Syzygium jambos (rose apple)

S . jambos is widely distributed pantropically, but is often rare where introduced and now has limited economic value. It has been spreading on oceanic islands where it is viewed with concern because of its perceived high impact on biodiversity. S. jambos casts a heavy shade, is able to regenerate under forest canopy, and is often found in monospecific stands. It is poorly dispersed and as a result it does not spread rapidly further afield. Once established it reduces plant species richness but tends to favour some native species and produces a habitat conducive to some endemic bird species. As it also suppresses light-demanding species, such as the invasive Lantana camara, in some instances S. jambos should not be eradicated but instead contained and managed to produce habitats favouring endemic species.

In most of the earlier literature the tree is referred to as Eugenia jambos L. or Jambosa vulgaris DC. There are over 500 species in the genus, and S. jambos may be confused with several of them, notably another weedy species, S. cumini. However, the rosewater smell of S. jambos fruit is distinctive for the species.

 

S. jambos is an evergreen small tree reaching a height of 7-12 m, with a generally short bole, to 50 cm diameter. Stems are terete, sometimes quadrangular when young, generally twisted at the base, with brown, furrowed, smooth bark. When young, it has an erect main stem but as it matures, it tends to produce arching branches resulting in a widespread crown with a diameter of over 20 m in multi-stemmed individuals. The leaves are opposite, lanceolate or narrow-elliptic, thinly coriaceous, cuneate at base, acuminate at apex, 10-22 cm long and 2.5-6 cm wide. They are somewhat leathery, glossy, dark-green above when mature but reddish when young, lighter green and obscurely glandular punctate beneath, petiole 5-6(-13) mm. Inflorescences short terminal or axillary corymbs, 5-10 cm long, 4-5(-10)-flowered; flowers large, 5-10 cm wide, white to greenish-white; calyx lobes 4, suborbicular, up to 10 x 7 mm; petals 4, suborbicular, 15-18 mm diameter, white to greenish-white; stamens about 400, up to 4 cm long; style up to 4 cm long; pedicel up to 1.5 cm long. The fruit is a drupe, up to 4-5 cm long, nearly round, oval, or slightly pear-shaped, and is capped with a prominent, green, tough calyx and style. The skin is smooth, thin, green at first, then pale yellow or whitish, sometimes pinkish, and covers a crisp, mealy, dry to juicy layer of yellowish flesh, that is sweet with a distinct scent of rose. Each fruit contains in its hollow centre one to four brown, rough-coated, medium-hard, polyembryonic, more or less rounded seeds, 1-1.5 cm in diameter. When they mature, they loosen from the inner wall and rattle when the fruit is shaken.

The native range of S. jambos is uncertain due to prehistorical introductions. It is thought to be native to South-East Asia, Indonesia, the Philippines and Malaysia, and its natural distribution probably did not extend to India where the tree is thought to have been introduced. Morton (1987) suggested that it was also introduced to the former Indochina region. It is likely that the species is now present in the majority of countries in the humid tropics, although generally very locally and often in low numbers.

In South-East Asia, indigenous people must have spread S. jambos beyond its native range, including to many of the offshore islands long ago, but no records exist. S. jambos was introduced into Jamaica in 1762 and subsequently to much of the neotropics from Mexico to Peru and to most of the Caribbean islands as a fruit tree. By 1821 S. jambos was introduced to tropical botanical gardens throughout the West Indies, including Puerto Rico (Wadsworth 1943). In Puerto Rico, it was originally planted for fuelwood and posts, but its sweet, fleshy fruits made it a horticultural favorite (Wadsworth 1943; Brown et al. 2006). In Puerto Rico it is now particularly common in secondary forest and riparian vegetation (Heartsill-Scalley and Aide, 2003) and it is spreading in Costa Rica (Di Stefano et al., 1998).

In Guatemala the tree has been planted as a living fencepost or in hedgerows around coffee plantations (Morton, 1987) and in many parts of the neotropics it forms dense stands and thickets. It was recorded in Florida, at Jacksonville, prior to 1877 and in California it was planted as far north as San Francisco as an ornamental. In 1825, eight saplings were taken from Rio de Janeiro to Hawaii by ship and in 1853 a United States warship delivered trees from Central America to the island of Hilo. It is thought to have been first planted in Queensland, Australia, in the 1890s (Morton, 1987).

In the Pacific, S. jambos has become invasive on the islands of Kauai, Molokai, Oahu, Maui and Hawaii (Smith, 1985). On the island of Pitcairn it is often reported as a major invasive (Diamond, 1994), but is more a feature of a largely abandoned agroforestry system rather than highly invasive of natural vegetation. In West Africa, it was reported as already cultivated in Ghana in 1893 and it is 'semi-naturalized' in some areas of tropical West Africa (Morton, 1987). It is now a major environmental weed on Indian Ocean Islands and is locally regenerating freely in the East Usambara forests and the islands of Zanzibar and Pemba, Tanzania. It was also introduced to the coastal plain of Israel where it is an ornamental (Morton, 1987).

Concern with this species has only arisen in countries where the awareness of biological invasion is high. In Dade County, Florida, USA, homeowners are encouraged not to plant S. jambos close to native plant communities (Anon., 2000). In Durban, South Africa, it is an escaped ornamental that has been listed as a Category 3 plant 'which have amenity value and which may be grown, but not planted, propagated, imported or traded', and cannot be grown within 30 m of a watercourse (Anon., 2001).

Natural Dispersal (Non-Biotic)

In areas with steep topography fruits can be locally dispersed by gravity down slopes. As the tree is common along some riparian forests, dispersal by water probably occurs. In Hong Kong, it is reported to be invading along some streamsides without anthropogenic disturbance (Leung et al., 2009).

Vector Transmission (Biotic)

It is thought to be dispersed by animals as clumps of naturally regenerated saplings are often found some distance from a seed source. Wadsworth (1943) suggested that bats and rodents may be involved, and on Pitcairn Island Polynesian rats are probably the principal disseminators.

Accidental Introduction

Seeds are dispersed by people, and children in particular, who eat the fruit more readily than adults. When the fruits are consumed during walks in the countryside, viable seeds are discarded mainly along paths.

Intentional Introduction

In recent decades there appears to have been only limited interest in the food or ornamental value of this species, or even no interest in other uses, such as a component of agroforestry systems, therefore very few intentional introductions must have occurred.

A variety of tropical crops are affected by the tree's shallow rooting or the heavy shade produced by spreading branches.

There are over 500 species in the genus, and S. jambos may be confused with several of them, notably another weedy species, S. cumini. However, the rosewater smell of S. jambos fruit is distinctive for the species.

It is unclear what was its original habitat in its native range, but it is now commonly found around homesteads and forms large stands, often pure, in many parts of the tropics. It has been used as an agroforestry tree to form field boundaries and provide shelter, is also known to occur under the canopy of mature disturbed forest, and is also found in pastures and waste places. In El Rodeo, Costa Rica, it has colonized old secondary forests and most preserved forests.

Genetics

The chromosome number is variable, 2n=28, 33, ~44, 46, ~54 and 66 having all been recorded by van Lingen (1991). As the quality and quantity of the fruit produced is variable, Burkill (1997) considered that there is scope for selection. Young seedlings have also been noted as being highly variable (Morton, 1987). Breeding of the tree for the production of essential oils or to enhance its ornamental value has also been suggested, but to-date no such programme has been initiated.

Physiology and Phenology

The tree grows in more or less synchronous flushes, one of which brings on flowering. Flowering occurs after a quiescent period, e.g. in spring in the subtropics, late in the dry season in East Java. The fruit ripens 3 months after bloom. In Puerto Rico and Jamaica, flowering and fruiting occurs nearly throughout the year, though infrequently in the summer (Little and Wadsworth, 1964; Morton, 1987). In the Bahamas and Florida, USA, the main reproductive season is from May through to July. In India, fruiting time varies according to regions; in the south flowering usually occurs in January, with fruit ripening in March and April, whereas elsewhere, fruit ripening takes place in April and May or in July and August. Some varieties produce fruit in February and March (Morton, 1987). In the East Usambara mountains of Tanzania there are two flowering seasons, December to January and September to October and fruits take a few weeks to ripen (Voigt, 1914). Van Lingen (1991) stated that fruits take 3 months to mature.

Reproductive Biology

Little is known about the reproductive biology of S. jambos. Sexual reproduction may start from as early as 4 years old and at higher altitude trees fail to flower and fruit. It is probably solely or mostly bee pollinated and produces a limited amount of fruits per tree, with mature trees yielding 2 kg of fruit each season (Morton, 1987), which probably reflects the low number of large fruit produced rather than pollination failure or fruit abortion. Chantaranothai and Parnell (1994) found that in cultivation, 73% of flowers set fruit and that apomixis and self-fertilization occur freely. The seeds are polyembryonic (producing 1 to 4 sprouts), germination requires 10 to 25 days to start and continues for up to 120 days (Voigt, 1914; Wadsworth, 1943; Schmitt and Riviere, 2002). After one week of drying, seed viability remains high (Schmitt and Riviere, 2002) but there is no seed bank. Trees coppice freely when cut and vegetative propagation can occur via layering. Various propagation techniques are described by Morton (1987). Establishment normally takes place in established vegetation where there is some shade and plenty of moisture.

Environmental Requirements

S. jambos flourishes in tropical and near-tropical climates only. It is found from sea level to around 900-1300 m in Jamaica, Hawaii and India but it is reported to reach an altitude of 2300 m in Ecuador. At its upper altitudinal limit the tree grows vigorously but does not bear fruit (Morton, 1987). It withstands temperatures down to freezing and is tolerant of wind and salt (van Lingen, 1991). Being commonly associated with riparian zones, it does best in damper habitats, yet Morton (1987) reported that it tolerates semi-arid conditions, although dry spells are detrimental. It copes with poor drainage as well as flooding and grows on various soil types, including sand and limestone. However, it is considered that a deep loamy soil with a pH of 5.5-7.0 is ideal (Smith, 1985; Morton, 1987). It tolerates shade but its growth rate tends to be small, about 10 cm per year in height increment in seedlings and saplings (Di Stefano et al. 1998).

S. jambos has few insect enemies, though the fruits are highly susceptible to fruit fly infestations (Leblanc and Putoa, 2000). In humid climates, the leaves are often coated with sooty mould growing on the honeydew excreted by aphids. The tree is also prone to various fungal diseases such as leaf spot caused by Cercospora sp., Gloeosporium sp., and Phyllosticta eugeniae; algal leaf spot (Cephaleuros virescens); black leaf spot (Asterinella puiggarii); and anthracnose (Glomerella cingulata). It is susceptible to root rot caused by Fusarium sp., and mushroom root rot (Armillariella (Clitocybe) tabescens) (Morton, 1987). In Brazil, where it is an ornamental and fruit tree, it is severely damaged by the neotropical rust fungus Puccinia psidii, and this causes premature defoliation, destructive dieback, and loss of flowers and fruits every year around Brasilia (Tessmann et al., 2001).

S. jambos has an indirect economic impact being a species highly susceptible to a number of fruit fly species. It is one of the preferred hosts of the Caribbean fruit fly (Anastrepha suspensa) that attacks several species of tropical and sub-tropical fruit trees. In some Pacific islands it is the host of Bactrocera spp. including Queensland fruit fly (Bactrocera tryoni) introduced in the late 1960s and known to be the most damaging fruit fly pest in Australia (Leblanc and Putoa, 2000). Similarly in Brazil, urediniospores of the rust fungus Puccinia psidii produced on S. jambos may serve as inoculum for other Myrtaceae, including some economically important species such as Eucalyptus spp. and Psidium guajava (Tessmann et al., 2001).

S. jambos has few insect enemies, though the fruits are highly susceptible to fruit fly infestations (Leblanc and Putoa, 2000). In humid climates, the leaves are often coated with sooty mould growing on the honeydew excreted by aphids. The tree is also prone to various fungal diseases such as leaf spot caused by Cercospora sp., Gloeosporium sp., and Phyllosticta eugeniae; algal leaf spot (Cephaleuros virescens); black leaf spot (Asterinella puiggarii); and anthracnose (Glomerella cingulata). It is susceptible to root rot caused by Fusarium sp., and mushroom root rot (Armillariella (Clitocybe) tabescens) (Morton, 1987). In Brazil, where it is an ornamental and fruit tree, it is severely damaged by the neotropical rust fungus Puccinia psidii, and this causes premature defoliation, destructive dieback, and loss of flowers and fruits every year around Brasilia (Tessmann et al., 2001).

S. jambos has an indirect economic impact, being a species which is highly susceptible to a number of fruit fly species. It is one of the preferred hosts of the Caribbean fruit fly (Anastrepha suspensa) that attacks several species of tropical and sub-tropical fruit trees. In some Pacific islands it is the host of Bactrocera spp. including Queensland fruit fly (Bactrocera tryoni) introduced in the late 1960s and known to be the most damaging fruit fly pest in Australia (Leblanc and Putoa, 2000). Similarly in Brazil, urediniospores of the rust fungus Puccinia psidii produced on S. jambos may serve as inoculum for other Myrtaceae, including some economically important species such as Eucalyptus spp. and Psidium guajava (Tessmann et al., 2001). On Pitcairn Island, the spreading, shallow and dense rooting system makes cultivation of gardens next to trees an arduous task and its heavy shading is deleterious to crop growth (Binggeli, 2001). In the Dominican Republic it became a serious weed in Pinus occidentalis forests, preventing the regeneration of the pine (Wadsworth, 1943).

On a number of oceanic tropical islands the presence and perceived spread of S. jambos has been viewed with some concern in recent years. A set of attributes (production of monotypic stands, ability to cast dense shade, and establishment of seedlings under shade) have been viewed as serious threats to native vegetation and associated fauna. On some small islands S. jambos now constitutes a high proportion of the woodland resource. By the 1990s, S. jambos covered a large area of Pitcairn Island and the species was viewed as a major threat to the biodiversity of the island because of its rapid spread into various native plant communities (Diamond, 1994). However, spread into semi-natural vegetation is limited and slow (Binggeli, 2001). It clearly impacts on native vegetation and monotypic stands generally have a very limited number of native species. It outcompetes native trees but also suppresses other invasive species such as Lantana camara. In doing so it allows some native species such as tree ferns (Cyathea medullaris) to regenerate under a S. jambos canopy. However, this regeneration will die unless the canopy opens up.

Once S. jambos produces mature monotypic stands, soil erosion becomes prevalent. The lack of ground vegetation and the near absence of small rootlets allow soil to be washed away even on moderately sloping hillsides. In extreme cases all soil may be washed away from surface roots. On Pitcairn Island, streams originating from unmanaged S. jambos stands have produced deep gullies down to the bedrock.

It is reported as one of the main invasive plant species in Kartala forest, Comoros islands (Yahaya Ibrahim and Mauremootoo, 2003). In the upland landscape of Hong Kong, where it is invading along some streamsides, it is reported by Leung et al. (2009) as the only species of possible current conservation concern. Environmental degradation caused by S. jambos in the Galapagos Islands is assessed by Watson et al. (2010).

The heavy shade cast by S. jambos stands reduces species richness, but does allow some native species to regenerate. It also appears to be an important substrate for some epiphytes. In Puerto Rico, it is the most important phorophyte for a species of orchid (Rodriguez-Robles et al., 1990). It also appears to be a favoured habit for some endemic species. In the Seychelles S. jambos is not listed, like other invaders, as a threat to orthopteroids, a group of large striking insects, instead it is given as one of the host species (Matyot, 1998). The endangered Rodrigues Warbler Acrocephalus rodericanus was mostly found in S. jambos-dominated woodland (Showler et al., 2002). 

Both the seeds and the roots are said to be poisonous. A hydrocyanic acid has been reported in the roots, stems and leaves and an alkaloid, jambosine, has been found in the bark of the tree and of the roots (Morton, 1987).

Fruits of S. jambos were once viewed as desirable and valuable, but today they are of limited economic value especially as trees are very low yielding. The fruits must be freshly picked, bruise easily and are highly perishable, and therefore they are rarely marketed (Morton, 1987). They are slightly bland and around the tropics fruits are mostly eaten out-of-hand by children. In some regions they are made into preserves, jellies or sometimes prepared in a number of other ways, often mixed with other types of fruits (Morton, 1987).

A yellow-coloured essential oil, important in the perfume industry, is derived from the leaves by distillation. In southern Mexico, S. jambos is one of the commonest shade-trees in coffee cultivation (Soto-Pinto et al., 2000). It has been used in agroforestry to provide shelter, as the species is rather wind resistant. It makes a good ornamental species with its regular shape, attractive foliage and striking appearance in bloom, it is a useful avenue tree along driveways, and urban situations. It is an excellent firewood and is commonly used for domestic fuel on Pitcairn Island. Wadsworth (1943) reported that large quantities of S. jambos wood were used as fuel for drying tobacco in Puerto Rico and branches used as tobacco poles in Cuba. Trees coppice profusely and the wood makes very good charcoal. The wood has been used in small quantities for a variety of minor products ranging from baskets to furniture and musical instruments. The heartwood is heavy and hard and is suitable for use as construction timber; however, the wood is very susceptible to termite attack and not durable in the soil.

Seedless fruits were formerly distilled to make 'rosewater' (Morton, 1987), said to be equal to the best obtained from rose petals. On Pitcairn Island, where honey production is one the few exportable products produced by the islanders, S. jambos is probably one of the main sources of nectar for bees. Morton (1987) reported that the honey has a good amber colour and that much was produced in the San Cristobal River Valley in Cuba. The bark contains 7% tannin on a dry weight basis and is used for tanning and dyeing purposes. A number of traditional medicinal uses have been reported (Morton, 1987) such as in tonics or diuretics, but their actual importance to human health is unknown.

Mechanical Control

Seedlings can be removed by hand-pulling (Soria et al., 2002).

Chemical Control

In the Galapagos, S. jambos has been successfully controlled by hacking the plant and applying one of three different herbicides: picloram/methyl metsulfuron, picloram alone in diesel, and pure glyphosate (effective but not very economical) (Soria et al., 2002). In Hawaii, glyphosate was applied to notches in the trunks but was found to be ineffective (Motooka et al., 1983).

Biological Control

S. jambos has never been considered as a target for biological control attempts.

S. jambos grows in more or less synchronous flushes, one of which brings on flowering. Flowering occurs after a quiescent period, e.g. in spring in the subtropics, late in the dry season in East Java. The fruit ripens 3 months after bloom. In Puerto Rico and Jamaica, flowering and fruiting occurs nearly throughout the year, though infrequently in the summer (Little and Wadsworth, 1964; Morton, 1987). In the Bahamas and Florida, USA, the main reproductive season is from May through to July. In India, fruiting time varies according to regions; in the south flowering usually occurs in January, with fruit ripening in March and April, whereas elsewhere, fruit ripening takes place in April and May or in July and August. Some varieties produce fruit in February and March (Morton, 1987). In the East Usambara mountains of Tanzania there are two flowering seasons, December to January and September to October and fruits take a few weeks to ripen (Voigt, 1914). Van Lingen (1991) stated that fruits take 3 months to mature.Little is known about the reproductive biology of S. jambos. Sexual reproduction may start from as early as 4 years old and at higher altitude trees fail to flower and fruit. It is probably solely or mostly bee pollinated and produces a limited amount of fruits per tree, with mature trees yielding 2 kg of fruit each season (Morton, 1987), which probably reflects the low number of large fruit produced rather than pollination failure or fruit abortion. Chantaranothai and Parnell (1994) found that in cultivation, 73% of flowers set fruit and that apomixis and self-fertilization occur freely. The seeds are polyembryonic (producing 1 to 4 sprouts), germination requires 10 to 25 days to start and continues for up to 120 days (Voigt, 1914; Wadsworth, 1943; Schmitt and Riviere, 2002). After one week of drying, seed viability remains high (Schmitt and Riviere, 2002) but there is no seed bank. Trees coppice freely when cut and vegetative propagation can occur via layering. Various propagation techniques are described by Morton (1987). Establishment normally takes place in established vegetation where there is some shade and plenty of moisture.In areas with steep topography fruits can be locally dispersed by gravity down slopes. As the tree is common along some riparian forests, dispersal by water probably occurs. It is thought to be dispersed by animals as clumps of naturally regenerated saplings are often found some distance from a seed source. Wadsworth (1943) suggested that bats and rodents may be involved, and on Pitcairn Island Polynesian rats are probably the principal disseminators.

Control of the tree as a weedS. jambos seedlings can be removed by hand-pulling (Soria et al., 2002). Wadsworth (1943) suggested that infestations in Dominican pine forest should be treated with sodium arsenite, but thought this method would be expensive and had not been tried. In the Galapagos, S. jambos has been successfully controlled by hacking the plant and applying one of three different herbicides: picloram/methyl metsulfuron, picloram alone in diesel, and pure glyphosate (effective but not very economical) (Soria et al., 2002). In Hawaii, glyphosate was applied to notches in the trunks but was found to be ineffective (Motooka et al., 1983). S. jambos has never been considered as a target for biological control attempts.

Whereas some authors maintain that the trees are rather variable, others do not mention this aspect. There are no cultivars. For successful selection or breeding work, a better insight into yield, yield-limiting factors, and variability of characteristics is necessary. Such work may also be directed towards enhancing the production of essential oils or the ornamental value. However, germplasm collections have yet to be established.

Concern with S. jambos has only arisen in countries where the awareness of biological invasion is high. In Dade County, Florida, USA, homeowners are encouraged not to plant S. jambos close to native plant communities (Anon., 2000). In Durban, South Africa, it is an escaped ornamental that has been listed as a Category 3 plant 'which have amenity value and which may be grown, but not planted, propagated, imported or traded', and cannot be grown within 30 m of a watercourse (Anon., 2001).Both the seeds and the roots are said to be poisonous. A hydrocyanic acid has been reported in the roots, stems and leaves and an alkaloid, jambosine, has been found in the bark of the tree and of the roots (Morton, 1987). On Pitcairn Island, the spreading, shallow and dense rooting system makes cultivation of gardens next to trees an arduous task and its heavy shading is deleterious to crop growth (Binggeli, 2001). In the Dominican Republic it became a serious weed in Pinus occidentalis forests, preventing the regeneration of the pine (Wadsworth, 1943).On a number of oceanic tropical islands the presence and perceived spread of S. jambos has been viewed with some concern in recent years. A set of attributes (production of monotypic stands, ability to cast dense shade, and establishment of seedlings under shade) have been viewed as serious threats to native vegetation and associated fauna. On some small islands S. jambos now constitutes a high proportion of the woodland resource. By the 1990s S. jambos covered a large area of Pitcairn Island and the species was viewed as a major threat to the biodiversity of the island because of its rapid spread into various native plant communities (Diamond, 1994). However, spread into semi-natural vegetation is limited and slow (Binggeli, 2001). It clearly impacts on native vegetation and monotypic stands generally have a very limited number of native species. It outcompetes native trees but also suppresses other invasive species such as Lantana camara. In doing so it allows some native species such as tree ferns (Cyathea medullaris) to regenerate under a S. jambos canopy. However, this regeneration will die unless the canopy opens up. Once S. jambos produces mature monotypic stands, soil erosion becomes prevalent. The lack of ground vegetation and the near absence of small rootlets allow soil to be washed away even on moderately sloping hillsides. In extreme cases all soil may be washed away from surface roots. On Pitcairn Island, streams originating from unmanaged S. jambos stands have produced deep gullies down to the bedrock.The heavy shade cast by S. jambos stands reduce species richness, but does allow some native species to regenerate. It also appears to be an important substrate for some epiphytes. In Puerto Rico, it is the most important phorophyte for a species of orchid (Rodriguez-Robles et al., 1990). It also appears to be a favoured habit for some endemic species. In the Seychelles S. jambos is not listed, like other invaders, as a threat to orthopteroids, a group of large striking insects, instead it is given as one of the host species (Matyot, 1998). The endangered Rodrigues Warbler Acrocephalus rodericanus were mostly found in S. jambos-dominated woodland (Showler et al., 2002).

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