Limitations to sustainable frankincense production: blocked regeneration, high adult mortality and declining populations.
Resins are highly valued non-timber forest products (NTFP). One of the most widely traded resins is frankincense, tapped from several Boswellia tree species (Burseraceae). Exploited Boswellia populations often show poor regeneration, but the demographic consequences of these bottlenecks are unknown. Here we report on the first large-scale demographic study of frankincense-producing trees. We studied 12 populations of Boswellia papyrifera in northern Ethiopia, varying in altitude and productivity. Six of these populations had been tapped before and were tapped during the study. Survival, growth and fecundity were determined for 4370 trees and 2228 seedlings, in 22.8 ha over a 2-year period. We also studied a remote population where no grazing and tapping took place. Matrix models were used to project population growth and frankincense production under four restoration scenarios. Population structures of both tapped and untapped populations showed clear gaps. Small seedlings were abundant in all populations, but none developed into persistent saplings. Such saplings were only present in the remote population. Fire and grazing are the likely causes of this regeneration bottleneck. Adult mortality was high (6-7% per year) in both tapped and untapped populations, probably caused by beetle attacks and fire. Unexpectedly, tapped populations presented higher diameter growth rates and fecundity compared to untapped populations. These differences are probably caused by non-random selection of exploited populations by tappers. Under the 'business as usual' scenario, population models projected a 90% decline in the size of tapped and untapped populations within 50years and a 50% decline in frankincense yield within 15years. Model simulations for restoration scenarios revealed that populations and frankincense production could only be sustained with intensive management leading to full sapling recruitment and a 50-75% reduction in adult mortality. Synthesis and applications. Regeneration bottlenecks and high adult mortality are causing rapid decline in frankincense-producing tree populations in Ethiopia. This decline is unlikely to be a consequence of harvesting and is probably driven by fire, grazing and beetle attacks. Fire prevention and the establishment of non-grazing areas are needed. Our results show that other factors than exploitation may seriously threaten populations yielding NTFP.