Frankincense tree recruitment failed over the past half century

Abstract

Boswellia papyrifera (Burseraceae) trees grow in dry woodlands south of the Sahara and produce frankincense, the economically important olio-gum resin used for cultural and religious ceremonies throughout the world and as raw material in several industries. Across its distribution area, this species is threatened by farmland expansion, fire, improper tapping and overgrazing. Most of its populations lack saplings and small-sized trees (e.g. <10cm). It is unknown whether the older, adult trees represent a single or several cohorts, representing single or plural regeneration and survival waves. To understand such long-term population dynamics, it is imperative to evaluate the age structure of the current populations. We used tree ring analysis to determine the age-diameter relationship. This study, (1) determines radial growth dynamics and age-diameter relationship of B. papyrifera, including verification of annual growth-ring formation, and (2) constructs the population age structure and discusses consequences thereof for population maintenance and long-term frankincense production. We could prove that B. papyrifera forms annual growth rings. The average radial annual growth rate of B. papyrifera is 1.15mm (s.d.=0.22) and varies significantly among the sampled trees. Age and diameter of B. papyrifera trees are significantly correlated. From the population-age structure, it becomes obvious that the current B. papyrifera populations lack successful recruitment since 1955, which we attribute to intensive grazing and fire associated with the escalating increase of human settlement in the area. Lack of recruitment leads to rapidly declining populations resulting in strongly reduced frankincense production. Management aimed at seedling survival and sustainable use of relic trees is urgent.