Introduction

Abstract

In view of the great forest destruction in the whole world the tropical forests are given more and more consideration. Destruction of tropical forests is mainly due to the felling of trees for the conquest of agriculturally profitable land (e.g. in Brazil) or to useless burning of forests by pyromaniacs (e.g. in Venezuela). Settlement on cleared land, construction of roads, and cementing of large areas as well as smog in the form of SO₂, NO_X and ozone escaping from factories or waste-gas of cars in industrialized regions play an increasingly important part in the destruction of the tropical flora. One of many examples is the flora of lichens in the city of Caracas, Venezuela, which was superabundant in the fifties, but has completely disappeared now. The waste-gas escaping from the unbounded phalanx of cars and its following conversion into ozone and triethyl lead must be the reason of this debacle, as the emission of the little industrial smoke is practically negligible. Another very interesting phenomenon disappeared in Caracas as a consequence of the high degree of air pollution. The rosettes of the epiphyte Tillandsia recurvata, Bromeliaceae, normally live on branches of trees and in Caracas abundantly occupied the overhead wire system in the air resembling musical symbols on lines as in a musical manuscript. It is commonly accepted that the seeds are dispersed by wind, but the way the rosettes are distributed on the wires makes bird dispersal probable. Possibly the seeds stick to the feathers of birds which later take a rest on the wires. However, this airy pattern of rosettes on wires was present everywhere in the whole of Caracas during the sixties, but at the present day it has disappeared completely due to air pollution. Epiphytes are particularly sensitive to polluted air and consequently are the first plants to dissappear. They may be considered bioindicators. Certain species of Loranthaceae (Phoradendron) have long ago disappeared in the small town of Maracay, Venezuela, for the same reason.

“Dispersal is but one of the factors determining the presence of plants” Van der Pijl