The defences of tropical forest trees

Plants are attacked by a wide range of herbivores and pathogens. These vary in size, specificity and many other characteristics. It is therefore not surprising that plants normally possess many different sorts of defence.

Some of these are outlined with examples from tropical forest trees below. More examples can be found in Chapter 5. Harborne (1993) and Bennett & Wallsgrove (1994) provide good reviews of plant defences.

Toughness/Fibre defence

In order to ingest and digest plant material, herbivores need to comminute leaves and other parts that they eat. Materials that increase the strength and toughness of the plant body increase the work to be done by the herbivore during feeding. Cell wall materials such as cellulose and lignin are the major structural components of plant tissues. The disposition of cells with thick walls is the major determinant of the mechanical properties of plant tissues (Lucas et al. 1995). In addition, structural compounds provide the fibrous portion of the plant material, a component that most herbivores are unable to digest. Therefore toughening also reduces food quality because the nutritious material is diluted by indigestible fibre.

mode of action

In most leaves it is the veins that are the toughest part (Lucas et al. 1991a; Choong etal. 1992; Choong 1996). The fibres in the vascular bundles provide toughening to the lamina, making it more difficult for herbivores to cut or tear pieces out of the blade. Sclerenchymatous bundle sheaths may protect veins from phloem-sucking insects and also effectively compartmentalise the lamina, preventing a chewing insect from having an unhindered path in its grazing and reducing the digestibility to larger animals.

distribution among tropical trees

In general, tropical forest trees have leaves of high toughness and fibre concentration (Coley & Barone 1996).

case studies

A range of generalist folivores from tropical rain forests have been shown to select low fibre/toughness leaves of relatively high protein content from among those available in their habitat; examples are howler monkeys (Milton 1979), leaf monkeys (McKey et al. 1981; Davies et al. 1988) and lowland gorillas (Rogers et al. 1990). Caterpillars avoid eating the tough veins of Castanopsisfissa (Choong 1996).

Spines defence

The presence of hard, sharp parts among the foliage may deter animals from coming too near the plant. The terms spine, thorn and prickle and others variously used for spiny plant parts await a universally agreed system of application.

mode of action

An animal risks injury when contacting a spiny plant. To be effective, the spine needs to be sharp and relatively hard in order to penetrate animal tissue. Plant spines are often hardened by accumulations of inorganic crystalline substances such as silica or calcium salts. Small animals such as insects may easily move between the spines, rendering them ineffective as defences against such organisms. Spines can also serve as anchorage mechanisms in climbing plants, and as modifiers of a plant's energy and gas exchange with the environment, but in tropical trees it seems likely that defence is the main role of armature.

distribution among tropical trees

Not many trees in the tropical rain forest are spiny. Perhaps the most notable group is the palms.

case studies

The prickles found on young shoots of Hawaiian treelets in the genus Cyanea have perplexed ecologists because the Hawaiian archipelago is devoid of indigenous mammalian herbivores. Flightless grazing ducks and geese, exterminated after Polynesian settlement of the islands, have been invoked as the selection pressure for these defences (Givnish et al. 1994).

Hairs defence

Plant trichomes can play a defensive role (Levin 1973).

mode of action

Hairs can be likened to spines at a reduced scale. A layer of hairs may physically obstruct insects and other tiny herbivores from attacking a plant. They may be hard and sharp through inclusion of crystalline materials, strongly hooked, sticky, stinging or contain toxins.

distribution among tropical trees

Densely pubescent leaves are not common among tropical rain-forest tree species. Of 205 species from Venezuela studied by Roth (1984), only 15 had a dense covering of hairs, and this was restricted to the leaf undersurface in all cases. Tropical trees possessing stinging hairs are confined to the Urticaceae tribe Urticeae and possibly a few Euphorbiaceae (Thurston & Lersten 1969).

case studies

Streblus elongatus has short razor-sharp silicified hairs on the undersides of the leaves (Lucas & Teaford 1995). The urticaceous genera Dendrocnide and Urera are the largest groups of stinging species among tropical rain-forest trees, with species of the former, notably Dendrocnide stimulans, inflicting stings of tremendous ferocity.

An interesting twist to the defensive role of pubescence was reported by Letourneau et al. (1993). They investigated the ant-protected euphorb En-dospermum labios in New Guinea. This species occurs as what were believed to be genetically determined glabrous and hairy morphs. The hairless morph suffered less herbivory that the pubescent one because glabrous plants were more likely to be inhabited by ants. Colony foundress queens did not appear to make plant-host choice based on pubescence, but they were more successful on the glabrous morph. Letourneau et al. (1993) hypothesised that parasitic flies had more chance of egg laying on queens that were delayed by the presence of indumentum in making their nest chamber in the stem. Later research revealed that the hairy phenotype was actually an induced response to attack by stem-boring insects (Letourneau & Barbosa 1999).

Crystal inclusions defence

Plant tissues can contain crystalline inclusions, generally of calcium oxalate, calcium carbonate or silica. These can vary in shape from rounded to sharply pointed structures.

mode of action

Sharp or pointed crystals can pierce animal tissues and membranes, causing injury. Otherwise the hardness of crystalline inclusions can wear teeth or mandibles of herbivores.

distribution among tropical trees

Many plants in the rain forest possess crystalline inclusions; for instance, the leaves of 20 out of 22 species investigated in Venezuela possessed crystals (Pyykko 1979). Approximately 300 out of 1300 species examined from South America (mostly Surinam) had silica grains in the wood (ter Welle 1976).

case studies

Lucas & Teaford (1995) proposed that siliceous deposits in leaves could cause microwear on the teeth of long-tailed macaques.

Gums, latexes and resins defence

Many plants secrete sticky fluids when parts are broken. These may be distasteful or toxic, but they can also deter herbivores through the physical property of stickiness or by hardening rapidly on exposure to air.

mode of action

Many plant species possess canal systems running through their tissues that contain resins, latexes or gums. When the plant is damaged the contents of the canals pour into the wound. They may even be under pressure and squirt out rapidly. Gluey secretions can gum up insect mouthparts. The latexes may also dry and harden, making removal difficult. Herbivores may waste time and energy cleaning themselves of sticky exudates from wounds that they create in the plant.

distribution among tropical trees

Exudates are relatively common among tropical trees. For instance, at four sites in Gabon the proportion of species producing latex or resin from the trunk ranged from 16 to 35% (Reitsma 1988). Families such as the Apocynaceae, Sapotaceae, Guttiferae and Moraceae, and Euphorbiaceae subfamily Crotonoideae are well known for latex production. The Bur-seraceae and Dipterocarpaceae are highly resinous. Farrell et al. (1991) noted that latex- or resin-canal-bearing taxa tend to be larger than sister groups without canals. This, they argued, reflected the evolutionary success of groups that evolved laticiferous defence.

case studies

Six out of seven tree species growing on Barro Colorado Island, Panama, inflicted with wounds to the trunk responded by latex or resin production which led to the formation of a scab of dried exudate over the wound, protecting it from infection (Guariguata & Gilbert 1996).

Non-protein amino acids defence

Plants may contain amino acids other than those usually incorporated into proteins by animals. These can be extremely toxic (D'Mello 1995).

mode of action

The non-protein amino acids when ingested by herbivores often act as analogues to specific amino acids used in protein synthesis. The plant amino acid molecules become incorporated into proteins, but the substitution leads to proteins that cannot fulfil their normal function, thus disrupting, sometimes irreparably, cell biochemical machinery.

distribution among tropical trees

The legumes are the major group that employs non-protein amino acids as a defence. These defences are often found in seeds, but may also occur in leaves and other plant parts.

case studies

Canavanine is a non-protein amino acid quite widespread in the legumes. It is an analogue of arginine. Hypoglycin A is found in unripe fruits and the funicle of the sapindaceous tree Blighia sapida. The seeds of Cycas circinalis contain a P-N-methyl amino acid that is associated with degeneration of the brain in Pacific Islanders who eat seeds improperly prepared. Seeds of Lecythis ollaria contain seleno-cystathionine, a toxic analogue of cys-

Table 2.7. Presence of alkaloids in mature leaves among tropical rain-forest tree floras

Number of species

Percentage

Site

tested

alkaloid-positive

Reference

Douala-Edea, Cameroon

70

16

Waterman & McKey

(1989)

Kibale, Uganda

20

50

ditto

Makokou, Gabon

160

7

Hladik & Hladik

(1977)

New Guinea

108

6

Hartley et al. (1973)

(as cited by Hladik & Hladik

(as cited by Hladik & Hladik

tathionine where selenium replaces sulphur in the amino acid molecule (Aronow & Kerdel Vegas 1965).

Alkaloids defence

Alkaloids are ^-heterocyclic molecules, generally with a biosynthetic origin in amino acids (Hegnauer 1988; Waterman 1996). They are complex and varied in structure and biochemical properties, but tend to be bitter to taste and are frequently toxic to animals.

mode of action

Toxic effects of alkaloids on animals include inhibition of DNA and RNA synthesis, inhibition of mitosis, breakdown of ribosomes and cell membranes and interference with nerve transmission.

distribution among tropical trees

Some surveys for alkaloid-bearing species have been conducted in tropical rain forests (Table 2.7). These have mostly shown quite low frequency of alkaloid-positive species. The one exception, although this has a smaller sample size than the rest, is the rain forest at Kibale, Uganda, where half the species tested gave positive results. Despite the general rarity of alkaloidal trees, lowland tropical rain forests have the highest frequency and mean concentration of alkaloids of any major vegetation type (Levin 1976). Alkaloids are found in many evolutionary lines and occur in about 20% of higher plant families, but abundant alkaloids are particularly associated with the Annonaceae, Apocynaceae, Lauraceae, Leguminosae, Rubiaceae, Rutaceae and Solanaceae among tropical trees (Hartley et al. 1973; Levin 1976). On the other hand, Guttiferae, Melastomataceae, Myrsinaceae, Myr-taceae and Sapindaceae are notable for the presence of few, if any, alkaloidal species.

case studies

Humanity has found uses for many plant alkaloids as medicines, stimulants and poisons. The arrow poison curare is derived largely from menisper-maceous vines, but the arborescent legume genus Erythrina contains alkaloids with a similar toxicology (Chawla & Kapoor 1997). The main ordeal poisons of Africa are another group of legume alkaloids from the genus Erythrophleum (Neuwinger 1996). Tropical trees have been domesticated for their alkaloidal properties among other things, for example caffeine (Coffea, Cola, Theobroma) and quinine (Cinchona).

Cyanogenesis defence

Cyanide is extremely toxic to nearly all living organisms. Cyanogenic glyco-sides generate cyanide when the CN group they contain is cleaved from the sugar moiety of the glycoside molecule. They provide the commonest form of cyanogenesis in plants.

mode of action

When a herbivore attacks a cyanogenic plant either enzymes that are released by the plant, or the digestive enzymes of the herbivore, break down the glycoside, releasing the poisonous cyanide. Cyanide interferes with the cytochrome system thus inhibiting the terminal part of the main respiration pathway in cells.

distribution among tropical trees

Cyanogenesis is not common among tropical trees. In a survey of 430 tree and shrub species from Costa Rica only 20 (4.7%) tested positive for cyanide production (Thomsen & Brimer 1997). Woody plants probably show a lower frequency of cyanogenesis than herbaceous species. Taxonomically, cy-anogenesis seems quite widely spread with families such as Annonaceae, Elaeocarpaceae, Euphorbiaceae, Flacourtiaceae, Leguminosae, Proteaceae, Rosaceae, Rubiaceae, Sapindaceae and Sapotaceae containing cyanogenic members (Hegnauer 1977; Thomsen & Brimer 1997).

case studies

Leaves of two Central American Acacia species were found to be toxic to the southern armyworm (Podenia eridania, Noctuidae) owing to the presence of cyanogenic glycosides (Rehr et al. 1973).

Phenolic compounds defence

Phenolic compounds are substances that possess one or more hydroxyl group (OH) bonded into an aromatic ring structure. A wide range of complex molecules, mostly synthesised via the shikimic acid pathway, can be included in the class of phenolic compounds. The simpler molecules include lignans, coumarins and flavanoids. Alkaloids and terpenes can also be phenolics. Tannins are a complex set of polyphenolic compounds. They cause the astringency found in many plant products and are the group of phenolic compounds most often associated with plant defence.

mode of action

Tannins possess the ability to cross-link protein molecules. This property is employed in their traditional use of tanning leather. Tanning makes the leather much less susceptible to microbial decay by binding the proteins in the animal skin. In plant leaves, the tannins, particularly the condensedtannin group, are believed to help deter herbivores by cross-linking plant proteins in the herbivore's mouth and gut and making them less accessible for digestion, and also by linking to the gut wall proteins and interfering with the process of nutrient uptake.

distribution among tropical trees

Tannins are commonly found in tropical trees. The leaves of tropical trees generally have higher concentrations of condensed tannins than those of temperate ones (Coley & Aide 1991), but assays of total phenolics do not show any major latitudinal difference. Woody plants are more likely to contain tannins than herbaceous ones (Mole 1993), possibly because of a biochemical link to lignin synthesis. This may also explain the association of tannins with more primitive angiosperm groups and their relative rarity in advanced clades such as the Asteridae. A few families, such as the Araliaceae and Moraceae, appear to have relatively few members that synthesise them.

case studies

There have been a number of studies of the role of tannins in the defence of juvenile stages of tropical trees, but little concerning adults.

Terpenes defence

Terpenes are perhaps the largest and most diverse group of plant secondary chemicals. They share a common biosynthetic origin in mevalonate and are based on a five-carbon molecule, which can be repeated from a few to many times to produce different chemicals. C10 molecules are referred to as mono-terpenoids, C20 as diterpenoids and so on. The larger polymers are the basis of plant resins and essential oils. Terpenoid glycosides, including cardiac glycosides, and saponins are among the most toxic of the terpenes found in plants.

mode of action

Terpenoids probably have a wide range of functions, but there is evidence that some are toxic, deterrent or inhibitory to herbivores and pathogens. The modes of action are varied, but include an evidently unpleasant bitter taste and irritation of animal tissues. Pyrethroids are a group of monoterpenoids that are effective 'knock-down' insecticides.

distribution among tropical trees

A number of families are particularly rich in highly resinous tropical trees, notably the Burseraceae, Dipterocarpaceae and Leguminosae. Tropical conifers are also very resinous. Essential oils are abundant in the Rutaceae.

case studies

The South American legume genus Hymenaea contains the terpenes cary-ophyllene and caryophyllene oxide. The former has been demonstrated to be an effective deterrent to insect herbivores, whilst the latter inhibits the growth of leaf-spotting fungi (Langenheim 1994) and makes leaves repellent to leaf-cutting ants (Hubbell et al. 1983) presumably because of its anti-fungal properties.

Ants defence

Myrmecophytic plants have a mutualistic relationship with ants. Ants are often highly aggressive to invaders of their territories, and myrmecophytic plants harness ant colonies to attack and deter potential herbivores.

mode of action

The trees elicit protection from ants by providing food and/or accommodation. Food is presented in the form of sugary secretions from extra-floral nectaries (EFNs) and food bodies (Fig. 2.21), typically found on young leaves and shoots. Many myrmecophytes provide domatia (little houses) for ants. These are usually pouch-like leaves or stipules, or hollow petioles or twigs. The ants seem particularly aggressive protectors around their nest site and can be effective defenders of host trees.

distribution among tropical trees

EFNs are quite common in tropical trees: 12% of species (91/741) at Pasoh, Peninsular Malaysia (Fiala & Linsenmair 1995), with most of the EFNs (70%) occurring on the leaf blades. The frequency of EFNs increased with tree stature at Pasoh (Table 2.8), possibly indicating that canopy trees can better spare the photosynthate delivered to the nectaries. An even higher proportion of species (33%) on Barro Colorado Island were found to possess EFNs (Schupp & Feener 1991), but the sample was strongly biased towards light-demanding species. Both studies found EFNs to be particularly common in advanced angiosperm clades, notably Dilleniidae, Rosidae and Asteridae. McKey & Davidson (1993) record more than 100 genera of tropical trees from 38 families that are regularly symbiotic with plant ants. Most of these provide domatia.

case studies

The benefit of ant-defence is clear from the data of Vasconcelos (1991) who demonstrated a 45-fold higher rate of fruit production in ant-inhabited individuals of the Amazonian melastome shrub Maieta guianensis compared with those from which the resident ant colony was removed.

Small ants may be better defenders against insect herbivores than large ones if reduced individual size translates into a higher density of defenders. This was hypothesised by Gaume et al. (1997), who showed that the African arborescent myrmecophyte Leonardoxa africana was well defended by the tiny ant Petalomyrmex phylax. Paired leaves, one with ants, one without, showed that the ants were very effective at keeping off marauding insect herbivores, including sap suckers that are usually not quantified in such studies (Fig. 2.22).

Figure 2.21 Food bodies being gathered by Crematogaster borneensis ants from under the stipule of a Macaranga trachyphylla plant. The holes in the stem are entrances cut by the ants to the hollow twig which acts as a domatium. Photo: Tamiji Inoue.

Table 2.8. Occurrence of woody plants with extra-floral nectaries (EFN) in Pasoh Forest Reserve, Peninsular Malaysia

See Table 2.1 for a definition of the stature classes.

Table 2.8. Occurrence of woody plants with extra-floral nectaries (EFN) in Pasoh Forest Reserve, Peninsular Malaysia

See Table 2.1 for a definition of the stature classes.

Stature class

Percentage of species with EFN (no. EFN spp./total no. spp.)

'shrub'

2.3

(1/44)

'treelet'

8.8

(9/102)

understorey

13.2

(34/257)

canopy

10.3

(29/286)

emergent

21.4

(12/56)

Data from Fiala & Linsenmair (1995).

Data from Fiala & Linsenmair (1995).

It is not just animals that ants will attack if they come into contact with their host plant. Ant exclusion from trees of Stryphnodendron microstachyum resulted in more leaf damage from pathogens (de la Fuente & Marquis 1999). Ants may also defend the host against other plants. Perhaps the most extreme example of this ant-mediated competition between plants has been reported in some neotropical spreading shrubs belonging to the Melas-tomataceae (Morawetz et al. 1992; Renner & Ricklefs 1998). Morawetz et al. (1992) described how Tococa occidentalis (which is actually Tococa guianen-sis according to Renner & Ricklefs (1998)) is found growing as clones in gaps in lowland forest in Peru. These clones are generally surrounded by clear space with no living plants. The ants (Myrmelachista sp.) that inhabit the hollow stems and leaf domatia of the Tococa actively attack living plants growing near the host. They chew the shoot tips and main nerves of the leaves and spray a chemical from their abdomens into the wounds that they have made (Morawetz et al. 1992). Plants treated in this way, including small trees, die rapidly. Morawetz et al. (1992) concluded that the ants were injecting a potent herbicide into the plants that they attacked. Renner & Ricklefs (1998) report similar observations for clumps of Tococa mixed with another myrmecophytic melastome (Clidemia heterophylla) in Ecuador.

It should not be assumed that ants are always beneficial. Very aggressive ant defenders could deter animals needed by the plant such as pollinators or seed-dispersers (Thomas 1988). Ant defence of Psychotria limonensis on Barro Colorado Island resulted in a 50% increase in fruit set but the amount of mature fruit removed was reduced by 10-20% (Altshuler 1999). In the South American myrmecophytic shrub Cordia nodosa, the common ant inhabitant of the hollow stems actively sterilises the tree by biting off flower buds (Yu & Pierce 1998). The apparent advantage to the ants of doing this is to maintain vegetative growth that allows more domatia and hence a bigger ant colony to develop. Fortunately for the Cordia, not all individuals are inhabited by the 'bad' ant.

Mites defence

Tufts of hairs in the axils of veins, often associated with shallow pits or pockets, are common on leaves (Walter 1996). These domatia are believed to be important as sites in which mites can hide.

Figure 2.22 Effects of ant exclusion on occurrence of insects over 4 days (upper) and on herbivory caused by chewing insects over 14 days (lower) on young leaves of Leonardoxa africana. After Gaume et al. (1997).

mode of action

Many mite species are major pests of plants, sucking the contents out of individual leaf cells. They can also act as disease vectors. However, studies have shown that the mite inhabitants of leaf domatia rarely include plant parasites. Instead, they are used by fungivorous and predatory species (Walter 1996). These mites may act like ant colonies, but at a further reduction of scale. Some may eat fungal spores and hyphae on the leaf surface before they have a chance to attack the leaf. The predatory species will devour the parasitic mites that would otherwise infest the plant. The presence of mite domatia has been shown to influence numbers of predatory mites (Walter 1996).

distribution among tropical trees

Domatia are frequently encountered on the leaves of trees in the tropical rain forest.

case studies

Agrawal (1997) compared two cultivars of avocado pear (Persea americana), one of which had domatia and one which lacked them. Herbivore numbers on the plants with domatia were consistently lower than those without, or where they were experimentally blocked. However, the differences failed to achieve statistical significance.

Was this article helpful?

0 0

Post a comment