Morphological Adaptation of Thymus (Lamiaceae) Species in Yakutia

Abstract

The variety of life forms and the structure of individuals and ontomorphogenesis of T. extremus, T. indigirkensis, T. brevipetiolatus, and T. pavlovii under the conditions of Yakutia have been studied. It has been established that the formation of two life forms and three types of biomorphs is characteristic for the species: a monocentric vegetatively immobile dwarf subshrub (T. extremus), an implicitly polycentric vegetatively semimobile dwarf subshrub and dwarf shrub (T. indigirkensis and T. brevipetiolatus respectively), and an obviously polycentric vegetatively mobile dwarf subshrub (T. pavlovii). The basis of the adult shrub is a monopodially sympodially growing compound skeletal axis which differs in individuals of Thymus by the method of growth and position in space. An analysis of the development of species in Yakutia allowed us to identify common and specific features. Common features for the species are the predominance of the dwarf subshrub living form and its morphological plasticity, a small variety and shortening of the morphogenesis phases, a shallow rejuvenation of the ramets, a prolonged monopodial growth of shoots (up to 10 years), and the predominance of rosette shoots in the structure of the individual. Differences are revealed depending on the conditions of specific habitats of species. When T. extremus grows on a flat surface and high sludge, a dense primary shrub is formed. In the free areas of the substrate with stepwise depressions of the microrelief, the primary shrub T. extremus is loose. In T. brevipetiolatus, which grows in the steppe on the hillside, the structure of the individual is formed only by rosette shoots, their prolonged monopodial growth is maintained due to the preservation (for 1–2 years) of dry leaves. For T. pavlovii growing on a mossy substrate, this is the characteristic lodging of shoots, their intensive rooting and strengthening of vegetative mobility, and a reduction in the duration of ontogenesis of individuals due to the rapid decay of maternal structures. The revealed features of Thymus development are the mechanisms of morphological adaptation, which provide species survival in different environmental conditions of Yakutia.