Dendrochronology and bark anatomy of the frankincense tree

M. Tolera Feyissa

Research output: Thesisinternal PhD, WU

Abstract

Boswellia papyrifera(Burseraceae) trees grow in drylands south of the Sahara. In Ethiopia, it grows in seasonally dry Combretum-Terminalia woodlands. It is a source of frankincense, an economically important olio-gum resin used for cultural and religious ceremonies throughout the world and as raw material in several industries. Ethiopia is a major exporter of frankincense. Currently, the populations of this species are threatened by farmland expansion, fire, overgrazing, improper tapping techniques and possibly also by climate change. Focussing on tree ring analyses and resin-production related bark anatomical features, this study had two objectives.

The first objective was to quantify the status of B. papyrifera populations with respect to radial stem-growth dynamics and size and age structure. Based on analysis of wood structure and crossdating of tree-rings series, it is shown that B. papyriferaforms annual growth rings and that the average age of sampled B. papyrifera trees is 76 years.More importantly, it is shown that the B. papyrifera populations lack trees that recruited over the last 55 years (1955-2010), and that the remnant trees established continuously between 1903 and 1955. This lack of successful recruitment for such a long period of time is attributed to continuous disturbances, such as fire and grazing accompanying new settlements of people into the area over the past decades. Radial growth patterns over decades suggest effects of heavy disturbances that the trees were experiencing. Remarkably, B. papyrifera trees showed a 2-3 year cycle in annual radial growth, and responded significantly to climate. As expected, radial growth increased with rainfall. An increase in ring width with maximum temperature may reflect radiation limits on growth. Radial growth decreased with increasing minimum temperatures, which may reflect temperature impacts on respiration. Overall, the predicted increase in temperature and rainfall for Ethiopia may not pose a direct threat for this species.

The second objective of this study was to describe the resin-secretory structure in the bark of B. papyrifera. The aim was to understand the relationship between structure and functioning of the secretory system with special reference to implications for frankincense yield and improvements of current tapping techniques. Resin canals of B. papyrifera form a three-dimensional network within the inner bark. In the wood, only few radial resin canals were encountered. The intact resin-producing and transporting network is on average limited to the inner 6.6 mm of the inner bark. Within the inner bark, the density of non-lignified axial resin canals decreases from the vascular cambium towards the outer bark. We also show that whole tree properties, such astotal resin-canal area in the bark, stem diameter, tree age, and the number of leaf apices impact frankincense yield.

Finally, this study provides recommendations for improving the existing tapping practice, aiming at maximization of frankincense yield at minimum damage costs to the trees. The new insights can also be used for selection and propagation of trees which are well suited for frankincense production. The information generated in this study is vital for planning sustainable management of the remnant trees and populations of B. papyrifera and the widely demanded frankincense.

Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Bongers, Frans, Promotor
  • Sass-Klaassen, Ute, Co-promotor
  • Sterck, Frank, Co-promotor
Award date18 Sep 2013
Place of PublicationS.l.
Print ISBNs9789461736444
Publication statusPublished - 2013

Keywords

  • boswellia
  • dendrochronology
  • growth
  • plant anatomy
  • resin canals

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