Trees (2004) 18:43–53 DOI 10.1007/s00468-003-0279-6 ORIGINAL ARTICLE Radim Adolt · Jindrich Pavlis Age structure and growth of Dracaena cinnabari populations on Socotra Received: 10 January 2003 / Accepted: 28 May 2003 / Published online: 26 July 2003 Springer-Verlag 2003 Abstract Unique Dracaena cinnabari woodlands on blood resin is available (Himmelreich et al. 1995; Socotra Island—relics of the Mio-Pliocene xerophile- Vachalkova et al. 1995), too few studies on growth sclerophyllous southern Tethys Flora—were examined in dynamic, phenology and ageing of arborescent Dracaena detail, especially with regard to their age structure. sp. have been implemented. Detailed statistical analyses of sets of 50 trees at four The age of fabulous dragon trees is thus still generally localities were performed in order to define a model clouded in overestimation by Humboldt (1814) who reflecting relationships between specific growth habit and hypothesized that a huge Dracaena draco with 15 m stem actual age. The problematic nature of determining the age girth from Orotava, Tenerife was several thousand years of an individual tree or specific populations of D. old. A more probable average figure of up to 700 years cinnabari is illustrated by three models relating to orders old was suggested by Bystrm (1960). However, later of branching, frequency of fruiting, etc. which allow the observations by Symon (1974) and Magdefrau (1975) actual tree age to be calculated. Based on statistical brought both a precise record of the age of cultivated trees analyses as well as direct field observations, D. cinnabari and also the first clues on determination of ageing. populations on Socotra do not regenerate to a great extent Nevertheless, no authentic study on ageing of natural and their age structure generally indicates overmaturity. populations of Dracaena tree species has been published. The unique Firmihin D. cinnabari woodland will reach The dragon tree group provides an example of two the stage of intensive disintegration within 30–77 years major biogeographical disjunctions between East and with 95% probability. According to our analysis of dead West Africa, being of Tethyan origin according to fossil trees, it is evident that, on average, D. cinnabari in and paleoclimatic data. Hooker (1878) first proposed that populations at Firmihin dies after reaching 17 orders of the dragon tree, together with other species of the peripheral branches. Macaronesian laurel forest, is probably a relic of an old vegetation which once existed in northwestern Africa. Keywords Age determination · Arborescent Dracaena · Axelrod (1975) proposed that subtropical elements, such Dracaena cinnabari woodland · Socotra Island · Yemen as Dracaena and Sideroxylon, found refuge in eastern and western Africa, as a consequence of the desertification of the Sahara in the late Oligocene (Quzel 1978). However, Introduction it is also clear that Dracaena sp. stretched throughout the Mediterranean region into southern Russia (Gwyne 1966). Dragon’s blood, a highly prized product of the ancient According to pollen analyses from the Neogene, two world (Milburn 1984), still makes the tree Dracaenas extinct Dracaena species have been identified. Pollen of relatively famous. This deep red liquid exuding from the first species, D. saportae Van Campo and Sivak, injured bark is known in Arabia as ‘dammalachawin’ or comes from Bohemia whilst the second, D. guinetii Van ‘cinnabar’. Local people from Socotra still use it to cure Campo and Sivak, was found in Tunez (Van Campo and gastric sores, dye wool, glue and decorate pottery and Sivak 1976). Current species of the dragon tree group are, houses (Miller and Morris 1988). Although comprehen- therefore, only a depleted and relic representation of the sive information on chemical features of the dragon’s Mio-Pliocene Laurasian subtropical flora. These species supposedly thrived at the edge of closed forests. It is R. Adolt ()) · J. Pavlis likely that they occurred in sunny and exposed areas of Faculty of Forestry and Wood Technology, rocky slopes, cliffs and escarpments (Axelrod 1975). Mendel University of Agriculture and Forestry (MUAF), The genus Dracaena comprises between 60 (Mabber- Zemedelska 3, 613 00 Brno, Czech Republic ley 1990) and 100 species (Bos in Kubitzki 1998). Recent e-mail: [email protected] 44 taxonomic ambiguity has caused its classification within clayey soil occur between rock crevices, but larger three families i.e. Agavaceae, Liliaceae and Dracae- shallow soil deposits have accumulated in depressions naceae, the latter as a family arching over the former where the gradient is slight (Popov 1957). A sufficient set confusion. Most of the Dracaena species grow as shrubs of climatic data is lacking; scarce information for a few or geophytes often having ornamental potential. There are years in the 1940s and in 2000 (PavliÐ 2001) is all that is only six species having the growth habit of a tree. These available. The island generally falls into an arid tropical arborescent taxa of Dracaena can be found: (1) on zone, but with a climate tempered considerably by north- Socotra—D. cinnabari Balf., (2) in south-western Ara- east and south-west monsoons, it is less torrid than that of bia—D. serrulata Baker, (3) in eastern Africa—D. ombet the adjacent mainland. Precipitation ranges between Kotschy & Peyr, D. schizantha Baker, (4) on Macarone- 200 mm for coastal plains and 1,000 mm (estimate sian islands and in Morocco—D. draco L., in the latter includes heavy horizontal rain-fall) for the highest location distinguished as subsp. ajgal Benabid et Cuzin, mountains (Popov 1957; Miller and Morris 2001). and (5) in Gran Canaria (Canary Islands) with the recently From a biogeographical viewpoint, Socotra is evident- described D. tamaranae A. Marrero, R.S. Almeida and M. ly more closely linked with Africa than Arabia but there Gonzles-Martn. are also interesting affinities with other island groups such All of these dragon tree species live in thermo- as the granitic Seychelles and even some remote Atlantic sclerophyllous plant communities of tropical-subtropical islands (White 1983). Its three main biogeographical regions which are rather xerophillous, sharing similar zones can be distinguished as: (1) coastal plains, (2) ecological requirements with a rainfall range of 200– limestone plateaus, and (3) the Hagghier Mountains. 500 mm and average temperatures of 18–20C (Marrero Species diversity projected through altitudinal vegeta- et al. 1998). They are found between 10N (Somalia) and tion zones is markedly influenced by orographical, 33N (Madeira), and there is also some correlation geological and pedological variability of the habitats. between latitude and altitude. Populations in Madeira The same is true on many of the Canary Islands (Sunding may be found at sea-level whilst those of Somalia never 1972). Some of the most dominant tree species often give occur below 1,400–1,800 m altitude (Bystrm 1960). the landscape a rather bizarre or prehistoric appearance Socotra Island, once linked to peninsular Yemen, (cf. Balfour 1888) with respect to their physiognomy and probably comprises the woodiest part of what was once structure. Prosperous shrubland and woodland can be called “Happy Arabia”. The island, with an area of found mainly at higher altitudes and in sheltered valleys 3,609 km2 (Aitken 2000) is situated on the African dominated by xenomorphic growth forms as a result of continental shelf 225 km east of Cape Guardafui, plant adaptation to local harsh conditions, i.e. desiccating Somalia. At times it has been described with mythical effects of strong winds and intensive solar radiation. The attributes such as Island of the Phoenix or The Abode of most evident adaptations evolved to assist the plants’ Bliss, the latter probably owing to its biodiversity wealth survival include the swollen and silvery trunks of related to an exceptional 8% forest coverage that is Adenium obesum ssp. socotranum (Vierh.) Lav. and uncommon on mainland Arabia (PavliÐ 2000). The Dendrosicyos socotranus Balf., and the distinctive um- prehistoric origin of local plants and their degree of brella or mushroom-shaped canopies of D. cinnabari endemism reaching some 35% (Miller and Cope 1996) Balf. and Euphorbia arbuscula Balf. (Mies and Beyhl place the island among the most environmentally signif- 1996). icant places on Earth. Vegetation of the island, repre- Five to six principal vegetation tiers are consistently sented by several distinctive formations, derives from the distinguishable on Socotra (Miller et al. 1999; Bucˇek Tethyan flora (Bramwell and Richardson 1973) and its 2001). Although prevalent physiognomic appearance is unique character is the result of a long period of isolation; related to shrubland category, woodland and even forest separation from Africa supposedly occurred in the late categories are also present throughout all the tiers: (1) Cretaceous (Mies 1996). coastal plains and low inland hills with prevailing Environmental conditions and quality throughout the formations of open deciduous low shrubland (widespread island are generally good and much better than those on Croton socotranus Balf. and Jatropha unicostata Balf.), the major part of mainland Yemen. The island is (2) highlands with woodland character (variety of Com- geologically characterized by the plutonic nucleus of the miphora sp. and Boswellia sp.), (3) limestone plateau with granitic Hagghier Mountains located in its centre, and by semi-evergreen forests (optimal conditions