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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

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    Conservation genetics of the frankincense tree
    Bekele, A.A. - \ 2016
    Wageningen University. Promotor(en): Frans Bongers, co-promotor(en): Rene Smulders; K. Tesfaye Geletu. - Wageningen : Wageningen University - ISBN 9789462576865 - 158
    boswellia - genomes - dna sequencing - tropical forests - genetic diversity - genetic variation - genetics - forest management - plant breeding - boswellia - genomen - dna-sequencing - tropische bossen - genetische diversiteit - genetische variatie - genetica - bosbedrijfsvoering - plantenveredeling

    Boswellia papyrifera is an important tree species of the extensive Combretum-Terminalia dry tropical forests and woodlands in Africa. The species produces a frankincense which is internationally traded because of its value as ingredient in cosmetic, detergent, food flavor and perfumes productions, and because of its extensive use as incense during religious and cultural ceremonies in many parts of the world. The forests in which B. papyrifera grows are increasingly overexploited at the expense of the economic benefit and the wealth of ecological services they provide. Populations of B. papyrifera have declined in size and are increasingly fragmented. Regeneration has been blocked for the last 50 years in most areas and adult productive trees are dying. Projections showed a 90% loss of B. papyrifera trees in the coming 50 years and a 50% loss of frankincense production in 15 years time.

    This study addressed the conservation genetics of B. papyrifera. Forty six microsatellite (SSR) markers were developed for this species, and these genetic markers were applied to characterize the genetic diversity pattern of 12 B. papyrifera populations in Ethiopia. Next to this, also the generational change in genetic diversity and the within-population genetic structure (FSGS) of two cohort groups (adults and seedlings) were studied in two populations from Western Ethiopia. In these populations seedlings and saplings were found and natural regeneration still takes place, a discovery that is important for the conservation of the species.

    Despite the threats the populations are experiencing, ample genetic variation was present in the adult trees of the populations, including the most degraded populations. Low levels of population differentiation and isolation-by-distance patterns were detected. Populations could be grouped into four genetic clusters: the North eastern (NE), Western (W), North western (NW) and Northern (N) part of Ethiopia. The clusters corresponded to environmentally different conditions in terms of temperature, rainfall and soil conditions. We detected a low FSGS and found that individuals are significantly related up to a distance of 60-130 m.

    Conservation of the B. papyrifera populations is urgently needed. The regeneration bottlenecks in most existing populations are an urgent prevailing problem that needs to be solved to ensure the continuity of the genetic diversity, species survival and sustainable production of frankincense. Local communities living in and around the forests should be involved in the use and management of the forests. In situ conservation activities will promote gene flow among fragmented populations and scattered remnant trees, so that the existing level of genetic diversity may be preserved. Geographical distance among populations is the main factor to be considered in sampling for ex situ conservation. A minimum of four conservation sites for B. papyrifera is recommended, representing each of the genetic clusters. Based on the findings of FSGS analyses, seed collection for ex situ conservation and plantation programmes should come from trees at least 100 m, but preferably 150 m apart.

    Dendrochronology and bark anatomy of the frankincense tree
    Tolera Feyissa, M. - \ 2013
    Wageningen University. Promotor(en): Frans Bongers, co-promotor(en): Ute Sass-Klaassen; Frank Sterck. - S.l. : s.n. - ISBN 9789461736444 - 136
    boswellia - dendrochronologie - groei - plantenanatomie - harsgangen - boswellia - dendrochronology - growth - plant anatomy - resin canals

    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.

    Dryland resources, livelihoods and institutions : diversity and dynamics in use and management of gum and resin trees in Ethiopia
    Teshale Woldeamanuel Habebo, Teshale - \ 2011
    Wageningen University. Promotor(en): Bas Arts; Frans Bongers, co-promotor(en): Freerk Wiersum. - [S.l.] : S.n. - ISBN 9789085859628 - 170
    bosbomen - gomleverende planten - acacia - boswellia - commiphora - bosproducten anders dan hout - bosbouw - bosexploitatie - bosbedrijfsvoering - ethiopië - forest trees - gum plants - acacia - boswellia - commiphora - non-wood forest products - forestry - forest exploitation - forest management - ethiopia
    Dry woodlands comprise the largest forest resources in Ethiopia. An important feature of these forests is their richness in Acacia, Boswellia and Commiphora (ABC) species that produce gum and resin. Gums/resins significantly contribute to rural livelihoods, the national economy, and ecosystem stability. Their contribution to local livelihoods is in terms of both cash income and subsistence value. In different parts of the country they contribute up to one-third of the annual household income. Currently, an estimated US2 million gum and resin are consumed locally, the rest is exported. During the 2007/08 fiscal year, Ethiopia earned a revenue of about US.7 million from this export. However, the woodlands and the ABC species are under intense pressure. Especially in the traditional production areas in north Ethiopia the pressure is high and the policies that were enacted to shape their use and management have not been very effective. The main objective of this study is to investigate how gum and resin utilization and management is carried out in the drylands of Ethiopia and what processes affect this. The following four questions were addressed: (i) What types of gum-resin woodland management and production systems are present in Ethiopia and how are they related to the land-use and socio-political conditions?, (ii) What dynamic processes in institutional arrangements and gum-resin production and management have occurred in various regions of Ethiopia?; (iii) How do multi-level formal and informal institutions interact and affect gum and resin production and management?, and (iv) How does gumresin utilization fits into the livelihoods strategies of households in the study areas? The study is based on a multi-theoretical approach giving attention to both diversity and dynamics in ABC woodlands production systems, institutional diversity and interaction regarding the governance of ABC resources, and the role of gum and resin in the livelihoods strategies of the households. The study design consisted of a comparative case study of three regions in north (Abergelle), northwest (Metema and Quara), and south Ethiopia (Borana). The three locations are characterized by ecological and socio-economic differences as well as a different history of gum and resin production. A two-phase research approach consisting of a base-line survey and a systematic household survey was used. The base-line survey served to assess the local socio-economic, institutional and land-use conditions; data were collected through open interviews with groups and key informants. The household survey served to obtain further detailed information on the ABC production conditions and the role of the products at household level. The survey included 327 respondents; it was follow-up by feedback meetings with groups of participants to check and validate the main issues that emerged from it. The qualitative data from key informant interviews and focus group discussion were transcribed, categorized, and interpreted. The data from household survey was analysed using descriptive statistics and mean comparisons in SPSS. Chapter 2 discusses the diversity in gum and resin management and production systems and how different exploitation arrangements are related to different phases of resource domestication and/or degradation. Seven presently existing production models are identified. In south Ethiopia pastoral people mainly collect the products in the form of ooze from natural vegetation. In north Ethiopia the production is part of mixed farming practices or is done by externally hired laborers. Production is done by tapping wild trees. Despite decades of production history in this region, the species is not cultivated and hardly domesticated in an ecological or biological sense. The production systems gradually evolved from openaccess extraction of wild trees to a controlled production in assigned forest plots. This institutionalisation of access rules concerns a process of domestication in a social sense. However, this process is not yet very effective; the ABC woodlands are often subject to serious degradation as a result of competing land-use practices and inappropriate social arrangements for production and trade of the gums/resins. These findings show that the nature of domestication in a social sense determines whether forests and/or specific forest resources can be further domesticated in an ecological and biological sense resulting in intensified management and resource enrichment, or whether they are subject to degradation. Chapter 3 and 4 elaborate how gum and resin production is shaped in the different parts of Ethiopia by the location-specific interaction between formal and informal institutions. Chapter 3 discusses how gum and resin production and marketing in Borana is related to the interplay between well-established traditional land-use institutions and external institutions. Both the traditional and external institutions do not explicitly control access to the gum and resin production system, but under traditional conditions gum and resin extraction was embedded in a strong customary system for controlled pastoral land use. The traditional institutions did not developed rules and norms regulating market access. The external institutions impacted gum and resin production mainly by creating access to markets, but this has not yet had much impact on the actual exploitation arrangements. The woodlands are experiencing increasing pressure due to the increase in non-traditional and non-gum and resin based livelihoods activities that negatively affect ABC woodlands. Also, the traditional natural resources management institutions are weakened due to modernization processes and contribute at present little to sustainable use and management of gum-resin resources. This situation calls for either revitalizing the traditional range land management system, or generating institutions specific to ABC species that integrate the customary and external institutions. Chapter 4 discusses the nature and interactions of formal and informal institutions concerning the AB resource use and management in the north and northwest Ethiopia. Existing government regulations recognize gum and resin production and marketing by both smallholders, cooperatives, and companies. However, in practice gum and resin production and marketing by smallholders is restricted. This is the result of informal bureaucratic institutions that act as rules-in-use regarding gum and resin production and marketing regardless of whether they contradict with the regulations of federal and regional states. Moreover, the customary rules and practices and the sectoral government policies often compete with the formal regulations for ABC species use and management. The interaction between government regulations and informal institutions is generally competing; this often results in indiscriminate tree cutting and woodland conversion. The situation requires harmonization of the formal and informal institutions and coordination of institutions across sectors. Chapter 5 discusses the relationship between gum and resin production and the livelihood systems of local producers. Both the livelihoods systems and the contribution of the multiple activities to cash and total income vary among the study areas. In Borana the use of gum and resin is part of a predominantly pastoral livelihood system with gum and resin acting as supplementary cash crops or safety nets in times of emergency. In Abergelle the production fits into a diversification strategy with gum and resin exploitation forming a component of a mixed farming system. In Metema local farmers were not involved in gum and resin exploitation; here production is a specialized activity of commercial enterprises using laborers from outside the region. The findings show that not only the value of the ABC resources, but also the degree of the embeddedness of the product in multi-livelihoods strategies of the households as well as the institutional arrangements that govern production system and market access are important regarding how these products fit into the livelihoods strategies of the households. Chapter 6 brings all the information together and further assesses the nature of the different institutional arrangements for gum and resin exploitation, and their dynamics and interaction. It also elaborates the relation between the status of ABC resource domestication and their exploitation arrangements. The process of organizing gum and resin utilization followed different pathways in north and south Ethiopia. In the south it started as the collection of products for chewing gum for subsistence use; later it was marketed as a coping mechanism during periods of livelihood stress. In contrast to these endogenous developments, in the north the production was introduced by external private and state companies. Only gradually also some informal systems of private exploitation evolved. After 1990 cooperatives took over many of the concession areas of the commercial companies. This cooperative movement also was introduced in the south. As a result of these location-specific dynamics in organizing the production, six exploitation arrangements evolved. These arrangements differ with respect to whether their organization is companybased, cooperative-based or privately based, and are characterized by different rules and regulations regarding access to resources and markets, and the type of labor used for production. In all study areas the exploitation arrangements co-exist with a growing importance of the cooperative arrangements. The institutional arrangements are not conducive to stimulate intensification of production, rather they may limit local participation and endogenous development of informal and location-specific institutions. Moreover, the effectiveness of the exploitation arrangements may be limited as a result from competing development policies and programmes aimed at other land-use sectors. These findings further illustrate that the limited progress in ABCs domestication greatly depends on the nature of institutional arrangements for access to resources and markets, the relation of formal and informal institutional arrangements, and development polices. In chapter 7 it is concluded that the use and management of the ABC species in Ethiopia is very divers both in terms of production systems, institutional arrangements for exploitation, and roles in local livelihoods. The nature of location-specific production systems is greatly affected by the local system for ABC governance. Such a system involves complex set of both formal and informal institutions at both government level and local level. The informal institutions do not only include customary institutions of local communities, but also informal rules-in-use of local bureaucrats. The historic process of institutionalisation of ABC governance differs between regions. Depending on local land-use conditions and government policies, different exploitation arrangements have been developed based on either company, cooperative or private control over the production, labor and marketing. But despite of this diversified stage of domestication in a social sense, the production systems are still in an early phase of domestication in ecological and technical sense and intensified production through tree cultivation or plantation establishment has hardly been developed. Several gum and resin production systems are even subject to serious degradation due to the inappropriate nature of, and sometimes even competition between, the exploitation arrangements, as well as the economic position of the ABC resources in relation to other forms of land-use. The complex pattern of institutions governing the production of gum and resin also impacts on the role that the resources play in local livelihoods. Both the role of gum and resin production in the prevailing land-use conditions and the degree of control on market and resource access determine how the gum and resin fit into the livelihoods strategies of the households. As the governance of gum and resin production involves a complex, diverse and dynamic web of formal and informal institutions, it will not be effective to stimulate production as a means for both sustainable forest use and livelihood improvement by a generic development policy. Rather a diversified and regional-specific approach is needed that builds upon the location specific characteristics of the gum and resin production systems and exploitation arrangements.
    Mycorrhizal symbiosis and seedling performance of the frankincense tree (Boswellia papyrifera)
    Hizikias, E.B. - \ 2011
    Wageningen University. Promotor(en): Frans Bongers; Thomas Kuijper, co-promotor(en): Frank Sterck. - [S.l.] : S.n. - ISBN 9789085859635 - 141
    boswellia - mycorrhizae - symbiose - zaailingen - vesiculair-arbusculaire mycorrhizae - waterbeschikbaarheid - waterstress - tropen - ethiopië - boswellia - mycorrhizas - symbiosis - seedlings - vesicular arbuscular mycorrhizas - water availability - water stress - tropics - ethiopia

    Arid areas are characterized by a seasonal climate with a long dry period. In such stressful

    environment, resource availability is driven by longterm and shorterm rainfall pulses.

    Arbuscular Mycorrhizal (AM) fungi enhance access to moisture and nutrients and thereby

    influence plant performance. In this dissertation I applied field observations and

    greenhouse experiments to address four questions: 1) What are the major environmental

    factors influencing AM incidence in the Boswellia-dominated dry deciduous woodlands?

    2) How do Boswellia seedlings respond when they are exposed to AM fungi and water

    pulses? 3) How do AM fungi, water deficit and soil fertility influence the growth and gas

    exchange of Boswellia and Acacia seedlings? 4) Does the AM symbiosis influence

    competition between Acacia and Boswellia seedlings at different water pulse levels?

    The present study showed that almost all woodland plants in northern Ethiopia are

    colonized by AM fungi. Root colonization levels in dry and wet seasons demonstrated that

    in the sites with the harshest conditions, AM plants and fungi respond to pulsed resource

    availability by temporally disconnecting carbon gain by the plant and carbon expenditure

    by the fungus. Consequently, we studied below-ground processes in conferring adaptation

    to highly pulsed resources in Boswellia seedlings. The strong interactive AM fungi and

    water pulse showed that mycorrhizal Boswellia benefits from drought pulses during the

    short rainy season. Boswellia acquires carbon and water after rain events and store

    probably carbon and water in coarse roots, suggesting conservative strategy. From this

    observation we carried out an experiment to test whether other trees (Acacias) than

    Boswellia in this habitat also show this conservative acquisition strategy, or whether more

    acquisitive strategies may also be beneficial under such climates.

    My study show that acquisitive and conservative species both benefit from the AM

    symbiosis, but that the acquisitive Acacias mainly benefit at higher water availability,

    whereas the conservative Boswellia benefits at water or nutrient-stressed conditions. I also

    investigate on how mycorrhiza and water availability affect competition between plants

    with different resource acquisition strategies in these drylands. Seedlings of Boswellia are

    competitively inferior to seedlings of Acacia, and neither the presence of AM fungi nor a

    stronger water limitation (through pulsing) affected this outcome.

    Physiological ecology of the frankincense tree
    Mengistu Woldie, T. - \ 2011
    Wageningen University. Promotor(en): Frans Bongers; G. Fetene, co-promotor(en): Frank Sterck. - S.l. : s.n. - ISBN 9789085859277 - 127
    boswellia - koolstof - ecologie - plantenfysiologie - tappen (rubber) - bosgebieden - koolhydraten - bladoppervlakte - bomen - harsen - harswinning - ethiopië - boswellia - carbon - ecology - plant physiology - tapping - woodlands - carbohydrates - leaf area - trees - resins - resin extraction - ethiopia





































    Keywords: Boswellia papyrifera, carbon balance, drylands, Ethiopia, frankincense, tapping

    The degradation of frankincense tree dominated woodlands has been attributed to climatic
    conditions and human activities. We lack however information on how such factors influence the
    resource balance and productivity of trees. The aim of this study was to evaluate the impact of
    resin tapping on the whole tree carbon gain, storage and allocation pattern of frankincense trees
    (Boswellia papyrifera (Del.) Hochst) in the dry woodlands of northern Ethiopia. I hypothesized
    that the intensive resin tapping of frankincense trees reduces tree vitality, particularly under
    relatively dry conditions. I established experimental plots in the highland woodlands of
    Abergelle and the lowland woodlands of Metema, and applied tapping treatments to similar sized
    adult trees (DBH 20 +/- 3cm). For these trees I also collected data on leaf gas exchange, crown
    traits, carbon storage, carbon allocation, growth and frankincense production during a period of
    two years (2008-2009).
    Trees follow similar leaf gas exchange patterns in contrasting environments, but differ in
    annual crown carbon gain between highland and lowland sites. Highland trees of Boswellia had a
    higher photosynthetic capacity, were exposed to higher light conditions, but had a shorter leaf
    lifespan than lowland trees. Integrating these effects, I showed that the annual crown carbon gain
    is higher in the highland trees than in lowland trees. Lowland trees are mainly constrained by
    clouded conditions and resultant low light levels during the wet season, limiting their carbon
    gain. Moreover, carbon gain was also restricted by atmospheric drought, and much less by soil
    water deficit during the growing season. The production of frankincense was not affected by the
    annual tree carbon gain implying that trees with smaller total leaf area may suffer sooner from
    carbon starvation by tapping.
    Tapping reduced storage carbohydrate concentrations in wood, bark and root tissues
    indicating that continuous tapping depletes the carbon reserves. A large part of the carbohydrate
    concentration in the plant tissues was starch. Boswellia trees have more total nonstructural
    carbohydrates (TNC) concentrations and pool sizes in wood than in root and bark tissues.
    Because tapped trees face depleting carbon storage pools during the dry tapping season and
    cannot fully replenish these pools during the wet season, tapped trees may face higher risks of
    carbon starvation compared to untapped trees in the long term.
    Estimated total annual carbon sinks to the different plant components were 38-68% of the
    annual carbon gain in both study sites. However, Boswellia trees also establish mycorrhizal
    associations which may consume an additional 20% of gross primary production. On a wholetree
    basis, the percentage of autotrophic respiration may exceed all other costs. The foliage
    construction costs and incense production are the second and third largest carbon sinks,
    respectively. Contrary to our expectation, the sum of all dry season carbon costs was higher than
    the total amount of consumed TNC during the dry season. The high carbon costs during the dry
    season imply that trees do not fully depend on TNC to pay for the carbon costs during the dry
    season. With the exception of carbon allocation to foliage production and maintenance, a higher
    gross primary production does not enhance an overall increase in carbohydrate investments in
    the other sinks. Therefore, the carbon allocation pattern is constrained not exclusively by the
    absolute amount of carbon gained but also by other factors.
    The results clearly indicate that continuous tapping depletes the amount of stored carbon,
    the leaf area production and the reproductive effort. These negative effects were however site
    specific and could possibly be apparent sooner for smaller trees than for larger ones. Thus,
    guidelines for resin tapping of Boswellia trees should consider tapping intensity, tapping
    frequency, environmental conditions and tree size and should focus on maintaining vital trees
    and populations for the future.










    The frankincense tree of Ethiopia : ecology, productivity and population dynamics
    Eshete Wassie, A. - \ 2011
    Wageningen University. Promotor(en): Frans Bongers, co-promotor(en): Frank Sterck. - S.l. : s.n. - ISBN 9789085859536 - 149
    boswellia - populatiedynamica - tappen (rubber) - modellen - harsen - klimaat - begrazing - soortenrijkdom - bosecologie - ethiopië - boswellia - population dynamics - tapping - models - resins - climate - grazing - species richness - forest ecology - ethiopia

    Keywords: Boswellian papyrifera, Frankincense tree, matrix model, population dynamics,
    population bottleneck, tapping.

    Combretum – Terminalia woodlands and Acacia – Commiphora woodlands are the two
    dominant vegetation types that cover large parts of the dry land areas in Ethiopia. Several of
    their tree and shrub species yield economically valuable products such as gum Arabic,
    frankincense and myrrh. Boswellia papyrifera provides the widely traded frankincense that
    accounts for >80% of the export revenues that the country is earning from gum and resin
    resources. Unfortunately, the Ethiopian dry woodlands and the B. papyrifera populations are
    disappearing rapidly due to the combined effects of over-harvesting gums and resins,
    overgrazing by livestock, recurrent fires, and excessive wood harvesting. The current lack of
    small saplings in the remaining populations of Boswellia suggests that the populations may
    not be sustained for the future.
    The main objectives of this thesis were to determine diversity and production patterns in B.
    papyrifera dominated dry woodlands, to show the regeneration status in various B. papyrifera
    populations, and to evaluate the effects of environment, frankincense harvesting, and grazing
    on the population dynamics of B. papyrifera. The main research questions were: (1) how do
    environmental conditions affect the tree/shrub species richness and production of Ethiopian
    dry woodlands? (2) what factors determine the frankincense production by B. papyrifera
    trees? (3) how do the vital rates and population dynamics of B. papyrifera vary across
    habitats that differ in soil conditions and biotic factors? (4) What are the major bottlenecks in
    the life cycle of the trees that hinder the sustainability of the remaining populations? To
    address these questions, tree populations were studied in the highlands of Abergelle and the
    lowlands of Metema. Metema also has a longer wet season length, higher annual rainfall and
    better soil fertility status than Abergelle.
    In total 36 and 22 tree and shrub species representing 20 and 9 families were recorded in
    Metema and Abergelle woodlands, respectively. The most dominant plant families were
    Burseraceae, Fabaceae, Combretaceae and Anacardiaceae. The vegetation at both sites was
    dominated by B. papyrifera. The two sites differed in species richness, diversity and
    production. Metema, the site with the longer wet season, had a higher species richness,
    diversity and production than Abergelle. The productivity of woodlands also increased with a
    higher clay content and greater soil depth. Populations structures indeed lacked the saplings,
    except for one very isolated population on a steep mountain slope.
    The studied frankincense trees produced 41 to 840 gram of frankincense during a year with
    seven collection rounds, and 185 to 1826 gram of frankincense during a year with 14
    collection rounds. The variation in frankincense production was large across individuals.
    Frankincense production increased with tree size, tapping intensity, and tapping frequency.
    The increase in production, however, levelled-off beyond a stem diameter of 20 cm, a tapping
    intensity of 9 spots, and a tapping intensity of 10 rounds. Growth rate, survival rate and
    fruiting probability varied across populations, but were not related to soil conditions or biotic
    factors. The growth rates of the 12 Metema populations varied between 0.86 to 0.98,
    suggesting that they were all decreasing. Matrix model analyses indicated that the mortality
    of adult trees was the major bottleneck for sustainable population growth, and that the lack of
    sapling recruitment was a second major bottleneck. These bottlenecks appear both in tapped
    and non-tapped stands. Remarkably, tapped stand showed higher growth rates than nontapped
    stands, probably because productive stands were selected for harvesting resin.
    All results suggest that the remaining populations of B. papyrifera will disappear in the near
    future if the current situation continues. Frankincense production is expected to halve in 15-
    20 years. Unexpectedly, tapping had no negative effect on vital rates, nor on population
    growth rates indicating that other factors are responsible for the decline of the populations.
    Adult mortality by insect infestation and windfall, and the negative impact of grazing and fire
    on the establishment of saplings need extra attention. Management should be directed
    towards releasing two major population bottlenecks (improve sapling regeneration, reduce
    adult mortability) to maintain the Boswellia populations and frankincense production in the
    future.

    The distribution and regeneration of Boswellia papyrifera (Del.) Hochst. in Eritrea
    Ogbazghi, W. - \ 2001
    Wageningen University. Promotor(en): M. Wessel; F. Bongers. - S.l. : S.n. - ISBN 9789058083685 - 131
    boswellia - gomleverende planten - plantengeografie - distributie - verjonging - eritrea - boswellia - gum plants - phytogeography - distribution - regeneration - eritrea

    Boswellia papyrifera (Del.) Hochst. is a deciduous gum-producing multipurpose perennial tree species growing in Sudanian and Sahelian regions. The tree is tapped on the stem for oleo-gum called olibanum (true frankincense). Land clearing for agriculture and un-regulated grazing are threatening the future of the natural Boswellia woodlands in Eritrea. Against this background, a study was carried out to investigate the distribution of the species and the factors determining its distribution in Eritrea, to study the structure and dynamics of Boswellia populations, including the natural regeneration, and to identify the factors causing the decline of Boswellia woodlands and measures which can reverse this situation. At macro-level, the distribution of the species was found to be limited to the southwestern and southern parts of the country between 800-1850 m altitude receiving a mean annual rainfall of 375-700 mm with a dependable length of growing period of 45-100 days. At micro-level, the abundance and distribution of the species was found to be affected in order of importance by altitude, land use intensity, soil organic matter, and to a lesser extent by silt and pH. Tree development studies showed that trees in the lowlands were twice as high as those in the highlands. The most important outcome of the population structure study is the lack of regeneration. Out of five areas investigated regeneration was only found at two sites where trees were not tapped and which were not accessible to livestock. Further research showed that the present system of intensive annual tapping throughout the dry season leads to low production of non-viable seeds and that where viable seeds are produced, seedlings and saplings are usually destroyed by livestock. Establishment of enclosures in which tapping and grazing is not allowed were found to be an effective measure to promote natural regeneration. Further research is needed to refine this system and to investigate the feasibility of replanting former Boswellia areas.

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