Overstory #138 - Tree Defences
It's a tough world. Trees face a constant battle in competing for light, water and minerals with surrounding plants. As if that were not enough, they also have to fend off the attention of living things, which view trees as good to eat and places to live. Insects chew away on all parts of a tree and are quite capable of completely defoliating it. Larger leaf-eating animals (which are usually on the ground since a belly full of compost heap is a heavy thing to carry around; leaf eating monkeys are an exception) chew away at the lower parts of the tree, although giraffes can reach up around 5.5 m. Whole armies of animals that can climb and fly will feed on the more nutritious flowers, fruits and the sugar-filled inner bark. The grey squirrel, introduced to Britain from N America in the 1880s, is a prime example. This rodent does extensive damage to hardwoods by stripping bark in spring to get at the sweet sap. It seems that dense stands of self-sown hardwoods have little sap and are largely immune (which may be why it does not cause problems in its native home) but well-tended planted trees have thin bark and a high sap content and are mercilessly attacked. So big is the problem that ash, lime and wild cherry may become more common in Britain because of their relatively low palatability to squirrels at the expense of palatable beech and sycamore.
Other animals are not adverse to making their homes from or in trees. Many birds, including various eagles, have been seen tearing off sizeable branches for nest making. Others live completely off the tree. Gall wasp larvae stimulate their host plant to grow galls on whichever part each species specialises in (leaves, flowers, fruits and stems). The nutritious tissues swell up around the grub and give it a home and food supply. Witches' brooms, the mass of densely branched small twigs that resemble a besom lodged in the canopy, are grown in the same way, induced by a range of different organisms: sometimes by fungi (e.g. Taphrina betulina in birches), mites, or even in N America by dwarf mistletoes (such as the Arceuthobium species). To this list of parasites you can add a number that normally live off the tree's roots and are only seen when they flower, such as the broomrapes (Orabanche spp.) and toothwort (Lathraea squamaria) of Europe. European mistletoe (Viscum album), in contrast, is only partly parasitic; it just takes water from the tree's plumbing and grows its own sugars by using its green leaves.
Epiphytic plants such as lichens and ivy merely use the tree as a place to grow up near the sun. In theory they take nothing from the tree. However, there is some evidence that tropical epiphytes, including a range of bromeliads and orchids, may intercept nutrients washed from the tree's leaves and branches by rain which would otherwise eventually reach the tree's roots. This 'nutritional piracy' may be significant to tree health in the tropics and explains 'canopy roots.'
As if all this was not enough, diseases also take their toll. Fungal diseases, especially of the roots and stems, are particularly important in tree health. Bacteria and possibly viruses also play a role. For example, 'wetwood' is a bacterial rot creating pockets of moist rot with plenty of methane.
Being firmly rooted in the ground, a tree cannot move to escape harsh conditions or the unwanted attentions of animals and diseases (seeds are the only parts with an option for movement). And there are no two ways about it, because trees are so long-lived they face a tremendous number of problems over their lives. They have therefore developed an impressive array of defences to protect themselves where they stand. The living skin of a tree is normally no older than two or three decades, and the leaves, flowers and fruits are usually even shorter-lived. Consequently, the defences of these parts are similar to those in non-woody plants. The real specialisation of defence comes in maintaining the woody skeleton which may persist for centuries or even millennia. We'll consider these defences in turn.
First-line defences: stopping damage
Physical defences: spines, thorns and prickles
Large herbivores are capable of eating copious quantities of leaves along with the odd twig. Indeed the low nutritional value of leaves requires large volumes to be eaten. The chief defences against these large herbivores are spines, thorns and prickles. Spines and thorns (which botanically are the same thing) are modifications of a leaf, part of a leaf, or a whole stem. In Berberis species (as in cacti) it is the leaf that is turned into a spine; this leaves the branch with no green leaves so new ones are produced from the bud in the leaf axil growing out this year rather than next as is normal. Holly (Ilex aquifolium) could be regarded as a half-way house with just the margin growing spines. In false acacia (Robinia pseudoacacia) it is just the stipules (outgrowths of the leaf base) that are modified into persistent thorns. Hawthorns (Crataegus spp.) and firethorns (Pyracantha spp.) have thorns above the leaf, showing that the thorn is a modified branch, grown out from the bud. Growth next year can still take place because these trees cunningly grow extra buds beside the thorns. Again there are half-way houses; some brooms (Cytisus spp.), blackthorn (Prunus spinosa) and other Prunus species produce thorns at the ends of normal branches.
Spines and thorns are expensive things to produce and will only be grown where they are needed. Since most leaf-eaters stand on the ground to eat, simply because they are too heavy to climb or fly, it is perhaps not surprising that trees like holly produce leaf spines mostly on the lower 2 m of the canopy. But why do thorns appear in some trees but not others? Peter Grubb of Cambridge University proposes a solution to what appears to be a haphazard distribution. First of all, physical defences are found in habitats where nutritious growth is scarce. This explains why thorny plants are found in deserts and heathlands (e.g. gorse). But it also explains why thorns are found on plants that invade gaps in forests, e.g. hawthorns, apples, honey-locust (Gleditsia triacanthos), and false acacia (Robinia pseudoacacia): food is scarce because other vegetation is out of the reach of large herbivores. The same principle applies to European holly, which, being an evergreen in a background of deciduous trees, provided scarce fodder in the winter. (Indeed around the southern Pennines in England, shepherds would cut the upper branches of holly (no prickles!) as fodder, particularly in spring when other foods were not available.) Finally the spines common around the growing points of palms and tree ferns are well worth the investment because they have only one growing point which, if damaged, means death to the plant.
Other physical defences
Trees have not been slow in evolving other physical defences. Hairs help deter insects attracted to young vulnerable growth. In some cases this is a physical barrier (like us having to chew through a pillow to get food), in others the hairs contain chemical deterrents (as in the nettle-like hairs of the Australian rainforest stinging trees, Dendrocnide spp.).
Other physical defences can be quite subtle. For example, because of the arrangement of their mouths, caterpillars have to eat a leaf from the edge; so holly has evolved a thickened margin to prevent the caterpillars getting a start. So why haven't more trees evolved a similar mechanism? Probably because the cost is only worthwhile in long-lived evergreen leaves and these are usually protected by chemicals instead. Physical defence can be yet more subtle. One example of many is the subterfuge indulged in by passion flower vines (Passiflora spp.), which are eaten by the caterpillars of heliconid butterflies. The butterflies rarely lay their yellow eggs where there are already plenty (this would be pointless competition for food amongst the caterpillars) so the vine grows imitation yellow eggs on its leaves!
Ants and other beasties
Around the world, there are many examples of tropical trees that use ants as their main defence. The ants run around the tree preventing birds and animals from nesting, dissuading herbivores, cutting away epiphytes and lianas, and in some cases killing anything growing within a 10 m radius of their tree. These ants usually have vicious stings as is rapidly found out if you lean against their tree! In return the tree provides food and often a home as well. For example, the 'ant acacias' of the New and Old World have swollen thorns in which ants hollow out nests. Nectar is delivered from leaf stalks ('extrafloral nectaries', i.e. nectaries not in flowers) and protein from bright orange bodies produced at the tips of the leaflets. Ant acacia leaves are less bitter than in other acacias: presumably the cost of looking after ants is cheaper than producing internal deterrent chemicals.
Employing guards can work on a much smaller scale. It is common to find little tufts of hair at the junctions where veins join together on the underside of leaves. A study on the European evergreen shrub Viburnum tinus found 10 species of mite - mostly predators and microbivores (i.e. eating microbes) - living in the crevices (or 'domatia') between these hairs. In other words, the plant is providing homes for mites that help keep other harmful organisms at bay.
Woody plants produce a great variety of chemical compounds to provide protection against other plants, diseases and herbivores big and small. These include alkaloids, terpenes, phenolics (e.g. caffeine, morphine, tannins and resins), steroids and cyanide producers (cyanogenic glycosides). These chemicals can be found in just about any part of the plant (for example, the waxy surface of apple leaves contains toxins to repel certain aphids). They can also be emitted into the air in considerable quantities which explains why pine woods smell so nice and the eucalypt-dominated bushlands of Australia have a blue haze. These chemicals seep out through the bark or the holes in a leaf (the stomata) or ooze out of special glands, often on teeth around the leaf edge (as found in pines, alder, willows, hornbeam, maples, ashes, elms and viburnums) usually when the leaf is young but sometimes (as in crack willow) even late in life. Are these emissions of expensive chemicals just inadvertent leaks or do they have a purpose? Certainly in oaks it allows the trees to communicate with each other: just how is revealed below.
Chemical defences work in several ways. Some are highly toxic and will kill attackers in small doses (referred to as 'qualitative' defences since they work by being what they are rather than how much is there). These include the alkaloids in rhododendron (honey from its flowers is poisonous to humans but is so bitter it's uneatable), and cyanide (hydrocyanic acid) produced, for example, by crushing leaves of cherry laurel (Prunus laurocerasus) and consequently used to effect in an insect-killing bottle. Other chemicals work by building up in the herbivore (called 'quantitative' defences since they become more effective the more there is). Classic examples are phenolic resins in creosote bushes and tannins found in a number of plants. Tannin is a 'protein precipitant' which makes food less digestible so herbivores end up starving and stunted. These often have the effect of causing the herbivore to move elsewhere. Incidentally, it is possible that the endangered British red squirrel is declining not through competition with the introduced N American grey squirrel but because of acorns. Acorns contain digestion inhibitors that greys can disarm but reds can't. Red squirrels do well in conifer plantations feeding on the more nutritious pine seeds and where there are no oaks to give the greys a competitive edge.
Plants are not immune to the effects of chemical defences. Spanish moss (Tillandsia usneoides) that festoons trees in southern USA and South America (and is not a moss but a sophisticated relative of the pineapple) never grows on pines, presumably because of the resins. Perhaps the best known example of anti-plant chemical defences was first reported by Pliny the Elder in the first century AD, who wrote that 'The shadow of walnut trees is poison to all plants within its compass'. Walnuts (and all trees in the genus Juglans) contain the chemical juglone, which, seeping from the roots, reduces the germination of competitors, stunts their growth and even kills nearby plants, resulting in open-canopied walnuts having very little growing under them. Tomatoes, apples, rhododendrons and roses are very susceptible but many grasses, vegetables and Virginia creeper (Parthenocissus spp.) will happily live under walnuts.
Defensive chemicals are costly to produce and store (many are toxic to the plant producing them). Oaks may put up to 15% of their energy production into chemical defences (which explains why stressed trees are most prone to attack: they have less energy available to produce defences). Thus, although there may be large pools of defences swilling around (preformed) where attacks are common and ferocious, it makes evolutionary sense in lesser situations to produce the chemicals in earnest only when they are needed (induced). Oaks (and a number of other trees) will tick along with low concentrations of tannins in their leaves but once part of the canopy is attacked, the tree will produce tannins in large quantity. More than that, the tannins released into the air are detected by surrounding oaks and they in turn will produce more tannins in preparation for the onslaught. In a similar way, willows in Alaska browsed by snowshoe hares produce shoots with lower nutritional concentration and higher levels of lignin and deterrent phenols, thus rendering them less palatable. These 'induced' defences may have an effect for some time; birches can remain unpalatable for up to three years after being browsed.
Defending the woody skeleton
At first sight it may seem that wood does not need much defending: it's pretty inedible stuff. The large quantities of cellulose (40-55%), hemicellulose (25-40%) and lignin (18-35%) are all tough carbohydrates that are quite hard to decompose. Moreover, wood is incredibly poor in protein and hence nitrogen (typically 0.03-0.1% nitrogen by mass compared with the 1-5% found in green foliage). Just how poor wood is as a food is illustrated by the goat moth, whose caterpillars burrow through wood and take up to four years to grow to maturity, and yet they can mature in just a few months if fed on a good rich diet. Despite the starvation diet that wood offers, there are many insects, fungi and bacteria that are capable of living off wood.
Keeping things out: resins, gums and latex
These fluids have a primary role in rapidly sealing over wounds (whether created by insects or by physical accidents) and in deterring animals from forcing their way in. Any animal rash enough to burrow is physically swamped and trapped, and may be overcome by chemical toxicity (though bark beetles are seen swimming through resin apparently unharmed, if a little hindered).
If you have leant against an old pine or handled a cone you will be well aware of the ability of conifers to produce copious quantities of resin. The typical 'pine' smell of conifer foliage comes from the resin. Yews (Taxus spp.) do not contain resin and consequently do not smell.
In most conifers, including pines, Douglas fir (Pseudotsuga menziesii), larches and spruces, the resin is contained in ducts that run through the bark and wood, tapering off into the roots and needles. Others such as the hemlocks, true cedars and true firs, have resin restricted more or less to the bark, although, like other conifers, they are capable of producing 'traumatic resin canals' in the wood after injury or infection. In the true firs (Abies) the resin is contained in raised blisters. Cells along the ducts or blisters secrete resin, creating a slight positive pressure, so if the tree is damaged the resin oozes out. Once in the air, the lighter oils evaporate, leaving a solidified scab of resin over the wound.
Fulfilling a similar function, a wide range of woody plants produce gums. The family of Anacardiaceae is notable for gum-producing trees including the varnish tree (Rhus verniciflua), a native of China, whose gum is used as the basis of lacquer. Gums are also found oozing from wounds in a variety of temperate trees such as those in the genus Prunus (the cherries, plums, etc.). These gums are carried in ducts, which, as in the conifers, are in the bark and often the wood where they follow the rays and grain. Traumatic canals can be formed in the wood of some hardwoods, for example, sweet gum (Liquidambar styraciflua) and cherries.
Latex (a milky mixture of such things as resins, oils, gums and proteins) is found in different plants from fungi to dandelions to trees. Many types of trees and shrubs have had their latex collected for making rubber (including the 'rubber tree', Ficus elastica, now grown commonly as a house plant). Around one third of rainforest trees have latex. The best commercial supply comes from Hevea brasiliensis in the spurge family (Euphorbiaceae) native to the Amazon and Orinoco river valleys of South America. The rich latex of this tree is about 33-75% water and 20-60% rubber. Latex is found in special ducts (lactifers) running through the bark in concentric circles. As with resin in conifers, the latex is under slight pressure, ensuring that any wound is sealed by coagulating latex (including those made deliberately to collect the latex, and which are treated with an anticoagulant to ensure a good collection).
The production of resins, gums and latex is often insufficient to seal over large wounds such as the breakage of large branches and the removal of areas of bark by, for example, squirrels. If the wound is kept artificially moist (by covering with plastic, lanolin or other non-toxic substance) so that the living cells of the rays do not dry out, a new bark will regenerate in the same season in many species. (Note, though, that in pruning off branches it is only the younger parenchyma cells of the sapwood around the edge of the stump that are capable of doing this, leaving a hole in the centre.) But if the newly exposed parenchyma cells are killed by toxic materials in paint or allowed to desiccate, then the wound can only be covered by the slow growth of callus tissue from the living cambium and rays around the edge. As the callus grows, it starts off uniformly around the wound but the sides tend to grow more rapidly resulting in a circular wound ending up as a spindle-shaped scar. Once the sides of the callus meet, the cambium joins so new complete cylinders of wood are again laid down underneath the scarred bark.
Unlike animals, trees cannot heal wounds; they can only cover them up. Once wood starts to rot it cannot be repaired. New wood can be grown over the top and look healthy but the rotting wood underneath is still there. Hence the wise saying quoted by William Pontey in The Forest Pruner (1810), 'An old oak is like a merchant, you never know his real worth till he be dead'.
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This excerpt was reprinted with the kind permission of the author and publisher from:
Thomas, P.A. 2000. Trees: Their Natural History. (c) Cambridge University Press.
About the author
Peter Thomas is a lecturer in environmental science at Keele University, UK, where his teaching encompasses a wide range of tree-related topics including wood structure and identification, tree design and biomechanics, tree ecology and identification, and woodland management. His research interests focus on the reconstruction of past environments from tree rings, the role of trees in nature conservation and the interaction of fire with trees. He can be contacted at: Huxley Building, School of Life Sciences, Keele University, Staffordshire ST5 5BG, UK; Email: firstname.lastname@example.org.
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