Some Acacia Curiosities; wattle they think of next?

Acacias are fundamental to Australian landscapes, though we sometimes forget here that they are equally characteristic of many African ones. 

Mulga Acacia aneura woodland, Chambers Pillar, central Australia.

Acacia woodland, Murchison Falls NP, Uganda.

The minor tensions generated between Australian and African botanists by our tendency to think of acacias as 'ours' came to a head in 2003 with a proposal from a couple of Australian botanists to change the type specimen of Acacia from an African one to an Australian one. This was breathtakingly cheeky, with very little precedent, and was based on two factors. The first was the growing acceptance that Acacia as traditionally used for plants across Africa, Australia and America (and a few other places) was an artificial genus, comprising at least five distinct genera. The second is the fact that the majority of Acacias are Australian (some 1000 of the 1300 known species) which would have involved a massive task in changing names here. After consideration by subcommittees the proposal to change the rules in this case was ratified by the International Botanical Congress in Vienna in 2005, but challenged on procedural grounds. After another sometimes bitter six years the original decision was upheld at the next Congress in Melbourne in 2011.

However for some it didn't actually end there, with a number of influential botanists preferring to go on using Acacia in its broad sense and ignoring the Congress rulings. I don't share the sense of triumphalism some in Australia were displaying after the decision, and have sympathy with these bolshie botanists - if we can change the rules for one circumstance, we can do it whenever it seems convenient in the future. I can't help but think this might come back to bite us one day.

For the record, African acacias are now either Vachellia or Senegalia. Both these genera are also significant in South America, along with smaller genera Acaciella and Mariosousa. A few species of Vachellia and Senegalia also occur in tropical Australia. Had the normal rules been followed, the rest of Australian acacias would now be Racosperma.

Anyway, what I really wanted to talk about was the way Acacia foliage in Australia has evolved so differently in Australia from elsewhere (and for now for simplicity I'm going to continue to refer to Acacia in the older broader sense but to avoid controversy I'll use lower-case acacia as a group name!). In both Africa and America acacias are typified by divided - pinnate - leaves.

acacias in northern Cameroon (above)
and near Machu Picchu (Peru) below.

 

Many Australian acacia species - though a definite minority - also have compound leaves, with foliage ranging from having just a few pinnae (leaflets) to scores.

Acacia spectabilis Goobang NP, central New South Wales

Acacia elata, coastal New South Wales

Acacia deanei Goobang NP, central New South Wales.

Most of these leafy acacias live in moister parts of Australia; an advantage of compound leaves is that they confer a larger surface area which enables higher levels of photosynthesis, but the trade-off is in greater water loss.

Accordingly a very large number of Australian acacias, especially in drier situations, have done away with their leaves altogether, at least as adult plants. However, it's of course not that simple - if you're a plant you need leaves to photosynthesise, and there's no using conserving water if you can't function. So the compromise solution has been the evolution of phyllodes in Australian acacias (phyllodes are not unique to acacias, but they are probably most dominant there). A phyllode is a petiole - a leaf stalk - which has shed its leaf, flattened out and taken chlorophyll on board to take over the photosynthetic role. In acacias the phyllodes tend to hang down or stand stiffly erect to minimise exposure to the sun, and are tough and leathery (like a eucalypt leaf) to minimise water loss. Here is a small sample of the numerous phyllode types that Australian acacias display.

Acacia anceps Eyre Peninsula, South Australia

Acacia floribunda, south coast New South Wales

Acacia hakeoides Goobang NP central New South Wales

Acacia inaequalitaria Kata Tjuta NP, central Australia

Acacia monticola Ormiston Pound, west MacDonnell Ranges, central Australia

Acacia pycnantha Canberra.
This is Australia's floral emblem, Golden Wattle

Acacia retivenea Bladensberg NP, tropical central Queensland

Acacia spondylophylla Ormiston Pound, west MacDonnell Ranges, central Australia

Acacia triptera Goonoo NP central western New South Wales

The hard spiky phyllodes on this last species, known as Spurwing Wattle, raise another interesting difference between Australian and other acacias; you'll note the Cameroonian and Peruvian species above both sport savage thorns (though they're less obvious in the Peruvian photo). Here are a couple more examples.

northern Cameroon

Acacia (Vachellia) rorudiana, Santa Cruz, Galápagos

Thorns, botanically, are modified pointed branches, complete with their own vascular system, which cannot be removed without tearing the wood; Australian acacias don't have them. It seems equally odd that they either never evolved them in the face of the large browsing herbivores which inhabited Australia until relatively recently while most of their relatives elsewhere did, or that they all lost them at some point in the past. Neither makes much sense to me. 

However there a number of spiky Australian acacias, most of which rely, like the Spurwing Wattle above, on toughened sharp phyllodes.

Acacia genistifolia Canberra

Acacia tetragonophylla south-west Queensland.
This is known as Dead Finish, the logic being that if it dies of drought, there's no hope for anything else.

Other plant parts also provide protection however for some species.

Acacia paradoxa Canberra.
In this one - Kangaroo Thorn - the spikes are stipules, growing from the base of the petiole.

Acacia spinescens Lincoln NP South Australia.
Here the branches themselves are spike-tipped. (Technically I suppose they could thus be thorns,
but that usually refers to smaller branches growing off the main ones.)

And there's another thing...

Acacia mearnsii Canberra.
Note the glands along the branch.

Such glands are common in African species, where they attract aggressive ants which defend the plant. In Australia however this does not seem to be the case, though you can find some websites which should know better asserting that it is - they have simply used African data. The Australian glands do exude small amounts of nectar (which acacia flowers do not) which attract a range of insects, including ants but not for the most part insect-hunting ants which would protect the plant. A lovely mystery to be solved...

And we could go on, but before I instead wrap up for today, let's return briefly to phyllodes. I've been asked, quite reasonably, how I know they aren't actually leaves? Well, there are doubtless some physiological reasons, but the best answer is "because we can see it happening".

Acacia rubida Redstem Wattle, Canberra.

Many acacias begin life with true divided leaves - perhaps because maximising photosynthesis is the top priority in the early establishment phase - then switch to phyllodes, water conservation being the long-haul imperative. In Redstem Wattle it is particularly evident. At the bottom of this sapling are the juvenile leaves, at the top are pure phyllodes. But halfway up, look carefully at the phyllode on the right of the stem. The compound leaf is still growing from the end of petiole, which is flattening and becoming a phyllode beginning from the base. On the bottom left petiole the process is just beginning.

The wonderful wattles - like everything else, there's much more to them than immediately meets the eye.

BACK ON WEDNESDAY