Wuchsformuntersuchungen im Páramo Costa Ricas: V. Untersuchungen an Escallonia poasana Donnell Smith, Coriaria thymifolia Humboldt & Bonpland und Myrrhidendron donnell-smithii Coulter & Rose

Abstract

Summary Three ligneous plants, Escallonia poasana, Coriaria thymifolia and Myrrhidendron donnell-smithii , growing in the paramo-vegetation of Costa Rica have been investigated with respect to their plant community relations and in regard to their growth forms. Escallonia poasana forms trees up to 12 m high or large shrubs at the edge of the cloud forest, but can also form islands of smaller trees within the Paramo vegetation or large stands of shrubs with broadly expanding plagiotropic branches. Initially the plants display a basi(meso-)tonic ramification and often a spherical outline. In pioneer sites, growth of basal prostrate branches with densely parted short shouts is supported, resulting in the formation of a low espalier-like shrub. A plant with this growth form is probably able to protect large areas of soil against desiccation and to prevent growth of competing plants to a considerable degree. In shady and protected sites, the basal ramification of the plant is usually restricted to two or three branches which at first grow more or less upright like the primary shoot. The erect primary shoot and also vigorous basal branches later tum to plagiotropic growth, developing short, parted, densely arranged shoots which spread horizontally and thus might impede the growth of neighbouring plants. In the bending zone of the main axis, one or several vigorous upright shoots are formed which soon overtop the main axis, but afterwards tum to plagiotropic growth themselves and repeat the mode of branching of the older stem. Thus the branches are arranged in horizontal tiers, giving the shrubs and trees of Escallonia poasana their characteristic outline. The treefoml of Escallonia poasana is the result of suppression of further development of basal shoots and from the fact that the lower branches of the vigorous stem die off. In protccted areas subterranean stolones can be formed in considerable amounts. By this means, even small groves can develop [rom one single plant. Flowering first takes place by single terminal flowers situated at the end of this year's leafy brachyblasts and then is repeated in a second period of flowering by single flowers terminating secondary short shoots arising from the leafaxils of the primary brachyblasts. Coriaria thymifolia is a pioneer shrub on volcanic soils. The shoots show a long-shooUshort-shoot differentiation which results in a formation of branches looking like fern-fronds. Rapid growth of the basal shoots of such a shape, which are often somewhat overhanging, enables the shrub to suppress the neighbouring plants. By means of subterranean runners it can spread rapidly over large areas. Myrrhidendron donnell-smithii , growing in the upper cloud forest as well as in smail depressions of the Paramo, forms slender stems of up to 5 m by a sympodial sequence of branches. Flowering starts at the age of 4-5 years. From this time on, the plant develops a temlinal inflorescence shoot each year, which is later overtopped and pushed aside by a continuing branch, arising from the axil of the uppermost frondose leaf and which flowers in the next year. The remaining part of the sympodial branches are 20-30 cm long and comprise 11- 14 intemodes. The slender tree streches its crown consisting of 6-8 frondose leaves to the upper level of the surrounding shrubs of the Paramo vegetation. Plants which are damaged by wind or other inlluences, but also older trees can develop branches from resting buds from the lower parts of the stem and thus gain a shrublike habit.