Ecology And Diversity Of Wall Flora Dwelling In Artificial Habitats With Special Reference To Various Successional Stages In Few Urban Sites Of Kamrup (Metro) District, Assam

Abstract

Wall vegetation includes an interesting group of plants growing on artificial habitats like walls which are not only tolerant to different types of abiotic stresses such as strong light, heat, wind, cold, drought etc. but also thrives in substrates very poor in nutrients. Human has started constructing mud, stone and brick walls for various purposes thereby creating a conducive environment for the invasion and establishment of different plants including medicinal plant species. Pioneer species after invasion on the bare wall are replaced by other species in some orderly sequence during the course of time.  Species living on rocks, stones or walls are also found to occur in rock and wall crevices where little amount of detritus and humus have accumulated. After surveying of various walls like retaining walls, garden walls, churchyard walls, building walls, fencing wall etc., it was observed that wall flora consists of different species which are at different stages of development and it includes mainly crustaceous lichens, different bryophytes, few ferns species including Adiantum sp., Pteris sp., many flowering species such as Ficus glomerata, Ficus religiosa, Eclipta alba, Portulaca oleracea, Ludwigia adscandans, Euphorbia hirta, Cleome sp., Ocimum sanctum, Sonchus sp. etc.  As the inclination of a mural site decreases, the number of species increases as the exposure of the sites is very important to invasion of different species in these artificial habitats. Species showing a marked preference for north-facing wall surfaces besides on South, West or East-facing walls because here the humidity and temperature ranges are usually small. There are pronounced and noticeable effects of atmospheric pollution on mural flora and is greatest on lichens followed by bryophytes. The pH values of the substrata which support mural vegetation usually lies between pH 7 and pH 9 in different stages of mural communities. Gradual decomposition of the wall by different agents and factors along with the accumulation of dust, soil particles and organic matter contributes minimum substrata for the development of a plant cover and thereby encourage the process of succession. In this regard, water, nutrient and light act as important limiting factor in development of mural communities in different sites and these dwelling communities possess special adaptations to tolerate fluctuations in the moisture content of the substratum and also to survive in habitats containing minimum nutrient content and sunlight availability. The anthropogenic influence on the communities is also strong which not only invites invasion of new species but may also gradually shift the direction of succession up to certain extent.