually diverse range of low shrubs, many of which are palatable, with occasional taller shrubs including [Hakea preissii, https://www.inaturalist.org/taxa/1033489-Hakea-preissii] and [Acacia xiphophylla, https://www.inaturalist.org/taxa/964520-Acacia-xiphophylla]. Fire is rare.”
 
Scaevola spinescens (currantbush, https://www.inaturalist.org/taxa/421592-Scaevola-spinescens) is

  • a shrub up to 3 m high,
  • drought-deciduous (unusual in the Australian flora of woody plants), and
  • nodal-spinescent, as well as being endozoochorous (a familiar combination in various parts of the world, but unusual in Australia).

Scaevola spinescens is locally an important forage plant for livestock, despite

  • the small size of its leaves,
  • the protein-poverty of the foliage (up to 13% protein on a dry matter basis), and
  • nodal spinescence.

Burnside et al. (1995) state:
“While currant bush is the most distinctive of the palatable species and is locally dominant within stands in good condition, the general vegetation comprises a rich mixture of both low and tall shrub species.”

Currantbush mixed shrubland can be described as follows:

Because of the wide range and density of drought-resistant, palatable shrubs, this vegetation is regarded as having pastoral value. However, readers should bear in mind that this is in the context of Western Australia, where all rangelands are poor by global standards.
 
With overutilisation by domestic livestock, certain ‘decreaser’ spp. become less common, e.g.

At the same time, certain other spp. (‘increasers’) become more common, including

“The country is very prone to invasion by woody increasers which make it difficult to walk through. Where invasion occurs, often near heavily-used watering points, there is no practical solution other than expensive mechanical removal. Fires are possible but only after a run of exceptional seasons – perhaps only once in thirty to fifty years.”
 
Of Hakea preissii, Burnside et al. (1995) state that invasion by this protea is ‘one of the most insidious. Low numbers can germinate almost every year, with higher numbers in some years. Growth rates are very slow and mortality is low.’

Hakea preissii is serotinous/bradysporous (https://en.wikipedia.org/wiki/Serotiny and https://en.wikipedia.org/wiki/Bradyspory). Conditions for the release of seeds, and subsequent successful germination and establishment, occur infrequently.

However, with the right combination of circumstances, H. preissii has explosive recruitment potential, such that ‘seedling storms’ may occur. There have been at least two ‘seedling storms’ within the past forty to fifty years (including one in the mid-sixties). These have led to a degraded community, dominated by H. preissii, with other unpalatable invaders including Senna spp. and Eremophila spp.

This change in species-composition has produced thickets of H. preissii, which have further reduced the opportunities for growth of annual plants, and hence have further reduced the chances of fire.
 
"Burning trials were conducted south-east of Carnarvon in 1985 where fire was found to be useful in killing [Hakea preissii], but of little use against other increaser species. [Eremophila crenulata], for example, survived even the most intense fires by resprouting from the base. Palatable species such as Maireana tomentosa and Enchylaena tomentosa were destroyed so that fire gave little advantage except to reduce the needlebush [Hakea preissii]."
 
"There are no known natural examples of fire-affected thickets of H. preissii. Therefore pastoral management has had little effect on currantbush mixed shrubland invaded by H. preissii. Changes for this community over perhaps a fifty to hundred year time-frame rely on fire, but opportunities are exceedingly rare. Shorter-lived spp. such as Senna spp. may die out, but it may take more than a century for H. preissii to die and for other spp. to emerge."
 
My commentary is as follows:
 
Australia is the most wildfire-prone continent, and much of the semi-arid vegetation here is dominated by pyrophytes, notwithstanding the bare ground separating the plants. Furthermore, Western Australia, south of the tropic, is completely devoid of ‘rainforest’ and ‘vine thicket’ types of vegetation, which are 'pyrofugic' (i.e. adapted to freedom from wildfire).

So, it is noteworthy to find an extensive vegetation type in which fire is naturally so rare that it can be considered absent, to all intents and purposes. This near-freedom from wildfire is partly the result of a relative lack of sclerophylly on the alluvially nutrient-enriched soils of the plain just east of Carnarvon.
 
Proteas, including most spp. of Hakea, are associated with nutrient-poor, sclerophyllous, wildfire-prone environments. Therefore, it is noteworthy that H. preissii plays such an extreme role in the successional cycle of currantbush mixed shrubland.

Both currantbush (Scaevola spinescens) and Hakea preissii are spinescent, but the types of spinescence are nearly ‘opposite’ in function:

This makes H. preissii so unpalatable and defensive that, quite unlike S. spinescens, it is virtually shunned by herbivores. This protea is deterrent to mammalian foliage-eaters, and this deterrence is nearly absolute in the case of ungulate livestock.
 
Imagine that someone had explained every aspect of this ecosystem to me, except the presence of H. preissii, and then asked me to predict which plants would encroach in response to overutilisation by livestock. I would have pointed not to any protea, but instead to various indigenous spp. of Acacia.

There are, after all, so many spp. to choose from, with various types of spinescence including nodal and foliar spinescence, the latter being convergent, in some cases, with that of H. preissii. There are plenty of spp. of Acacia that have phyllodes similar to, and similarly spinescent to, the leaves of H. preissii.

If those were unsuitable, I would point to the various spp. of Acacia that have spine-tipped green stems instead of leafy phyllodes. These form the same kind of combination of foliar lignification and spinescence presented by H. preissii.

It is surely significant that, in many other parts of the world, it is indeed legumes - and particularly acacias - that increase in response to overutilisation by livestock. Acacias fit the bill for recovery of the vegetation, partly because they can fix atmospheric nitrogen. This contrasts with proteas, which - instead of having N-fixing nodules on their roots - have proteoid cluster roots (https://en.wikipedia.org/wiki/Cluster_root), adapted to oligotrophy (https://en.wikipedia.org/wiki/Oligotroph).

I cannot, offhand, think of any genus of shrubs, under semi-arid climates elsewhere on Earth, that has cluster roots. So, it would have been surprising enough to find any protea in a flora such as that of the currantbush mixed shrubland; but to see a protea play such a striking role here is food for thought, indeed.
 
I realise that there are several other examples of indigenous Hakea encroaching, in natural vegetation. However, it is one thing for Hakea to encroach in e.g. Tasmania, which is mesic, somewhat leached, and fire-prone. It is quite another for Hakea to encroach in a near-tropical, semi-arid environment, in which the flora lacks any other elements of kwongan (https://en.wikipedia.org/wiki/Kwongan), and the particular vegetation type is as free from wildfire as any vine thicket.

Hakea seems out of place in both its cluster roots and its serotiny/bradyspory. For example, I do not recall offhand a single species of plant in arid or semi-arid southern Africa that is serotinous, let alone with substantial woody follicles (https://www.agric.wa.gov.au/rangelands/needle-bush-hakea-preissii-western-australian-rangelands).

Hakea sericea and congeners, similar to H. preissii apart from requiring far rainier climates, have seriously invaded fynbos (https://en.wikipedia.org/wiki/Fynbos) in South Africa. However, it is unthinkable that they would invade any southern African vegetation comparable with currantbush mixed shrubland - a vegetation type which would hardly seem out of place in southern Africa, apart from e.g. the phyllodes on several spp. of Acacia).

Publicado el julio 29, 2022 12:54 TARDE por milewski milewski

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