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Site Limitations to Tree Growth

Alicia Chacalo(1), Jaime Grabinsky(2), Alejandro Aldama(3)

Universidad Autónoma Metropolitana - Azcapotzalco,
Av. San Pablo-180. Col Reynosa Tamaulipas. 02200
México D.F.

Departamentos de Energía(1), Ciencias Básicas(2) y Sistemas(3), respectively

A paper from the Proceedings of the 9th Metropolitan Tree Improvement Alliance (METRIA) Conference held in Columbus, Ohio, August 8-10, 1996.

Abstract

Based on a tree inventory data developed in 1993 and 1994 in Mexico City, we identified the major problems of street trees growing in difficult sites. The relationship between tree health and site characteristics, neighborhood socioeconomic status, and tree density per block analyzed.

Introduction

Contrary to popular belief, the Metropolitan area of Mexico City is not the world's largest metropolis, but it is one of 14 cities with more than 10 million inhabitants. With a population of 16.7 million people, it is smaller than Tokyo and equal to New York and Sao Paulo.

Mexico City's growth rate has fallen from 3.5% in the 1970's to 1.8% currently; the population increase is attributed to immigration and the high birth rate among the immigrants.

According to the United Nations Population Fund, 16% of the country's population live in conditions of extreme poverty, without access to any social services and approximately 40% are defined as poor. Poverty and lack of social services drive the rural migration to Mexico City. The increased population has expanded the Mexico City's boarders.

The city consisted of 34,000 blocks in 1980, increasing to 55, 000 blocks by 1994; a 62% increase in 14 years. (INEGI, Governmental Institute for National and Local Economic and Geographic Information).

Pollution levels are high year-round because the valley in which Mexico City is built at 2240 m above sea level, is surrounded by tall mountains.

New urban areas are heterogeneous by income level, but all are uniformly poorly planed and are typified by small green spaces. It is estimated that Mexico City's green areas have decrease 3.7% annually. This rate of decrease is highest in low income areas, being replaced by buildings and roads (Ezcurra, 1990). In neighborhoods where residential buildings are being rapidly converted to businesses, the trees and the green areas are vanishing.

Many environmental issues affecting tree growth and urban forestry programs could be mentioned, but I would like to remark on four important facts:

1. Mexico City was a lake in pre-Hispanic times.

2. Utility lines are mainly external.

3. The Spanish building style, which uses a lot of concrete, closed patios, fences, and narrow sidewalks, creates a constant struggle between the trees and the infrastructure.

4. Mexico City is in an area with a wide potential natural species diversity. The climate and soil are conducive to year-round tree growth.

Mexico City Tree Inventory

During 1993 and 1994 three professors from Metropolitan Autonomous University, 17 students and two research assistants, did a Tree Inventory in Mexico City. A representative sample of 1261 sidewalk trees in 240 blocks was measured and evaluated to describe the condition of trees. The data obtained were species, height, diameter, number of stems, site characteristics, health conditions and required treatments.

Main Problems

The main problems identified in the tree survey are: planting in inappropriate locations, inappropriate species selection, and inadequate, inappropriate tree maintenance, all of which are indicators of a lack of planning (Fig. 3). The first part of this study was published in the July 1994 Journal of Arboriculture.

We then analyzed the data by borough. We included a precise count of the number of trees per block and ascertained relations between site characteristics and health, dendrometic measures and species of the trees. This second part, has been recently published in the Journal Ciencia Forestal (Journal of Forestry Science).

Sidewalk Width. For each tree we measured the sidewalk width. The modal size was 1.6 to 2 meters; which is not sufficient space for trees. In 60 cases there was not even a sidewalk! Streets with narrow sidewalks, may represent an uncorrectable condition to tree growth.

Utility Lines. Utility lines were present in 48% of the trees and represented 24% of all the interferences.

Pavement. Pavement accounted for 19% of the interferences but affected 37% of the trees.

Soil Compaction. Soil compaction was severe around 34% of the trees. Around 13% of the trees it couldn't even be measured, because the trees were surrounded by concrete. In all, growth of 47% of the trees was greatly inhibited by this problem.

Multiple Interferences. Thirty-six percent of the trees inventoried had 3 or more interferences.

Size of the Trees. The Mexico City has a young tree population: 66% of them had a diameter of less than 20 cm. A large planting effort has been conducted by the government recently, but survival has been low. Even worse, many trees that do survive are planted on inappropriate sites. The government's political goal of planting millions of trees per year, is being translated into a greening of Mexico City, providing many benefits from the trees in urban landscapes, but also creating plenty of future problems. Young trees classified in good condition during the inventory are expected to live just a few years. In the near future this will mean higher costs for replacement.

The possibility of having high quality landscapes in Mexico, with high quality trees appropriately selected for the site, may be more and more difficult to achieve. It would be desirable to base an urban forestry program on planting high quality, properly sited trees with an adequate maintenance budget, but that situation does not exist presently. However, it would be detrimental and expensive to eliminate poor trees or improperly sited trees. We can conclude that arborists must be in charge of an urban forestry program rather than politicians. We need more training opportunities for arborists.

Site Quality. From the measurements and observations, an evaluation of the site quality was obtained. More than half of the trees are located on poor to fair quality sites, and an average of four maintenance procedures per tree were recommended.

Wounding. Wounding occurred frequently on poor quality sites. Many trees had multiple wounds; more wounds were found than the total number of trees inventoried. Wounds were classified as poor, medium or severe; the predominant wound category was severe.

Correlations. Factors affecting tree health were not always obvious. A high correlation was found between sites quality and tree health, but occasionly we had good quality trees in very poor sites.

An interesting result was the lack of correlation between socioeconomic level and tree health condition. We expected better trees in wealthy areas; a simple consequence of having more available resources.

A similar condition was found for sidewalk width and tree condition. We expected a high positive correlation between tree health and sidewalk lawn panel space. However, the results didn't show that. We observed that even if the sidewalk is wider:

1. Trees are planted very close to the curb.

2. Street vendors affect the health condition of the trees and

3. The sidewalk design is exactly the same for narrow and for wide sidewalks. When excavations are needed the same quantity of roots are damaged in both cases.

Number of Trees per Block. Great disparities exist in tree density among blocks and boroughs. More than a hundred trees were found in some blocks while no or few trees were found in others; 21% of the blocks had less than six trees. The same density disparities can be easily observed in open spaces.

Conclusions

Evaluation of the trees shows that some arboricultural practices have not achieved a professional status, even though the amount of work devoted has been considerable.

The study revealed problems that might have been prevented with a proper selection of species, a better matching of the species to the site and a more environmental sensitive urban planning that takes into account that living organisms grow.

Nowadays, the whole responsibility for street trees in Mexico City resides with the government. More active participation from non-government organizations, private nurseries, professionals and universities is needed. A more sensitive population, will demand higher quality urban trees and urban landscape.

The quality of the landscape, which is closely related to the quality of life, needs to be considered a priority, not as a luxury. We need to think that the quality of urban landscape is strongly related to the quality of the urban trees.

Literature Cited

Chacalo, A., A. Aldama and J. Grabinsky. 1994. Street Tree Inventory in Mexico City. Journal of Arboriculture. 20(4):222-226.

Chacalo, A., J. Grabinsky y A. Aldama. 1996. Inventario del arbolado de alineación de la Ciudad de México en: Ciencia Forestal en México 21(79):101-120.

Cutler, D. 1995. Jodrell Laboratory, Royal Botanic Gardens, Kew. Personal communication.

INEGI. 1994. Manzanas de la Ciudad de México. Comunicación Personal. México.

Ezcurra, E. 1990. De las chinampas a la megalopolis. El medio ambiente de la cuenca de México. Fondo de Cultura Económica, colección la ciencia desde México no. 91. México. 119p.

We would like to thank Dr. Gary Watson for his help in editing the English of this paper.



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