A study of the ecology of the sandarac gum tree, Tetraclinis articulata (Vahl) Masters, from a site in Malta

Abstract

The aim of this work was to study the ecology of the sandarac gum tree, Tetraclinis articulata (Vahl) Masters, from a site in the Maltese islands. For this purpose, the largest population of Tetraclinis articulata was chosen. This is found in the area known as Il-Miizieb at Mellieha. The distribution of Tetraclinis articulata was studied and the approximate position of the individual trees was marked on a site map. Tetraclinis articulata is found in the valley beneath the main road, on the cliff top, and on the slopes of the valley leading to the small road. It forms the higher layer of a garigue dominated by Erica multiflora towards the edge of the cliff top, but is absent away from the edge. It forms small patches of maquis on the rocky slopes leading to the small road. Population growth was assessed by counting individuals in 1999, and then again in 2002, and comparing this to the population size in 1992. This population is growing on its own. The approximate height of all trees was measured. In April 2002 this population consisted of 88 individuals. 67% had a height of less than or equal to 1.5 m. The highest individuals had a height of approximately 6 m. Phenology was studied. Tetraclinis articulata produces male and female cones in autumn, as early as September. By July of the following year, fertilized female cones are mature, and seeds start to be released. By October, all seeds have been released. Growth was investigated by measuring the height and the trunk circumference of twelve trees before and after a definite period of time. Tetraclinis articulata is a slow-growing tree and in this population, growth rate, in terms of increase in height, is between 7.5 and 10 cm/year. The organisms interacting with Tetraclinis articulata were investigated. No epiphytes and no visible parasites were observed on this tree. The plant species growing in the same ground area and within a distance of 0.5 m from the outer circumference of a number of chosen trees were recorded. The vegetation growing further from these trees was studied by recording the plant species growing beneath line transects of specific lengths. Quite a number of plants grow in the vicinity of Tetraclinis articulata and thus compete with it for resources. Erica multiflora seems to be associated with Tetraclinis articulata. Two other plants often found in the vicinity of Tetraclinis articulata are Urginea maritima and Asparagus aphyllus. On the rocky slopes, Tetraclinis articulata grows on and between the rocks, and forms small patches of maquis together with Ceratonia siliqua. The latter seems to be associated with Tetraclinis articulata on these slopes. The dominating presence of Erica multiflora in the vicinity of Tetraclinis articulata makes this formation different from most other Tetraclinis articulata formations found in other countries. This local formation has affinities with a number of already established associations, especially with Ceratonio siliquae-Tetraclinetum and also some with Genisto moulleronii-Ericetum multiflorae, whose formations are found in Morocco. Most probably, this local formation does not belong to any established association. The animals, which interact with Tetraclinis articulata, were investigated using 20 - 30 minute intensive searches on individual trees once a month for a period of one year. The different parts of the tree were observed. Leaf litter was collected from beneath the trees. A number of animals - insects, arachnids, crustaceans, molluscs, reptiles and birds, interact with Tetraclinis articulata. They use this tree for shelter, as a safe place for nesting and laying eggs, as a place to catch prey, and as a direct source of food. The number of herbivores on Tetraclinis articulata does not seem to be very high, most probably due to the presence of chemicals with antifeedant attributes in different parts of the plant. Most of the animals interacting with Tetraclinis articulata do not depend entirely on it, however some may be more closely associated with it. An example is a particular spider, which uses this plant for building nests and laying eggs. Analysis of the soil, where this plant is growing, and climate analysis were carried out. Here Tetraclinis articulata is growing at altitudes of up to 140 m, in semi-arid conditions with mild to warm winters, in a predominantly sandy loam soil, which is slightly alkaline and non-saline, and has a low organic matter and nitrate content and a high phosphate content. Tetraclinis articulata is therefore not very edaphically demanding, partly due to arbuscular mycorrhizae. The present distribution of Tetraclinis articulata is not due to edaphic conditions, which might indicate that this plant prefers slopes, especially South-facing slopes, to horizontal ground. Finally, the area where this Tetraclinis articulata population is found, needs to be protected, since it is experiencing a high degree of human disturbance.