Bark Recovery of Medicinal Trees After Bark Harvesting
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Sustainable Harvest Methods for Medicinal Bark from Trees (a Research and Development Case Study from Benin) by * Patrick VAN DAMME & Claire DELVAUX KEYWORDS. — medicinal trees, ethnobotany, ethnomedicine, domestication, forest management SUMMARY. — Medicinal trees are very important for local medicine. In Benin, there is an important market demand for medicinal tree bark for treating a variety of diseases. Overharvesting and unsustainable bark collection methods, however, are negatively influencing the often scarce resource basis. Over a period of ~2 years, we monitored and evaluated tree response to debarking of 12 endemic medicinal tree species (in the Monts Kouffé area, central Benin) in an effort to develop and define harvest methods that should enable trees to survive even under continuous and intensive harvesting. This article will highlight different trees’ dynamics in overcoming debarking, and address issues of how to teach proper collection/debarking techniques to local populations who depend on the latter trees for their treatments, but also for income generation through selling of tree (bark) products. TREFWOORDEN. — etnobotanie, bosbeheer, duurzame oogst, medicinale planten, domesticatie SAMENVATTING. — Duurzame oogst van medicinale boomschors – een onderzoek in Benin. — Medicinale boomsoorten spelen een belangrijke rol in de volksgeneeskunde in de Derde Wereld. Ook in Benin is er een belangrijke markt voor boomschors die aangewend wordt om een veelheid aan ziekten te genezen. De methodes die gehanteerd worden om die schors af te oogsten zijn echter veelal niet duurzaam. Bovendien worden doorgaans te veel bomen afgeoogst zodat er eerlang niet veel meer te oogsten zal vallen… Bij het begin van de werkzaamheden die hier worden beschreven werden in totaal 12 verschillende endemische boomsoorten uit Centraal-Benin (Monts Kouffé reservaat) ontschorst. Gedurende 2 jaar hebben wij dan de dynamiek die tot herstel/hergroei van boomschors leidt, gevolgd. De resultaten tonen dat de soorten verschillende strategieën hebben om ontschorsing te overleven en/of nieuwe schors aan te maken. Vanuit die verschillende dynamieken werden verschillende groepen gedefinieerd die moeten toelaten lokale ontschorsers c.q. gebruikers te adviseren over hoe intensief iedere soort mag ontschorst worden. De tekst gaat ook in op dit laatste aspect: hoe de lokale bevolking te sensibiliseren tot meer duurzaam beheer van 'hun' medicinale bomen. MOTS-CLES. — ethnobotanique, gestion forestière, dynamique de croissance, écorce, domestication RESUME. — La récolte durable de l’écorce d’arbres médicinaux – 5 ans de recherche au Bénin (Monts Kouffé). — Plusieurs espèces d’arbres/arbustes médicinales sont exploitées par les populations locales au Tiers Monde. Beaucoup en utilisent l’écorce qui est souvent collectée de manière peu durable, ce qui porte préjudice à cette ressource naturelle qui est souvent menacée par une surexploitation. Pendant 2 ans nous avons suivi la dynamique de recroissance de l’écorce sous différents régimes d’écorçage. Les résultats montrent qu’il y a différents mécanismes de réaction. C'est ainsi que nous avons peu définir plusieurs classes d’espèces qui ont toutes besoin d’une approche spécifique de prélèvement de l’écorce. L'article donne aussi quelques conseils sur les méthodes à suivre si l'on veut guider la population locale dans une meilleure gestion de leurs espèces médicinales. Introduction Two of the many threats to medicinal plant species are the loss of local knowledge on their often multiple uses, and loss of species from the wild due to overharvesting. Every year, the sum of human knowledge about the types, distribution, ecology, and methods for managing and extracting the useful (by)products of these species declines rapidly (Hamilton, 2004). This loss of local knowledge has been ongoing for hundreds of years (and can be explained by * Laboratory of Tropical and Subtropical Agronomy and Ethnobotany, Department Plant Production, Ghent University, Coupure links 653, 9000 Ghent (Belgium). 1 growing acculturation, and pressure on traditional societies as exerted by dominant western societies). In recent decades, many ethnobotanical and ethnopharmaceutical studies have been undertaken to try to document and describe the traditional herbal products used by traditional societies, and also to validate their use (Light et al., 2005). Global demand for herbal medicine is not only large, but also steadily growing (Srivastava, 2000; Light et al., 2005). This has caused, and is still causing some valuable indigenous plant species to become threatened or even extinct (Williams et al., 2000; Shingu, 2005). Prunus africana offers a recent example of this. Indeed, bark of African stinkwood as it is called, is harvested for the European medicinal market (prostate-related diseases). Formal market demand in the north has risen over the last decade, and is currently threatening natural Prunus tree stands. In the case of Cameroon, many trees have died as a result of girdling caused by bark removal on Mount Cameroon but also in some other areas within the range of this species. The bark from the trees on Mount Cameroon is transported to the Plantecam factory at Mutengene where it is extracted to produce a powder for export to a company in France. A great deal of attention, and funding has been paid by international conservation organizations to investigate and address this harvest and perhaps for this reason the species has received a high conservation rating, with its IUCN status defined as ‘lower risk, near threatened’. Concerns are, however, that in other areas less attention will go to sustainable harvesting of P. africana so that the species may eventually become extinct in certain areas. Apart from trying to domesticate the species, the solution might also be to develop sustainable bark harvesting methods. Apart from being a source of chemicals with medicinal properties, medicinal plants c.q. trees can also have other uses and the threats from overharvesting may at least also partly be due to collection for other than medicinal purposes (Hamilton, 2004). There thus is a continuing and even ever-growing need for defining adequate conservation strategies, and developing and promoting sustainable harvesting of medicinal tree species through the use of improved harvesting techniques, but also through domestication and cultivation of the best accessions, and their introduction into existing livelihood c.q. exploitation systems (Cunningham, 1991; Schippmann et al., 2002; Botha et al., 2004; Light et al., 2005; Belcher et al., 2005, Geldenhuys, 2007). Sustainable medicinal tree species management is complex for two main reasons. First, the exploitation of these species has a variable effect on the plants themselves, depending on the parts harvested. For instance, harvesting flowers and fruits has a significant impact on species regeneration and on population viability and survival (Peters, 1994; Hall & Bawa, 1993; Witkowski & Lamont, 1994; Endress et al., 2004; Gaoue & Ticktin, 2008). On the other hand, harvesting bark or root is more damaging in terms of tree survival (Cunningham, 1991; Peters, 1994; Witkowski & Lamont, 1994; Davenport & Ndangalasi, 2002; Geldenhuys, 2004; Vermeulen, 2006). Second, most medicinal plants are multipurpose species: different kinds of gatherers are interested in the same species for different end uses (Shackleton et al., 2002). More in general, sustainable management of medicinal perennials requires knowledge of how individual trees belonging to different species will respond to different harvesting techniques (Geldenhuys, 2004). The same applies to how Non-Timber Forest Product trees (NTFPs) will behave under increased and continuous harvesting (Ticktin, 2005). In this paper, we will present the results of our study on the impact of different bark harvesting intensities on tree survival in 12 medicinal species from Benin. Bark here is considered as all tissues outside the vascular cambium regardless of their composition (Junikka, 1994). The complexity of the bark tissues derives from the combined presence of dead and living tissues. The rhytidome is the dead outer part of the bark. The latter protects 2 the tree against insect attacks but also against fire and fungi. The living tissue, usually called phloem, constitutes the inner bark. The latter is also called non-collapsed secondary phloem because it is the part of the secondary phloem that contains open and non-collapsed sieve elements (Trockenbrodt, 1990). Through these non-collapsed sieve elements, sap is transported from leaves to roots. It is easy to understand that a simple wound in the bark can easily disrupt the physiological functioning of the tree. Continuous development of new vascular tissues (Aloni, 1987), however, allows to regenerate wounded tree parts thus maintaining all metabolic processes. Bark from numerous species has long been used for the treatment of several diseases such as fungal skin infections (Milicia excelsa), fevers (Alstonia constricta), malaria (Cinchona officinalis) or benign prostatic hyperplasia (Prunus africana) (Neuwinger, 2000). Bark regeneration dynamics may vary among species depending on the ability of species and individual trees to survive harvesting and recover from the inflicted wounds. Throughout Africa, only a few studies have assessed the ability of trees to regenerate bark upon debarking/harvesting. In Cameroon, Cunningham & Mbenkum (1993) showed that P. africana achieves complete bark regrowth after ring-barking.