Simarouba amara

Simarouba amara is a species of tree in the family Simaroubaceae, found in the rainforests and savannahs of South and Central America and the Caribbean. It was first described by Aubl. in French Guiana in 1775 and is one of six species of Simarouba. The tree is evergreen, but produces a new set of leaves once a year. It requires relatively high levels of light to grow and grows rapidly in these conditions, but lives for a relatively short time. In Panama, it flowers during the dry season in February and March, whereas in Costa Rica, where there is no dry season it flowers later, between March and July. As the species is dioecious, the trees are either male or female and only produce male or female flowers. The small yellow flowers are thought to be pollinated by insects, the resulting fruits are dispersed by animals including monkeys, birds and fruit-eating bats and the seeds are also dispersed by leaf cutter ants.
Simarouba amara has been studied extensively by scientists in an attempt to understand the tree and also to gain a better understanding of the ecology of the rainforest in general. Many of these studies were conducted on Barro Colorado Island in Panama or at La Selva Biological Station in Costa Rica. Of particular interest is how it competes with other species and with individuals of the same species at different stages in its life cycle. The seedlings are normally limited by the amount of light and nutrients found where they are growing and the saplings are considered relatively light demanding compared to other species. Young individuals are more likely to survive when they grow further away from their parents and when there are few other individuals growing near to them, which may be due to them being able to escape diseases. Plant physiologists have investigated how the leaves of the tree differ depending on their location in the forest canopy finding they are thicker in the canopy and thinner in the understory. They have also measured how the water potential of their leaves changes and when their stomata open and close during the day; the findings suggest that rather than closing their stomata to control water loss, it is controlled by the leaf area instead. Population geneticists have examined the way in which its genes vary, at both the local scale and across its range using microsatellites. It is genetically diverse, indicating gene flow occurs between populations and seeds can be dispersed up to 1 km. The leaves of S. amara are eaten by several species of caterpillar, particularly those in the genus Atteva. Several species of termite and ants live on or around the tree and lianas and epiphytes grow on the tree.
The bark of S. amara has been used by people in its range to treat dysentery and diarrhea, as well as other diseases, and was also exported to Europe in the eighteenth century to treat these illnesses. A number of compounds have since been isolated from the bark and have been shown to have antimicrobial effects. Local people use the wood of the tree for various purposes and it is also grown in plantations and harvested for its timber, some of which is exported.

Description

Simarouba amara grows to heights of up to 35 metres, with a maximum trunk diameter of 125 cm and a maximum estimated age of 121 years. It has compound leaves that are each around 60 cm long, the petioles are 4–7 cm long and each leaf has 9–16 leaflets. Each leaflet is 2.5–11 cm long and 12–45 mm wide, with those towards the end of the compound leaf tending to be smaller. The flowers occur on a staminate panicle that is around 30 cm in length, which is widely branched and densely covered in flowers.
The flowers are unisexual, small and pale yellow in colour. They are thought to be pollinated by insects such as small bees and moths. On Barro Colorado Island, Panama, it tends to flower during the dry season from the end of January to the end of April, persisting for 11 to 15 weeks each year. In Costa Rica, it flowers slightly later, between March and July, peaking in April. Fruits form between 1 and 3 months after pollination occurs. The fruits are brightly colored green to purplish-black, approximately 17 mm long and contain large seeds, they occur in groups of 3–5 drupes. The seeds cannot stay dormant and are dispersed by vertebrates. Each seed weighs approximately 0.25 g.
It is an evergreen species, with a new flush of leaves growing between January and April, during the dry season, when the highest light levels occur in the rainforest. This phenology is thought to allow S. amara to photosynthesise most effectively, since the new leaves are more efficient than those they replace. It has visible, but indistinct growth rings that are on average 7 mm wide. A study of individuals in Panama found that they grow on average 8.4 mm in diameter each year, in Costa Rica, growth rates as fast as 18 mm per year have been recorded, and the stem grows constantly throughout the year. The xylem vessels in mature trees range from 20 to 90 μm in diameter, with around 50 vessels present per mm² of branch. The density of the wood is between 0.37–0.44 g/cm³, lower than many other species in the rainforest.
It is a fast-growing, light-demanding and shade-intolerant species. Saplings are typically one straight pole, with several compound leaves and only one point of growth. This allows the sapling to achieve the greatest vertical growth with a minimum amount of biomass. They start to branch once they are 2–5 m tall.
A study in the forest dynamics plot on BCI found that between 1982 and 2000, around 65% of individuals died, with mortality highest amongst small individuals. Large trees are relatively rare, averaging 2.4 trees per hectare, compared to 40 trees per hectare of >1 cm dbh.

Taxonomy

Simarouba amara was first described by Jean Baptiste Christophore Fusée Aublet in French Guiana in 1775 and is the type species of the genus Simarouba. In 1790, William Wright described Quassia simarouba, which Auguste Pyrame DeCandolle suggested was the same species as S. amara. However, because S. amara was described as monoecious by Aublet and Q. simarouba was described as dioecious by Wright, they were still regarded as separate species in 1829. By 1874, when the Flora Brasiliensis was published, they were considered synonymous.
Among the six species of Simarouba, two besides S. amara occur on the continent: S. glauca and S. versicolor. S. amara can be distinguished from the other continental species by having smaller flowers, anthers and fruit, and straight, rather than curved petals. The leaves of Simarouba amara subsp. opaca are not glaucous on their underside, whereas those of Simarouba amara subsp. typica are.

Structure	S. amara	S. glauca	S. versicolor
Flower	3–5 mm long	4–7.5 mm long	4–7.5 mm long
Anthers	0.4–1.2 mm long	1.3–2.0 mm long	1.0–1.5 mm long
Petals	Straight, dull yellow-green to white	Curved, brighter yellow with a touch of orange or red	Curved
Fruits	1.0–1.5 x 0.6–1.0 cm	2.0–2.5 x 1.2–1.5 cm	2.0–2.5 x 1.5–2.0 cm

Common names

Simarouba amara is known by many common names in the Neotropics. In Bolivia it is known as chiriuana, in Brazil as marupa, marupuaba, parahyba, paraiba and tamanqueira. In Colombia it is called simaruba, in Ecuador as cedro amargo, cuna and guitarro, in French Guiana as simarouba, in Guyana as simarupa, in Peru as marupa, in Surinam as soemaroeba and in Venezuela cedro blanco and simarouba. In Jamaica, it is called damson.
In Europe, it was known by various names during the nineteenth century when it was used as a medicine; these names included bitter ash, bitterwood, mountain damson and stave-wood.

Distribution

The natural range of S. amara is in the Neotropics, the ecoregion of Central and South America. Its range extends from Guatemala in the north, to Bolivia in the south and from Ecuador in the west, to the east coast of Brazil. It has been introduced to the islands of Dominica and Puerto Rico in the Caribbean Sea, becoming naturalised in Puerto Rico. On BCI, mature trees are found at a frequency of 5 per hectare, in Ecuador at 0.7 per hectare and in French Guiana at 0.4 per hectare. Genetic analysis of populations suggests that it has always been relatively common within its range. It grows in rainforests and in savannahs. The seedlings of S. amara are rare in primary forest due to their light-demanding habit.

Genetics

s of S. amara display high levels of heterozygosity indicating that it is genetically diverse. This is consistent with the tree outcrossing over large distances by long-distance pollen flow and that there has been sufficient long distance gene flow between populations to counteract the effects of genetic drift. A study of 478 plants from 14 populations across South America found that 24% of all alleles occurred in only one population. A study of 300 plants on Barro Colorado Island found that the heterozygosity at 5 microsatellite loci varied between 0.12 and 0.75. 8 out of the 50 alleles scored occurred in only one plant.

Reproduction

Individuals do not typically reproduce until they have a trunk diameter of 30 cm. Once mature, the trees produce flowers each year, but not all females produce fruit each year. Their flower morphology is typical of being pollinated by generalist small insects such as bees and moths. It has been reported to be pollinated by non-sphingid moths, but other authors have questioned whether this is correct.

Seed dispersal

The seeds of S. amara are dispersed by vertebrates, mainly large birds and mammals, including chachalacas, flycatchers, motmots, thrushes, howler monkeys, tamarins and spider monkeys. Leaf cutter ants have also been observed to disperse the seeds and dense seedling carpets form in areas where they dump waste material but most of the seedlings die and dispersal by the ants is thought to be unimportant in determining the long-term patterns of recruitment and dispersal. Seeds that are eaten by monkeys are more likely to germinate than seeds that have not. Fruit-eating phyllostomid bats have also been noted to disperse their seeds; this may aid the regeneration of forests as they disperse the seeds of later successional species while they feed on S. amara.
Based on inverse modelling of data from seed traps on BCI, the estimated average dispersal distance for seeds is 39 m. Studying seedlings and parent trees on BCI using DNA microsatellites revealed that, in fact on average, seedlings grow 392 m away from their parents, with a standard deviation of ±234 m and a range of between 9 m and 1 km. In the forest there are many seeds and seedlings beneath reproductive females; genetic data indicate that seedlings are unlikely to be from nearby adults, but rather dispersed there by vertebrates that have fed on one tree and then moved to feed on another, defecating while in the canopy and depositing the seeds.