Effect of sulfur dioxide on epiphytic lichens and bryophytes.

Abstract

The effect of sulfur dioxide on epiphytes was investigated by methods comprising field studies, transplantation and laboratory experiments. In the field studies, an inventory of the epiphytic flora for the region of Wawa, Ontario, was made on the basis of the investigation of 48 sites. A total of 110 epiphytic species (71 lichens, 29 mosses and 10 liverworts) were recorded. The distribution of sulfur dioxide emitted by an iron sintering plant in the area, as well as the pH and sulfate content of surface-water, soil and vegetation, were determined. The fallout pattern of SO2 and the chemical indices (pH & SO4-) of water and soil were correlated with the epiphytic flora of the region. With a rise in the concentration of SO2 in air and of SO4 - in water and soil, a marked reduction in the epiphytes, both as to the number of species and their abundance, was noted. There was also a significant decrease in the sulfur content of the vegetation with an increase in the distance from the center of pollution. According to the pattern of pollution, the investigated region can be delineated into five zones, corresponding with the epiphytic flora. From this survey it has become apparent that there are various degrees of toxiphoby among epiphytes (from highly toxitolerant to highly toxisensitive species), and that there are several species of lichens and mosses, the absence of which can serve as indicators of, SO2-pollution. In the transplant experiments, forty-two circular discs (diam. 4.8 cm), bearing 19 species of mosses and lichens, were cut from the bark of trees growing in unpolluted areas near Ottawa, Ontario. These discs were transplanted onto trees in the region of Sudbury, Ontario, where the atmosphere is heavily polluted by sulfur dioxide. After one year, the majority of the epiphytes growing on these transplanted discs were either dead or seriously damaged, and only a few species such as Bacidia chlorococca and Parmelia sulcata appeared to be toxitolerant. Thalli of Parmelia caperata and P. sulcata, collected from such discs, showed abnormal features on microscopic examination such as: (i) marked reduction in the thickness of the thallus; (ii) formation of a thin layer of a whitish, crystalline, water-insoluble and acetone-soluble substance on the upper surface of the thallus; (iii) plasmolysis, chloroplast damage, and formation of oil-globules in Trebouxia cells, the algal symbiont; and (iv) formation of chlamydospore-like bodies by the fungal symbiont, especially in the lower cortex of the thallus. From these observations it is evident that the epiphytes are affected to various degrees, both externally and internally, by SO2-pollution. In the laboratory experiments, thalli of Xanthoria fallax, X. parietina, Parmelia caperata and Physcia millerana were exposed to 5 ppm sulfur dioxide for 24 hours under various conditions of humidity. Abnormalities such as bleaching of the chlorophyll, permanent plasmolysis, and the formation of sporadic brown spots on the chloroplasts were observed in the algal cells. Sulfurous acid and Mg++ were detected in the extracts of the SO2-exposed thalli. Sulfate concentration increased in thalli exposed to SO2 in increased humidity. The absorption spectrum of chlorophyll extracted from SO2-exposed thalli showed a maximum absorption at 667 mmu characteristic of phaeophytin-a, thus indicating the degradation of chlorophyll-a into phaeophytin-a under the influence of SO2. These observations are of interest with regard to the well known sensitivity of lichens to atmospheric pollution.