Nutrient status and plant growth effects of forest soils in the Basin of Mexico.

The nutrient status of forest soils in the Mexico City Air Basin was evaluated by observing plant growth responses to fertilization with N, P or both nutrients combined. P deficiency was the most frequent condition for soil from two high pollution sites and N deficiency was greatest at a low N deposition site. Concentrations of Pb and Ni, and to a lesser extent Zn and Co, were higher at the high pollution sites. However, positive plant growth responses to P and sometimes to N, and results of wheat root elongation bioassays, suggest that heavy metal concentrations were not directly phytotoxic. Further studies are needed to determine if heavy metal toxicity to mycorrhizal symbionts of eucalyptus (Eucalyptus camaldulensis Dehnh.) from high pollution sites may explain the P deficiency and stunted growth. P deficiency is expected to limit the capacity for biotic N retention in N saturated forested watersheds in the Basin of Mexico dominated by Andisols.

Decline of sacred fir (Abies religiosa) in a forest park south of Mexico City.

Decline of sacred fir (Abies religiosa) trees in the high elevation forest park, Desierto de los Leones, located south of Mexico City, is described. Trees located in the windward zone (exposed to air masses from Mexico City) were the most severely affected, especially trees at the distal ends of ravines. Examination of tree growth rings indicated decreases in ring widths for the past 30 years. Polluted air from Mexico City may be an important causal factor in fir decline. Drought, due to excessive removal of soil water, insects, mites and pathogens, and poor forest management are possible contributing and interactive factors in fir decline.

The Valley of Mexico: summary of observational studies on its air quality and effects on vegetation.

Present day Mexico City was established on the banks of the now dead Lake Texcoco by Aztec Indians in a.d. 1325. Over time, increasing population growth, urbanization, vehicular traffic and the number of diverse stationary sources, coupled with its topography and the resultant frequency of prevalent stagnant air masses make Mexico City one of the most polluted areas in the world. Use of biological indicator plants since the early 1970s has shown the critical nature of phytotoxic photochemical oxidants in the region. More recent empirical studies with bean, soybean and pine species confirm these observations and raise concern regarding the geographic magnitude of the problem. Surface measurements of air quality in the region are inadequate and require much additional support. Ultimately, ambient air quality measurements must be coupled with the observations on responses of sensitive vegetation. While the results presented in this paper are primarily observational or qualitative, they hopefully serve the purpose of bringing attention to a critical air quality issue in a developing country.