Urban forest characteristics
In total, 1,640 trees were surveyed during 2017 and 2018. Of the sampled trees, 106 species, 72 genera, and 44 families were found. Overall, 70% of all the sampled species were introduced, and 30% were native to the Trans-Mexican Volcanic Belt. Evergreen species represented 64% and deciduous species represented 36% of the tree species. According to species biogeographical origin, most tree species were sub-tropical (45%), followed by temperate and tropical species (27.5% each).
Of the 25 urban forest variables from field data, PCA revealed five significant principal components. The new, uncorrelated, orthogonal principal components (PC) derived urban forest variables. Urban forest variables that contributed significantly to the variance captured by a particular component were used to interpret the principal components as follows: (PC1) Evergreen-subtropical canopy, (PC2) Introduced-evergreen richness, (PC3) Temperate basal area and canopy, (PC4) Deciduous basal area and canopy, (PC5) Tropical basal area and canopy.
The five principal components accounted for 78% of the data's cumulative variation; PC1 and PC2 accounted for 37% of the variation. The PC1 explained 21% of the variation in urban forest characteristics and was most strongly related to the canopy cover of evergreen, subtropical and native trees, and the basal area of subtropical and evergreen trees. The PC2 explained 16% of the variation and was most strongly related to the richness of introduced, evergreen and subtropical species. PC3 accounted for 15% of the variation and was formed by the canopy cover and basal area of temperate trees, the richness of tropical species, the basal area of natives and the number of trees. PC4 explained 14% of the variation and included the canopy cover and basal area of deciduous trees. Finally, PC5 accounted for 12% of the variation and represented the basal area and canopy cover of tropical trees (Table 1).
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PC1
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PC2
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PC3
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PC4
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PC5
|
|
Table 1
Summary of PCA correlation of variables and cumulative variance explained for each principal component based on the 25 urban forest variables derived in Mexico City
After rotation sum of squares
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5.27
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4.01
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3.40
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3.44
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2.98
|
|
Total cumulative variance explained
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21%
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37%
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52%
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66%
|
78%
|
|
Canopy cover evergreen species
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0.87
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0.09
|
0.17
|
-0.09
|
0.11
|
Structure and composition
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Canopy cover subtropical species
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0.85
|
0.11
|
-0.12
|
0.23
|
-0.17
|
Canopy cover native species
|
0.77
|
0.01
|
0.37
|
-0.08
|
-0.18
|
Basal area subtropical species
|
0.77
|
0.23
|
-0.14
|
0.36
|
0.06
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Basal area evergreen species
|
0.75
|
0.13
|
0.29
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-0.10
|
-0.09
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Basal area (m2) per tree
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0.73
|
0.12
|
0.27
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0.32
|
0.35
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Species richness per plot
|
0.13
|
0.93
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0.29
|
0.16
|
0.12
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Composition
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Species richness introduced
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-0.02
|
0.89
|
-0.08
|
0.18
|
0.14
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Species richness evergreen
|
0.21
|
0.87
|
0.23
|
-0.16
|
0.17
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Species richness subtropical
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0.31
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0.82
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-0.10
|
0.21
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-0.17
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Basal area temperate species
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0.14
|
-0.08
|
0.84
|
0.03
|
0.03
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Structure and composition
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Canopy cover temperate species
|
0.12
|
-0.04
|
0.84
|
0.12
|
-0.04
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Species richness tropical species
|
-0.07
|
0.33
|
0.74
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0.12
|
-0.02
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Basal area native species
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0.63
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-0.03
|
0.63
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-0.10
|
-0.09
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Number of trees
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0.37
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0.44
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0.41
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0.43
|
0.01
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Basal area deciduous species
|
0.12
|
0.01
|
0.02
|
0.88
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-0.02
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Structure and composition
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Canopy cover deciduous species
|
0.03
|
0.01
|
0.17
|
0.88
|
-0.06
|
Basal area tropical species
|
0.10
|
-0.07
|
-0.03
|
0.04
|
0.91
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Structure and composition
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Canopy cover tropical species
|
0.06
|
0.00
|
-0.01
|
0.02
|
0.90
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Basal area introduced species
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0.41
|
0.19
|
-0.17
|
0.50
|
0.53
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No significant correlation
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Canopy cover (m2) per tree
|
0.60
|
0.13
|
0.37
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0.46
|
0.20
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Canopy cover introduced species
|
0.34
|
0.16
|
-0.15
|
0.62
|
0.44
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Species richness native species
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0.25
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0.35
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0.64
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0.02
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-0.01
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Species richness temperate species
|
-0.17
|
0.35
|
0.02
|
-0.12
|
0.59
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Species richness deciduous species
|
-0.10
|
0.44
|
0.20
|
0.62
|
-0.05
|
The number of significant principal components and significant loadings were determined based on the broken-stick criterion. Variables ranked according to their coefficient values. Coefficients in bold indicate a significant correlation between loadings and their correspondent component.
Urban biotopes classification
The optimal number of clusters was identified at two (Silhouette width=0.59), and 7 (Dunn=0.09), and both results were evaluated. The uniformity of cluster sizes in clustering solutions varied between clustering methods (Fig. 2). For the solution of two clusters, Ward's method and average linkage produced more balanced clusters of 97 and 223 and 25 and 295 samples, respectively, while complete and single linkage produced unbalanced clusters with 9 and 311, and 1 and 319 samples, respectively (Fig. 2). For the solution of 7 clusters, Ward's method produced clusters that ranged in size between 9 and 110 samples per cluster. Complete linkage and average linkage clustering gave more unbalanced solutions with cluster sizes ranging from 2 to 217 samples per cluster. Single linkage clustering produced the most unbalanced solutions with clusters ranging from 1 to 284 samples and six clusters with less than 20 samples (Fig. 2). Accordingly, the Agglomerative Coefficient (AC) showed the strength of the clustering structure obtained by Ward's method (AC=0.99 for two clusters, AC=0.98 for seven clusters), complete clustering (AC=0.96 for two clusters, AC=0.95 for seven clusters), and average clustering (AC=0.92 each), as compared to single linkage clustering (AC=0.83 each).
Considering the results of the four agglomerative clustering methods, Ward's method was selected to derive the urban forest biotopes. A dendrogram was produced and illustrates the hierarchical and agglomerative clusters derived (Fig. 3). Classifications resulting in two clusters were interpreted as "broader-level biotope groups" characterized by their canopy percentages. Then, seven clusters nested within the two broader biotopes were identified as "finer-level biotope classes" and interpreted as biotopes defined by urban forest and environmental characteristics. No tree canopy biotope class was directly extracted from field and spatial data hexagons without trees and tree canopy and was characterized with zero canopy cover and the average values of its environmental characteristics.
Broader-level biotope groups
Three biotope groups were identified as 1) defined by impervious surfaces, 2) defined by the canopy and urban forest characteristics, and 3) defined by the absence of trees. The biotope group defined by impervious surfaces had an average of 85.5% (±24.7) impervious surfaces, 18.5% (±18.7) of canopy cover, 1.1% (±5.7) of pervious surfaces, and the PC scores in this group were represented by negative values, indicating the low influence of urban forest components. The nested finer-level biotopes within this group are biotopes 1 to 3 (Fig. 3). The biotope group defined by canopy cover and urban forest had an average of 46% (±34.3) canopy cover, 58% (±37.9) of impervious surfaces, and 5% (±19.1) of pervious surfaces; all urban forest variables were important in the formation of this cluster. Biotopes 4 to 7 were grouped in this cluster (Fig. 3). Finally, the biotope group defined by the absence of trees had 0% canopy cover, 82% (±22.2) of impervious surfaces, and 2.3% (±1.0) of pervious surfaces.
Finer-level biotope classes
Urban tree canopy across urban biotope classes ranged from 0 to 63% per unit (hexagon). Biotope 7 (63.2% ±36.1%) and Biotope 6 (60.2% ±29.8%) had the highest tree canopy cover, and all urban forest variables were important in the formation of those clusters; these biotopes differ considerably in their percentages of soft and impervious surfaces. Biotopes 3 and 1 had the lowest tree canopy cover (6.8% ±8.5, and 9.5% ±9.4, respectively), and urban forest characteristics were not meaningful in defining these clusters. The average impervious surface cover between biotopes ranged from 34% (±28.0) in biotope 7 to 95% (±7.4) in biotope 1. The density of dwellings/ha ranged from 269 (±151) in biotope 2 to 690 (±366) in biotope 7. Pervious surface cover ranged from 2.3% (±1.0) in biotope 8 to 25.6% (±41.1) in biotope 7. Biotopes 1 and 2 were represented with more than 90% by Phaeozem, and biotope 3 by Solonchak, whereas the rest of the biotope classes were a mix of soil types. Tables showing the average values of biotic and abiotic variables per cluster are found in the Supplementary Material.
Biotope 1, "Average 95% impervious surfaces", had a high percentage of impervious surfaces (95.2% ±7.4%) and was formed without the influence of any of the urban forest variables, as indicated by their negative component scores. Biotope 1 had 9.5% (±9.4) canopy cover, 0.1% (±1.0) of pervious surfaces, and a high density of dwellings of 623 per hectare (±304). Phaeozem was the dominant soil type (98.8% ±2.6).
Biotope 2, named "Average 90% impervious surfaces", had 91.6% (±19.3) of impervious surfaces, 6.8% (±8.5) of canopy cover, and no pervious surfaces. None of the urban forest variables was important in this cluster. This biotope had a dwelling density of 604 (±259) and Solonchak was the dominant soil (99.4% ±0.8%).
Biotope 3, "Average 90% impervious surfaces, introduced trees", was represented by an average of 87.3% (±17.6) of impervious surfaces, and PC2 (Introduced-evergreen richness) was the urban forest variable with more influence in the formation of this cluster with a component score of 0.15 (±2.1). It had 0% of pervious surfaces, a low density of 269 dwellings/ha (±151), and Phaeozem was the dominant soil type (98.0% ±4.2%).
Biotope 4, or "Average 70% impervious surfaces, introduced trees" had 31.9% of canopy cover (±22.3) and was mainly represented by PC2 (Introduced-evergreen richness) (0.35 ±3.9). It had 68.5% (±36.0) of impervious surfaces, 3.6% (±10.2) of pervious surfaces, a density of 633 (±337) dwellings/ha, and Phaeozem soil (65.0% ±47.3).
Biotope 5, "Average 50% tree canopy, temperate trees", was characterized by an average tree canopy cover of 54.7% (±27.8), ranging between 13% and 98%, and dominance of temperate tress as per PC3 (Temperate basal area and canopy) (5.7 ±6.0). The biotope had an average of 49.0% (±36.9) of impervious surfaces, 16.5% (±30.3) of pervious surfaces, 472 dwellings/ha (±335), and two types of soil, Phaeozem (62.6% ±48.2%) and Andosol (36.3% ±48.5%).
Biotope 6, or "Average 60% tree canopy, evergreen trees", had a range of tree canopy cover between 5% and 100% (average 60.2% ±29.8), is also characterized by evergreen and subtropical tree canopy (PC1, 6.6±4.0), and basal area of tropical trees (PC5, 5.1±3.1). This biotope on average has 52.6% (±38.1) of impervious surfaces, 0% of pervious surfaces, a density of 514 (±358) dwellings/ha, and 76.9% (±42.0) of Phaeozem soil.
Biotope 7, namely "Average 60% tree canopy, evergreen-subtropical trees", had 63.2% (±36.1) of canopy cover, and a strong influence of PC1 (Evergreen-subtropical canopy) (14.2±16.6). Biotope 7 had 34.4% (±28.0) of impervious surfaces, 25.6% (±41.1) of pervious surfaces, the highest density of dwellings/ha (690 ±366), and two soil types, Phaeozem (44.2% ±52.4%) and Andosol (44.0% ±52.1%).
Biotope 8 or "Average 80% impervious surface without trees", was not derived from the cluster analysis as it was directly interpreted as a biotope class from hexagons without trees and canopy cover. This biotope was characterized by 82.6% (±22.2) of impervious surfaces, 2.3% (±10.4) of pervious surfaces, and a dwelling density of 578 (±353). Phaeozem and Solonchak were the types of soils present in this biotope, with 66.9% (±47.0) and 32.1% (±46.7), respectively.