There are dozens of Boswellia species around the world, but only a handful are significant sources of frankincense. Two of the best quality species, Boswellia carterii and Boswellia frereana, grow in the Cal Madow and its surrounding ecosystems. Both Boswellia carterii and Boswellia frereana are members of the Burseraceae family, also known as the Incense Tree family, so-called for the aromatic resins these species produce.
Tree Biology Breakdown
Both B. carterii and B. frereana are small trees, from 1.5 meters up to 8 meters in height. B. carterii may be single trunk or multi-trunked near the base, while B. frereana is nearly always multi-trunked. Both trees have compound leaves with multiple leaflets. B. carterii produces white flowers while B. frereana produces red to reddish-green flowers, suggesting possible pollination differences. A congener, Boswellia sacra, found in Oman and Yemen, was found to be pollinated by bees, wasps, flies, butterflies, and ants. Both species are presumably wind-dispersed, and possibly fruit in August.
B. carterii ranges from 5-1500 meters in elevation, but generally prefers 500-1250 meters. B. frereana on the other hand is found between 5-750 meters but is found primarily between 5-500 meters. Both seem to prefer growing on exposed rocks, particularly of volcanic origin, often on cliff faces or in gullies. They show an affinity for water, generally growing in water drainages.
The most significant cause of natural mortality is probably a boring beetle, locally called xare. The beetle is possibly of the Cerambycidae or Buprestidae families. Two types of xare are known, one which burrows into the center of the tree and one that primarily attacks the cambium. Trees may also be killed by strong winds that blow them off of the rocks.