Urban foresters require methodologies that help to select tree species for urban environments, mainly in places where there is a large number of potential species such as the Valley of Mexico. We applied the Analytic Hierarchy Process (AHP) to select suitable native tree species that are under-represented or non-existent in Mexico City trees. Through bibliographic research, the selection criteria and the list of trees to be evaluated were selected; later, a group of specialists in urban forestry and arboriculture determined by pair-wise comparison matrices the specific weight of each selection criterion, while a set of taxonomists evaluated the rating of each attribute for each species. Finally, for practical purposes, the synthesis of both evaluations resulted in a ranking of 15 tree species according to their degree of aptitude suggested for Mexico City. According to results,
Selection of tree species for urban forests must be undertaken strategically to optimize the ecosystem benefits they provide (
In many cities, one of the essential components of the urban structure is green areas, and different studies have been carried out to determine the most suitable tree species (
An appropriate selection process that incorporates expert’s opinions, the available scientific information, and the conditions of the plantation site is fundamental to avoid conflicts in the future, extend the tree life, and maximize tree benefits (
The Analytical Hierarchical Process (AHP) is a multicriteria analysis technique that breakdown multiple datasets in a pair-wise comparison matrix, which is used to calculate the geometric mean and normalized weight of parameters parts to support complex decision making such as the selection of suitable trees in urban areas (
Mexico City’s territory is divided into urban land (UL) with an area of 60,867.9 ha and conservation land (CL) with 87,294.4 ha (
A conventional four-level hierarchy model was constructed to integrate all the AHP stages properly. In the first level, the overall objective of the process was established. In the next level, the selection of the most critical criteria to solve the main problem were selected. In the third level, each criterion was broken down into a series of subordinate criteria or subcriteria to provide further specificity to the resolution of the problem. Finally, the last level identified the alternatives (tree species) to be evaluated for planting purposes in an urban environment. The purpose of breaking down the problem into four stages is to analyze in an isolated manner all the elements that contribute to its resolution (
The first level of the decision-making process was the selection of small, medium and large native trees suitable for urban conditions of Mexico City. This level was determined based on the configuration of the urban landscape with a small dominating number of species, the large number of native species growing in natural conditions of the Valley of Mexico, and the recommendation of increasing native species on the urban plant selection (
Criteria and subcriteria were based on previous research on the selection of trees for urban environments (
The relative weights of both criteria and subcriteria were estimated by assigning them numerical values of the so-called fundamental scale of absolute numbers (1-9), based on the degree of importance they have on the decision (
In the case of the criteria, each one was compared against the remaining criteria and for the subcriteria, the comparisons were made only between the elements within each criterion,
From the previous procedure, the consistency index was calculated to determine the strength of the comparisons made by the participants and to validate that the judgments do not have errors, nor contradictions among them. Based on the result of this index for each questionnaire, only those whose result was equal to or less than 0.10 were considered as recommended by
A total of 13 bibliographic sources were reviewed to identify the taxonomic tree diversity near Mexico City (Valley of Mexico), resulting in a total of 1851 species (
Subsequently, those species that appeared consistently in most of the 12 primary bibliographic sources were selected, giving higher weight to those that appeared in any of the combinations of listings 1, 2, 11 and 12, as they are native species with an adjacent distribution to the study area (listings 1 and 2) and for belonging to regions with ecological affinity to the area (listings 11 and 12). It was considered that these species could have a high potential to settle and survive under harsh conditions. This last procedure generated a list including 74 tree species; later, because the plantation spaces in the city are diverse in size, the species were categorized according to height, with those up to nine meters being considered as small, those from nine to 15 meters as medium-sized, and those more than 15 meters in height as large. Mature tree size is the first characteristic to consider based on the plantation site. Finally, the 15 most suitable species were selected to fulfill the purpose of this study; five from each height category. The final number of species were considered based on the required resources and time to qualify of all attributes for all the chosen species during the experts’ evaluation step.
The 15 selected tree species were evaluated (rated) on their suitability for planting by a group of six botanists specialized in the taxonomy and with experience in the Valley of Mexico flora for the suggested attributes (subcriteria). The information was gathered through an evaluation format where the ratings were expressed based on a nine-level qualitative scale, with one assigned as the lowest value and nine the highest to rate the performance of the species about a specific attribute. For example, concerning the drought tolerance sub-criterion, taxonomists rated 7, 5 and 1 to
Finally, to obtain the final score that indicates the degree of suitability of the tree species for the conditions of Mexico City’s urban environment, the following equation was used (
where
The AHP goal of this study was the selection of suitable native trees for planting in Mexico City conditions (first level). Among the six criteria (second level) involved in the selection of tree species, the opinion of the experts through the pair-wise questionnaires indicated that for Mexico City’s conditions the most important one was the environmental. In the case of the 31 subcriteria (third level), pest and disease resistance, soil compaction tolerance, microclimate regulation, non-aggressive rooting systems, crown shape and maintenance costs result as the most important in each criteria group (
The bibliographic research showed up a list of 74 potential species for Mexico City, but only 15 alternative species grouped by height were analyzed for ranking the suitability degree with the AHP approach (
The AHP allowed the qualification of suitable native trees to be planted in green areas of Mexico City. The methodology facilitates the incorporation of bibliographical information, the experience of specialists, and a multicriteria approach to qualify the suitability of potential native species. Of the final 15 species analyzed,
Species that tolerate stressful conditions such as
The introduction of the species
Species selection based on scores derived from the analysis of performance concerning specific criteria had a 6.44 rating as the highest score; however, the average was around 5.28, so a threshold could be established for better management of species. For example, the species
Regarding the species that obtained the three highest scores, namely
The National Forestry Commission (
Among the critical points to consider to achieve a healthy urban forest, the species diversity is recognized as a crucial factor, mainly because experience has shown that in the face of specific pests and unusual climate patterns, one cannot depend on a single species (
The use of the AHP approach emphasizes that the identification of appropriate criteria and subcriteria for species selection made necessary to incorporate all the actors involved in the different levels and stages of green area planning and maintenance (
Within the subcriteria of tolerance to the urban environment, tolerance to soil compaction stood out as the one with the greatest weight; in fact, soil compaction is the main interference that occur between trees and urban infrastructure conditions and that creates problems in the medium and long-term (
Finally, crown shape and maintenance costs turned up as the most crucial subcriteria of the aesthetic and economic criteria, respectively. Problems in this regard can be attributed to a lack of planning, training, budget and proper maintenance practices. Generally, the inappropriate selection of species to be planted results in establishing species with crowns that are mostly incompatible with the urban infrastructure, as they may, for example, interfere with aerial service lines, a problem that results in constant and poorly executed pruning, so-called topping, which leads to frequent expenses (
The incorporation of AHP into decision making gave different advantages over other methodologies for the selection of species in urban environments with the specific criteria for each case. For example, the weighting of criteria reflects the actual decision-making process in which each factor to be considered has a different degree of importance (
The implementation of a multicriteria methodology (AHP) helps to adequately address the complex nature of selecting the most suitable tree from among a large number for some cities. The selection process contributes to the potential incorporation of an excellent availability of native species. Although most of the tree species evaluated in this work are not propagated at present, this should not be a constraint on promoting their use by local government and municipalities. Finally, the flexibility of this methodology could allow the combination with new criteria or tools to strengthen the results during its practical implementation.
Urban area of Mexico City.
Structure of the hierarchy for Mexico City tree species selection. (1): Drought resistance; (2): High-temperature resistance; (3): Low-temperature resistance; (4): Pest and disease resistance; (5): Salinity resistance; (6): Air pollution tolerance; (7): Wind tolerance; (8): Soil compaction tolerance; (9): Shade tolerance; (10): Poor soil tolerance; (11): Ability to retain suspended particles; (12): Noise absorption; (13): Plant palette diversification; (14): Microclimate regulation; (15): Visual beauty; (16): Lifespan; (17): Plasticity; (18): Growth rate; (19): Suitability for restrictive soil volumes; (20): Non-aggressive rooting systems; (21): Species with low VOC emissions; (22): Phenology (Deciduous/Perennial); (23): Resistance to limb breakage; (24): Flowering and fruiting; (25): Fragrance; (26): Foliage color; (27): Crown shape; (28): Bark texture; (29): Establishment costs; (30): Maintenance costs; (31): Disposal costs. (A to O): Species evaluated for Mexico City’s green areas.
Criteria and subcriteria used to guide Mexico City tree species selection. (1):
Criteria | Subcriteria | Source |
---|---|---|
Environmental | 1. Drought tolerance | 1, 2, 6, 7 |
2. High temperature resistance | 1, 2, 6, 7 | |
3. Low temperature resistance | 1, 2, 6, 7 | |
4. Pest and disease resistance | 2, 5, 6, 7 | |
5. Salinity resistance | 1, 6, 7 | |
Urban environment tolerance | 6. Air pollution tolerance | 1, 4, 5, 7 |
7. Wind tolerance | 1, 5, 6 | |
8. Soil compaction tolerance | 6 | |
9. Shade tolerance | 1, 6 | |
10. Poor soil tolerance | 1, 2 | |
Functional | 11. Ability to retain suspended particles | (new) |
12. Noise absorption | 3 | |
13. Diversification of plant palette | 3, 7 | |
14. Microclimate regulation | 3 | |
15. Visual beauty | 3 | |
Desirable species | 16. Life span | 1, 2, 3, 7 |
17. Plasticity | 5 | |
18. Growth rate | 1, 2, 3, 5, 7 | |
19. Suitability for restrictive soil volumes | (new) | |
20. Non-aggressive rooting systems | 1, 7 | |
21. Species with low VOC emissions | (new) | |
22. Phenology (deciduous/perennial) | 1, 3, 7 | |
23. Resistance to branch breakage | 2, 5 | |
Aesthetics | 24. Flowering and fruiting | 1, 2, 3, 6, 7 |
25. Fragance | 1 | |
26. Foliage color | 1, 2, 3, 6 | |
27. Crown shape | 2, 3, 6 | |
28. Bark texture | 2, 3, 6 | |
Economics | 29. Establishment cost | 4 |
30. Maintenance cost | 4 | |
31. Removal and cleanup cost | 4 |
Example of a comparison matrix to determine the weight of the criteria for Mexico City tree species selection (consistency index = 0.1)
Criteria | Environmental | Urbanenvironmenttolerance | Functional | Desirablespeciesqualities | Aesthetic | Economic | Weight |
---|---|---|---|---|---|---|---|
Environmental | 1 | 1 | 1/5 | 1/7 | 8 | 2 | 0.1169 |
Urban environment tolerance | 1 | 1 | 1 | 1/3 | 3 | 1 | 0.1249 |
Functional | 5 | 1 | 1 | 1 | 8 | 3 | 0.2822 |
Desirable species qualities | 7 | 3 | 1 | 1 | 7 | 3 | 0.3511 |
Aesthetic | 1/8 | 1/3 | 1/8 | 1/7 | 1 | 1/5 | 0.0288 |
Economic | 1/2 | 1 | 1/3 | 1/3 | 5 | 1 | 0.0962 |
Bibliographical sources to identify species with potential for tree diversification in Mexico City. (1):
Source | Source topic | No. species |
---|---|---|
1, 2 | Native tree species with adjacent distribution to the study area | 122, 79 |
3 | Tree species used for different purposes, mainly reforestation | 168 |
4 | Native tree species of Mexico | 70 |
5, 7 | Frequent tree species in the green areas of Mexico City | 131, 57 |
6 | Tree species with adjacent distribution to the study area and ornamental potential | 65 |
8 | Tree species recommended for cities | 27 |
5, 9 | Tree species recommended for Mexico City | 53, 59 |
10 | Tree species of Mexico | 300 |
11, 12 | Tree species with ecological affinity to the study area | 487, 233 |
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Relative weight of the most important criteria and subcriteria. (1): The estimated weight of the criteria is shown in descending order; (2): the subcriterion corresponds to the highest weight in each criteria group.
Criteria | Weight (1) | Subcriteria | Weight (2) |
---|---|---|---|
Environmental | 0.2576 | Pest and disease resistance | 0.3179 |
Urban environment tolerance | 0.2337 | Soil compaction tolerance | 0.3111 |
Functional | 0.1884 | Microclimate regulation | 0.3285 |
Desirable species qualities | 0.1791 | Non-aggressive rooting systems | 0.2019 |
Aesthetic | 0.0817 | Crown shape | 0.3342 |
Economic | 0.0596 | Maintenance costs | 0.6349 |
Degree of the suitability of native species by height type using AHP, to be suggested in Mexico City’s green areas.
Height | No. | Species | Score |
---|---|---|---|
Large(>15 m) | 1 | 6.46 | |
2 | 5.92 | ||
3 | 5.37 | ||
4 | 5.02 | ||
5 | 4.88 | ||
Medium-sized(9-15 m) | 1 | 5.67 | |
2 | 5.18 | ||
3 | 5.11 | ||
4 | 4.95 | ||
5 | 4.85 | ||
Small(<9 m) | 1 | 5.75 | |
2 | 5.40 | ||
3 | 5.34 | ||
4 | 4.79 | ||
5 | 4.56 |