Frankincense tapping reduced photosynthetic carbon gain in Boswellia papyrifera (Burseraceae) trees

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Whole-crown carbon gain depends on environmental variables and functional traits, and in turn sets limits to growth sinks of trees. We estimated the annual whole-crown carbon gain of trees of the species Boswellia papyrifera, which are tapped for frankincense, by integrating leaf photosynthetic rates over the total leaf area and leaf life span. We examined the effect of tapping on total leaf area and leaf photosynthesis and, in turn, on carbon gain and resin yield for trees of a dry highland population and a wetter lowland population. Highland and lowland trees had similar total leaf area, but highland trees had higher photosynthetic rates per unit leaf area than lowland trees since they received more light and had higher photosynthetic capacities. Highland trees therefore achieved a higher annual carbon gain than lowland trees, despite a shorter rainy season and shorter leaf lifespan. Intensive tapping reduced crown leaf area and the carbon gain in the lowland trees, but not in highland trees. These results highlight how the interplay between local conditions and functional traits determine regional variation in tree productivity. However, such differences in productivity and carbon gain did not influence frankincense yield across sites. We conclude that tapping B. papyrifera trees reduces annual carbon gain but the extent differs among different populations.