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A second line of defense is not as readily apparent until contact is made with a protected plant

Kirk Andersen, Garden Collection Manager from the Living Desert and Zoo in Palm Springs explains the many ways a tree has developed in order to survive in the African savannah. 

 

Drought is not the only killer of trees.  Foraging animals can defoliate a tree, leaving it unable to take up the nutrients it needs to survive.  Acacia trees have developed several innovative ways to keep their leaves.

 

Read this fascinating story of the giraffe, the acacia tree, and survival.

---Anne K Moore, March 6-2009---

 

 

NO PLACE TO RUN, NO PLACE TO HIDE

Acacia Defense                                                    Kirk Andersen, Living Desert and Zoo

 Palm Springs, California

 

Defensive tactics of prey species often rely on speed or camouflage to elude predators. In the following encounter, the predator can reach nearly twenty feet in height, run 35 miles an hour, and consume up to 140 lbs a day. The prey can be twice or more the height but can neither run nor hide. The predator, an African giraffe (Giraffa camelopardalis) stalks its favorite prey – one of the many thorn trees (Acacia spp.) that constitute a major component of Africa's savannas and woodlands. The leaves and seedpods provide nutritionally superior forage. The characteristic flattop shape of these trees is a result of being browsed by mammal herbivores, the bottom of the canopies being equal to the reach of the tallest browser.  What's a 'defenseless' plant to do?

 

Acacias rely on multiple defense mechanisms to deter herbivores such as giraffes. The most readily apparent comes in the form of thorns; paired, up to four inches long, hide- piercing, stiletto-like or recurved, claw-like and flesh-grabbing arrayed amongst the branches. Umbrella thorn (Acacia tortilis), a dominant tree of the eastern and southern savannas is the only acacia to have both straight and curved thorns. The Afrikaans name of Ōhaak-en-steekÕ or Ōgrab and stabÕ is most appropriate. As one of the gardeners to tend to The Living DesertÕs African acacia collection over the past twenty years, many times I've felt like a pincushion and scratching post. I can personally attest to the pugnacious effectiveness of this line of defense. Tender new growth on the branch tips is eaten thorns and all, before the thorns have a chance to harden or lignify. The giraffe with its desensitized, twenty-inch long tongue is able to strip leaves from the thorny branches and crush the thorns with its molars. Thorns do not prevent tissue and foliage loss but they do lessen it by restricting bite size and retarding bite rates, in some cases slowing the rate of consumption by three times the rate of thornless branches.

 

A second line of defense is not readily apparent until contact is made with a protected plant. Ants – biting, stinging, swarming, and ready to give their lives in defense of the colony and turn a mouthful of foliage into a painful experience. Many plants draw ants into their canopies with the lure of nectar found in nectaries on the leaves. But certain species of acacia such as the whistling thorn (Acacia drepanolobium) go one step further by supplying room as well as board in the form of swollen thorn bases. The ants hollow out the thorns while green and become nest sites and living quarters, domatia. The colony readily defends its home tree against any and all intruders be they mammal, insect or plant. Pheromone scent trails rally the troops to battle and may serve as a warning to nearby browsers that these leafy morsels are served with a bite. While this indirect defense of ant-plants, known as myrmecophytes, keeps immature giraffes at bay, the angry ants do not as easily dissuade older ones and a second line of defense fails to deter hungry giraffes.

 

Once the perimeter canopy has been breached itÕs time for chemical defense in the form of tannins. Tannins are water soluble, carbon based compounds found in plants and important to man. Their chemical and physical properties allow the binding of alkaloids, gelatin and other proteins in the making of leather, or tanning. They are also used in food processing, fruit ripening and the making of wine and cocoa. The presence of these bitter tasting compounds makes the plant less palatable to herbivores.

 

Tannins inhibit digestion by interfering with protein and digestive enzymes and binding to consumed plant proteins making them more difficult to digest. Herbivores have various strategies for dealing with tannins, some more successful than others – too much absorbed across the gut wall can be fatal. The concentration of tannins varies widely from plant to plant and even within the same plant depending on when and where in the canopy. It doesnÕt seem to follow the theories of resource allocation, which holds that tradeoffs should occur concerning growth, spine formation, chemical defense and fruit or flower production. Is it a matter of energy costs or is it possible that the plant keeps the playing field uneven by varying levels of tannins so that the herbivores digestive tract is unable to adapt to a constant level of toxin or threat?

 

Under certain conditions, acacias build up levels of a toxin, known as Prussic acid or hydrocyanic acid or hydrogen cyanide (HCN). Higher levels occur when the plants cell walls rupture due to stresses that retard the plantÕs growth such as freezing or drought. This allows the cyanogenic compounds found in the outer epidermal cells to combine with enzymes in the mesopyll cells of the leaves, forming HCN. High levels are also associated with the new growth following an extended drought or freeze but usually dissipate quickly. Ruminant animals are more susceptible due to certain enzymes found in their digestive tract. This potent poison interferes with oxygen use at the cellular level causing death by asphyxiation.

 

 Once again, the giraffe appears to reap the bounty provided by the acacia woodland heedless of the defensive postures the plants assume. Upon tearing away at the protein-rich foliage, the torn leaf surfaces emit the gaseous hormone ethylene, alerting other plants within 50 yards to increase tannin production in order to thwart the foraging mega-herbivore. Sensing the menu change the giraffe moves upwind dining on plants that have failed to catch the drift. More than the adjoining vegetation may detect this call to arms. Passing carnivores pick up the invitation, resulting in a hasty retreat or, at the very least, a change of plans for the browser.

 

If resistant defense traits fail to discourage herbivory there is always tolerance, compensating for lost tissues with regrowth. Often, branches that have been pruned and stripped of leaves grow back more vigorously than unbrowsed branches. Tipped branches send out many side shoots, each with new leaves that seasonally provide even more nutritious browsing than before, while increased leaf surface allows increased energy production. Repeatedly done this would be analogous to a gardener shearing a hedge.  This mutuality outcome works out for both parties, over the short term anyways. Browsed stems are more likely than unbrowsed ones to be dead the next year and repeated heavy tissue loss and replacement could ultimately deplete the trees available resources unless some compensatory source of nutrients becomes available, e.g. increased dung production, which fertilizes the tree.

 

Living in arid and semi-arid lands where rainfall is erratic, resources limited and pressure from herbivores on the sparse vegetation can be high, acacias as well as other plants must try to protect what they've got. It has been demonstrated that seedlings tend to be thornier than older trees (age effect), and the upper canopies, out of browsing range, are the least prickly. Branches experiencing herbivory are induced to produce longer spines spaced closer together. Tannin production may or may not be stepped up. On the flip side, at least in the case of the whistling thorn, with mammal herbivores physically excluded, valuable resources are allocated elsewhere and the plants defense strategies are relaxed. After five years of separation from browsers, spines were 35% to 40% shorter. The level of investment in rewards offered the ants also relaxed by about 25% for both inflated thorns and active nectaries.

 

The induction and relaxation of defensive mechanisms allows plants to best match the strategy with the current threat and conserve costly resources in the absence of those threats.

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