their shapes are different, and sold resins from both species as the same thing. However, the chemical compositions of their essential oils are completely different. Recent studies indicate that frankincense tree populations are declining, partly from overexploitation. Heavily tapped trees produce seeds that germinate at only 16% while seeds of trees that had not been tapped germinate at more than 80%. In addition, burning, grazing, and attacks by the longhorn beetle have reduced the tree population. Conversion (clearing) of frankincense woodlands to agriculture is also a major threat. Chemical composition These are some of the chemical compounds present in frankincense: acid resin (6%), soluble in alcohol and having the formula C20H32O4 gum (similar to gum arabic) 30–36% 3-acetyl-beta-boswellic acid (Boswellia sacra) alpha-boswellic acid (Boswellia sacra) incensole acetate, C21H34O3 phellandrene olibanic acid Among various plants in the genus Boswellia, only Boswellia sacra, Boswellia serrata and Boswellia papyrifera have been confirmed to contain significant amounts of boswellic acids. History Frankincense has been traded on the Arabian Peninsula for more than 5,000 years. Frankincense was also traded from the Horn of Africa during the Silk Road era. Greek historian Herodotus wrote in The History that frankincense was harvested from trees in southern Arabia. He reported that the gum was dangerous to harvest because of winged snakes that guard the trees and that the smoke from burning storax would drive the snakes away. Pliny the Elder also mentioned frankincense in his Naturalis Historia. Frankincense was reintroduced to Western Europe by Frankish Crusaders, and other Western Europeans on their journeys to the Eastern Roman Empire where it was commonly used in church services. Although named frankincense, the name refers to the quality of incense brought to Western Europe, not to the Franks themselves. Southern Arabia was an exporter of frankincense in antiquity, with some of it being traded as far as China. The 13th-century Chinese writer and customs inspector Zhao Rugua wrote that or (Chinese: / ) comes from the three Dashi states (Chinese: - Caliphate (Arab Muslims)) of Maloba (Murbat), Shihe (Shihr), and Nufa (Dhofar), from the depths of the remotest mountains; the trunk of the tree is notched with a hatchet, upon which the resin flows out, and, when hardened, turns into incense, which is gathered and made into lumps; it is transported on elephants to the Dashi ports, then on ship to Sanfoqi; which is why it was known as a product