|Bursera simaruba (Gumbo-limbo)|
Burseraceae is a moderate-sized family of 17-18 genera and about 540 species of flowering plants. The actual numbers differ according to the time period in which a given source is written describing this family. The Burseraceae is also known as the Torchwood family, the frankincense and myrrh family, or simply the incense tree family. The family includes both trees and shrubs, and is native to tropical regions of Africa, Asia and the Americas.
Just as the family size (in terms of genera and species) differs according to the time period of the study, so too does its placement in the higher ranks. Nevertheless, the family is a genetically supported monophyletic group currently and frequently cited within the Sapindales and is recognized as a sister group to the Anacardiaceae.
The Burseraceae members are characterized by the non-allergenic resin they produce in virtually all plant tissue and the distinctive smooth, yet flaking aromatic bark,. The origins of the family can be traced to the Paleocene (~65 Mya) when Beiselia mexicana first diverged in Mexico. The subsequent divergences in the family lineage and migration of the species in the Eocene (~53 Mya) from North America have led to the current distributions of the species that are primarily associated with the tropics. Though the family likely originated in North America, the greatest generic diversity presently is in the Southern Hemisphere. Tabonuco (Dacryodes excelsa) and gumbo limbo (Bursera simaruba) represent the economic, ethnobotanical, and ecological significance of the Burseraceae in the Western Hemisphere, while frankincense (Boswellia sacra) and myrrh (Commiphora myrrha) represent the same in the Eastern Hemisphere.
The Burseraceae trees or shrubs are characterized by resins (having triterpenoids and ethereal oils; that are present within the plant tissue from the vertical resin canals and ducts in the bark to the leaf veins. In fact, the synapomorphy of the Burseraceae is the smooth yet peeling or flaking aromatic bark. The clear, non-allergenic  resins may smell like almonds, but at least the most well known resins, frankincense and myrrh, have an odor that is distinct from almonds, smelling like incense. The leaves are generally alternate, spiral, and odd-pinnately compound with opposite, frequently long-petiolulate, entire to serrate, pinnately veined leaflets whose symmetry is distinctive in some genera. However, some members are known to have trifoliate or unifoliate leaves. The leaf and leaflet stalks and axis may be brown and scurfy, while the leaf base is swollen and may be concave adaxially. The family members tend to be without stipules. The determinate, axillary inflorescences carry small, radial, unisexual flowers. The plants tend to be dioecious. The flowers may have 4-5 faintly connate but imbricate sepals with an equal number of distinct, imbricate petals. Also, the stamens, that may contain nectar discs, have distinct glabrous filaments that come in 1-2 whorls and in numbers equaling or twice the number of petals; the tricolporate pollen is contained within 2 locules of the anthers that open longitudinally along slits. The gynoecium contains 3-5 connate carpels, one style, and one stigma that is head-like to lobed. Each locule of the superior ovary has 2 ovules with axile placentation that are anatropous to campylotropous. The 1-5 pitted fruit is a drupe that opens at maturity. The endosperm is usually lacking in the embryo.
Tribes, subtribes, and genera
There is some discrepancy in the literature about the size of the Burseraceae. Records say that the family has 17  to 18  genera and 500  to 540  to 726 species. Other authors cite different numbers: 16-20 genera and 600 species; 20 genera and 500-600 species; According to a pollen studies and molecular data, the family is split up into three tribes: Protieae, Bursereae, and Canarieae. The Protieae is composed of Protium (147 species and largest in this tribe), Crepidospermum, Garuga, and Tetragastris. The Bursereae, which is further split into subtribes Boswelliinae and Burserinae, contains Commiphora (nearly 200 species and largest in the family), Aucoumea, Beiselia, Boswellia, Bursera, and Triomma. Finally, the Canarieae is composed of Canarium (75 species and largest in this tribe), Dacryodes, Haplolobus, Pseudodacryodes, Rosselia, Santiria, Scutinanthe, and Trattinnickia. The morphology of the fruit, which is a drupe, helps to distinguish between the three tribes. Though the groupings have slightly changed since the 1990s, Protieae is described as having a 2-5-parted drupe  with either ‘free or adhering parts’ which are ‘not fused in the endocarp’; Bursereae is described as having a drupe with parts that are fused in the endocarp but an exocarp with dehiscing valves; and the Canarieae as simply having a drupe with parts that are fused in the endocarp.
Bursereae subtribe Burserinae
Bursereae subtribe Boswelliinae
According to the literature, the Burseraceae has not been lumped with other families nor split up into several others. However, it has jumped orders several times. For example, it seems that in the early 19th century the family was placed in the Burserales, with Anacardiaceae and Podoaceae. In the mid-19th century and early 20th century the family was placed in the Geraniales. Then by the mid- and late-20th century the family moved to the Rutales. Finally, in the late 20th century the family was and still is located within the Sapindales. Families that are consistently found in the same order as Burseraceae (except when it was in the Burserales) include Rutaceae, Meliaceae, and Simaroubaceae. It is only in recent studies that Burseraceae and the Anacardiaceae were seen as sister groups.
The Sapindales are contained within the Malvids of the Rosid clade within the Eudicotyledons. The order contains 9  to 15 families, 460 genera, and anywhere from 5,400  to 5,670  to 5,800 species. The nine currently recognized families include Aceraceae, Anacardiaceae, Burseraceae, Hippocastanaceae, Julianaceae, Meliaceae, Rutaceae, Sapindaceae, and Simaroubaceae. The Sapindales is a clade supported by DNA-based analyses on rbcL, atpB, and 18S sequences. Within the Sapindales are two clades that contain gum and resin: 1) the Rutaceae-Meliaceae-Simaroubaceae clade; and 2) the Burseraceae-Anacardiaceae clade. Therefore, the Burseraceae are not the only family with this characteristic. The synapomorphies of the Sapindales include pinnately compound, alternate and spiral leaves that may be palmately compound, trifoliate, or unifoliate, and small 4-or 5-merous flowers having a characteristic nectar disk and imbricate petals and sepals. Some of these characteristics also occur in the Rosales. However, the Sapindales and Rutales may actually form a complex since many families jump between them. Indeed, rbcL sequence studies seem to indicate that a Sapindalean/Rutalean complex exists and may better represent the relationships of the families than the separate orders would. A study based on chloroplast-encoded gene rbcL reconstructed cladograms that include families within both the Sapindales and Rutales. One such cladogram indicated that the Sapindales are robust and that the Burseraceae (and Anacardiaceae) are within a single clade. This grouping seems to make sense as both the Burseraceae and Anacardiaceae have secretory canals in the phloem, resin canals in the leaves and are unique in the Sapindales for having biflavones in the leaf tissue. However, the two families have several distinguishing characteristics. The resin of the Burseraceae is non-allergenic and there are two ovules per carpel, whereas the resin of the Anacardiaceae can be allergenic or poisonous and there is one ovule per carpel. The Burseraceae-Anacardiaceae clade is sister to a robust cluster of three other families, the Sapindaceae-Aceraceae-Hippocastanaceae clade. The Rutaceae-Meliaceae-Simaroubaceae clade is sister to the Burseraceae-Anacardiaceae and Sapindaceae-Aceraceae-Hippocastanaceae clade. The rbcL technique is supported and considered acceptable until other methods become better developed for the analysis.
The Burseraceae are distributed throughout the world and primarily in the tropics, especially Malaysia, Africa, Meso- and South America. The three tribes can be linked to a specific region of the world although this is not obligatory. For example, members of the tribe Protieae are generally found in South America, those of the Bursereae are found in Africa and Mesoamerica, while members of the Canarieae are found in Malaysia. However, each tribe has a representative genus that is present in all the tropical regions: Dacryodes (Canarieae), Protium (Protieae), and Commiphora (Bursereae). The Burseraceae are found in a variety of habitats including hot, dry desert and savannah as well as in coastal mangrove forest and rain forest habitats. One study found that the family originated in North America during the Paleocene (~65 Mya), when the earliest fossils of the Sapindales are found. During the Early to Middle Eocene (~53 Mya) family members dispersed to eastern Laurasia (i.e. Europe and Asia) via the Boreotropical Land Bridge (BLB) and the continents in the Southern Hemisphere, which is now the area of the greatest generic diversity of this family.
More specifically, the earliest diverging genus was Beiselia (of the Bursereae subtribe Boswelliinae) in either North America, Mexico, or the Caribbean in the Paleocene (~65 Mya). Similar results from other studies  find that Beiselia mexicana, a native of Mexico is basal to the remaining Burseraceae members. These results may indicate that the family originated in Mexico. The next divergence was in the Early Eocene (~53 Mya) when Burserinae (i.e. Commiphora) diverged and emigrated from North America into Africa, Madagascar, and India. Commiphora dispersed throughout Africa during the Middle Eocene (~44 Mya) and from Africa to Madagascar during the Oligocene (~30 Mya) via the Mozambique Channel Land Bridge; the spread to India was more recent (~5 Mya). The Canarieae and Boswelliinae (subtribe of Bursereae) dispersed from western Laurasia and spread eastward during the Eocene; fossils of Canarium, for example, from the Czech Republic date to the Late Oligocene (23 Mya). Finally, Protieae originated in North America like the rest of the family, then migrated to Africa and Asia through the Tethys seaway in the Late Eocene (~37 Mya), but then made its way back to South America via long-distance dispersal. By the late Oligocene (~23Mya) all three Burseraceae tribes were extant and dispersed throughout the Northern Hemisphere.
The mechanism of seed dispersal via animal link vectors (endozoochoric dispersal) may explain how most Burseraceae members were able to expand their range so efficiently across the globe. Beiselia, Boswellia, and Triomma have dry fruits better suited for wind dispersal, but most Burseraceae members have fleshy, edible fruit that is eaten by many animal dispersers. The seeds may provide a high reward in fat (24-73%) and protein (2.7-25.9%) if digested, but many animals eat just the fleshy part of the fruit and either discard the endocarp right away or excrete it some time later. Some known Burseraceae fruit consumers include hornbills (Buceros bicornis, Ceratogyma atrata, C. cylindricus, Penelopides panini), oilbirds (Steatnoris caripensis), fruit pigeons, warblers, vireos, orioles, flycatchers, tanagers, woodpeckers, loeries, primates (Cercopithecus spp., Lophocebus albigena), lemurs (Varecia variegate subsp. Variegate), and sun bears (Helarctos malayanus). There is also a possibility that the fruits may also have been water dispersed.
Economic significance and ethnobotanic uses
There are several representative species within the Burseraceae that typify the economic and ethnobotanic significance of the family. First, Dacryodes excelsa of the Canarieae is an important old-growth species found in the Caribbean. Second, Bursera simaruba of the Burserinae is a fast-growing ornamental that is one of a few representatives of the primarily tropical family in the United States. Finally, the namesakes of the family Boswellia carterii (frankincense) and Commiphora abyssinica (myrrh) are important economically and medicinally in several parts of the world. Though this is a small subset of the large number of potentially important species, these four members exemplify the wide use and importance of the Burseraceae. The latter three are frequently cited in the literature for their renowned importance.
Commonly known as Tabonuco (or gommier, also candlewood), Dacryodes excelsa is a large dominant tree found in Puerto Rico and other parts of the Caribbean. The seeds of the tree are a source of food for birds. Like all members, the tree releases sap from the bark when wounded. The clear sap oozes from the tree and hardens to a white, aromatic waxy resin that can be used to make candles and incense. Before the arrival of the Spaniards to Puerto Rico, the native Taínos used the resin to make torches. The wood itself is useful for construction, furniture, making boxes, small boats, and a variety of other wood-based products; the utility of the wood is comparable to that of mahogany and birch. In addition, species like Canarium littorale, Dacryodes costata, Santiria laevigata, and Santiria tomentosa from Malaysia as well as Aucoumea klaineana and Canarium schweinfurthii from Africa also produce valuable wood for construction projects and carpentry.
The species in the Bursera (esp. the elephant tree) can be found primarily in Mexico where they are used to make varnish. The Maya also used a Bursera sp. to make incense. However, the Bursera may also be considered an ornamental genus and a common representative of the family in the United States, especially in Florida (B. simaruba) and the Southwest (B. odorata, B. microphylla). Naked Indian (also known as gumbo limbo), or Bursera simaruba, in particular, is found in Florida, Mexico, the Caribbean, Venezuela, and Brazil. The tree is also named the ‘tourist tree’ for its very distinctive flaking red bark; apparently, the tree occurs in tropical areas where many white tourists go on vacation. The resin from this tree can be used to make varnish and turpentine. In addition, the resin may also be used in the same form as tiger balm (containing Cinnamomum camphora of the Lauraceae) to relieve sprains and muscle aches. The leaves containing hexane can be used to make tea to relieve inflammation. The bark serves as an antidote to skin irritation caused by Metopium toxiferum (also known as poisonwood, Florida poison tree, and hog gum) of the Anacardiaceae. The gumbo limbo grows quickly and can be used to make a living fence especially out of cut limbs that are placed straight into the ground or for restoration projects as a pioneer species. The tree is highly tolerant of high intensity wind such as hurricane-force winds and is therefore planted in areas where hurricanes occur frequently like Florida and the Caribbean. The seeds of this species are also a source of food for birds.
Frankincense, or olibanum, (Boswellia carterii) and myrrh (Commiphora abyssinica) may readily be associated with the aromatic resins they produce. These resins are extracted via tapping, or cutting of the bark to make it release sap. The liquid sap hardens and is gathered, sold as it is or further ground up and mixed with spices, seeds, and roots to make various odors for incense. The two species are native to parts of Northeast Africa (Somalia, frankincense; Somalia and Ethiopia, myrrh) and Arabia (Oman and Yemen, frankincense), but their distribution and utility spreads beyond these regions to India and China. The best frankincense is found in Oman and the incense is widely used in worship in India. The ancient Egyptians prized frankincense for the resin they used to make the characteristic dark eyeliner and myrrh as an embalming agent for deceased Pharaohs. At this time, myrrh was worth more than gold. These days, the resins from these trees are frequently used in Chinese herbal medicine and Indian Ayurvedic medicine to treat several ailments. Ingesting frankincense in small amounts usually in the form of a pill mixed with other ingredients promotes blood flow and the movement of the ‘qi’ (pronounced ‘chi’; meaning ‘life force’ or ‘spiritual energy’). Ingesting myrrh in a similar way also promotes blood flow and stimulates the stomach and digestion. It is also reportedly useful to treat diabetes, menopause, uterine tumors, amenorrhea (loss of menstruation or failure to menstruate), and dysmenorrhea (menstrual cramping). Both frankincense (containing triterpene acids)  and myrrh are used to relieve pain and inflammation as in arthritis and asthma.
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