| To document possible effects from urban emissions on lichen communities, three field studies were conducted in oak and conifer forests near Mexico City. To evaluate physiological responses from air pollution, seven local lichen species were fumigated with sulfur dioxide, a pollutant recognized as harmful for lichens and abundant for decades in the Mexico City's air. In chapter one, the epiphytic lichen community on sacred firs (Abies religiosa) of Desierto de los Leones, a protected area 8 km southwest of the city, is described comparatively with a similar one in El Chico National Park, a less polluted site ca. 100 km north of Mexico City. The first community had 47% fewer species, 62% less lichen cover, and a species-abundance pattern that reveals a highly disturbed community. The differences between these communities are greater at the trunk base than at the height of 2 m, and much stronger from a species to species perspective.;Two transplant experiments involving the lichens Usnea ceratina Ach. and Everniastrum neocirrhatum (Hale M. Wirth) Hale ex Sipman are described in chapter two and three. In the first one, lichen samples from oaks (Quercus rugosa) of El Chico National Park were transplanted for 54 days into an oak forest near the south margin of Mexico City. Lichen net photosynthesis and chlorophyll b decreased ca. 30 and 25%, respectively, near the city. There was no interspecific difference in the photosynthetic response, but E. neocirrhatum was more sensitive to chlorophyll b degradation near the city. Chlorophyll a decreased ca. 15% near city. Changes in carotenes depended on species and time rather than on location.;Chapter three describes a similar experiment conducted in sacred fir forests. In this case, the lichens experienced a 30% decrease in net photosynthesis and 18% degradation in chlorophyll b near Mexico City. Chlorophyll a and carotenes did not vary significantly. From the two transplant experiments, we conclude that levels of ozone in southwest Mexico City are high enough to account for the lichen responses.;The sulfur dioxide experiment is described in chapter four. Seven lichen species from El Chico National Park were fumigated twice with 0, 0.5, 1.0 and 1.5 ppm SO2. Six of them showed decreased net photosynthesis during the first or second fumigation. The decline in photosynthesis was linearly dependent on the pollutant concentration. Parmotrema stuppeum and Sticta beauvoisii, which are scarce near Mexico City, and Teloschistes flavicans, which is absent near the city, were the most sensitive lichens. Lichen chlorophylls, carotenes and dehydration rates did not vary in general. The results support the inference that SO 2 may have contributed to the lichen decline in forests near the city. |
