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 �haak-en-steek� or �grab and stab� is most appropriate. As
one of the gardeners to tend to The Living Desert�s 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 it�s 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 doesn�t 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 plant�s 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.