**3.2. Tropical forest**

The most recent plant community is the tropical forest (see distribution in **Figure 1**), which originates toward of the end of Cretaceous period when angiosperms take over the plant diversity [66]. In general, plants at tropical forest have a great variety of chemical compounds for defense, principally blends (**Figure 2**, **Table 1**).

In tropics, there is a high herbivory variation explained by multiple syndromes in plant defense strategies, driven by leaf nutritional quality, in relation to nutrition defense [67], where most extreme plants can combine high chemical defense, low nutritional quality and asynchronous leaf expansion, reduces to minimal its vulnerability [68].

At tropical forest, chemical plant defenses have diverged recently and increased their diversity, because there is a high herbivore pressure due to high insect diversity [69]. Tropical forest may hold more than 650 tree species per hectare, in that species interact each, and pests may promote plant diversity including that in leaves of a unique tropical tree, there are hundreds of different chemical defensive compounds. In addition, herbivore diversity and abundance, rates of herbivory, and host specificity are higher in the tropical than temperate plants (see **Table 1** for comparison) [70]. For example, in Amazonian forest canopy, there are concentrations of one to two orders of magnitude in value of foliar phenols, lignin, and cellulose [71].

Tropical forest has been considering an unproductive habitat where plants need investment in defensive traits because they cannot utilize molecules and energy simultaneously to defense, growth, and replacing loss tissues [6, 45, 72]. Then, synergistic interactions among various defensive traits offer an effective resistance, which is reflected with an increase in the simultaneous expression for direct and indirect defenses [73]. When plants exceed the capacity to store, constitutive secondary metabolites could avoid autotoxicity [74].

Mixtures of defensive compounds allow plant increase resistance, including attack from new herbivore-related congeners, considering that species interactions are stronger in tropics [75].

In general, plant species in tropical forest have a high defensive diversity, in which plant species are chemically unique in their communities [76]. It should be noted that chemical compounds implicate in defensive traits, and different interactions between molecules to perform defenses are equally distributed at family, genus, and species level [71].

For example, in tree genus *Inga*, there are a great variety of defensive traits, like phenolics, which includes polygalloylated compounds, polymers of flavan-3-ols with different substitutions, triterpene saponins, and the amino acid tyrosine. Moreover, plant can identify the agent, amount, and timing of damage and produce a particular induced response, and its response differs in low- and high-risk environments [77].

In a temperate deciduous forest at Powdermill Nature Reserve, the leaf damage caused by herbivores and in majority of individual had a low rate than 2% that can be due to low herbi-

Also, in stressful environment like high-elevation alpine plant communities with low temperatures, plant species have asexual reproduction by rhizomes resulting in clones. Clonal species have developed a tolerance strategy against herbivores and reduced investment in

The most recent plant community is the tropical forest (see distribution in **Figure 1**), which originates toward of the end of Cretaceous period when angiosperms take over the plant diversity [66]. In general, plants at tropical forest have a great variety of chemical compounds

vore densities and poor degrees of specialization thereof [64].

**Figure 2.** Characteristics related to defense mechanisms in plants of different environments.

for defense, principally blends (**Figure 2**, **Table 1**).

chemical defense [65].

98 Pure and Applied Biogeography

**3.2. Tropical forest**

Amino acid tyrosine can be redirected into other primary and secondary metabolites, and its accumulation in excess in young leaves may not be adaptive as they would persist once the leaf was full size and protected by toughness [78].

Another genus well characterized about its defensive chemical compounds is *Piper*, which is broadly represented at tropical forest in the world [79]. The most bioactive compounds reported by *Piper* are amides, a group nitrogen-based compounds stored at leaves and fruits to defend that genera against herbivores [80]. In *Piper,* prenylated benzonic acid, chromene, and dimeric chromane at concentrations higher than 10% of dry weight of leaf material that compounds have synergistic or additive effect against herbivore attack also have been reported. In addition, concentration of these metabolites is correlated with increasing elevation in relation with UV exposure and photoactive properties, and more toxic plants support a lower diversity of specialist herbivores [81].

Sometimes, plant species can response locally to different herbivores, as *Datura stramonium*: the plant can be eaten by generalist and specialist herbivores at great geographic range and produces the alkaloid atropine and its derivate, less toxic, scopolamine. The secondary plant compound is more effective against herbivores specialist, but the precursor is still effective against generalist. Then, when there is a community of generalists, *D. stramonium* produces atropine [82].

Another example is *Zamia stevensonii*, which produces azoxy glycosides (AZGs), highly toxics with mutagenic and carcinogenic properties. AZGs are an excellent defense against generalist herbivores, but are not sufficient to specialist [83].

Another important group of chemical compounds, relevant in defensive plant traits, is phenols, including tannins, which at media concentration reduces herbivory, through reduction of digestibility of plant tissues, and increases immune responses [84].

Terpenes also are present at plants in tropical forest; in that, these compounds protect against abiotic factors such as light, heat, and drought and against herbivores. In Borneo rain forest, foliar terpene presence in 73 of 75 plant species has been analyzed (97%), 15 monoterpenes and 65 sesquiterpenes. This suggests that terpenes can be a favorable selective trait in rainforest [46].

Currently 25,000 structures of terpenes approximately have been reported; some of them are volatile and can be synthesized de novo or are stored in leaves, stems, and trunks and are released in response to attack [85]. One plant can release a highly complex blend, which can include up to 200 volatile terpenes, and its effect is due to direct toxicity, repulsion to herbivores, or attraction of herbivore enemies [85, 86].

Among defensive traits in tropical forest, some are strongly correlated with herbivore damage: leaf size, shearing resistance, cellulose, and ash content. Then, large leaves are more susceptible to herbivory. Other three factors—shear toughness, cellulose content, and ash, which is a mixture of calcium oxalates and phytoliths—reduce herbivore damage acting as structural defenses. These strategies are very efficient and have a relatively low energetic cost [45].

Interestingly, lianas have increased cover and abundance. That plant forms are genetically predisposed to reduce structure and defense traits for investment more in chemical implicates in growth and light capture, wherewith lianas response to stress conditions, like warmer and drier conditions [87].

At tropical forest are common indirect defenses to reduce herbivore attack. In that way, plants provide house, nourish or attract organisms like ants or parasitoids [88], by production of refuges or nesting sites, extrafloral nectar, food bodies or/ and volatile compounds (VOCs) [89].

For example, extrafloral nectar production increases in herbivory and diminishes in the herbivore absence, because that is the secret to attract predators like ants, who defend their food sources and parasitoids. Extrafloral nectar consists in sugars, proteins, lipids, mineral nutriments, and antioxidants and can attract organisms like mites, ladybird beetles, wasp, lacewing larvae, and spiders [90, 91].

VOCs also attract other organisms to improve defense, can attract pollinators, repel herbivores, and are used by plants for communication among them [92, 93] to alert of a possible future attack [94].
