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Table 1 Tree species gradation based on air pollution tolerance index, morphological parameters and socio-economic importance

From: Roadside air pollution in a tropical city: physiological and biochemical response from trees

Grading characterParametersPattern of assessmentGrade allotment (point)
ToleranceAir Pollution Tolerance IndexLess than 121
Between 10 and 162
Above 173 (Maximum)
MorphologicalType of treeDeciduous, Semideciduous0
Evergreen1 (Maximum)
Tree size (height)Small: less than 30 ft (< 9.14 m)0
Medium: 30–70 ft (9.17–21.34 m)1
Large: above 70 ft (21.36 m)2 (Maximum)
Canopy structureSparse, irregular, globular0
Spreading crown, open, semi dense1
Spreading dense2 (Maximum)
Laminar structure 
 (a) Leaf sizeLess than 225 mm21
225–2025 mm22
2025–4500 mm23
4500–18,225 mm24 (Maximum)
 (b) TextureSmooth0
Rough, coriaceous, leathery1 (Maximum)
 (c) HardnessSoft, delicate, fragile0
Hardy, robust, sturdy1 (Maximum)
Socio-economic valueFood, fodder, timber, medicinal, raw material, erosion control, shade/shelter, ornamental, reclamationLess than three uses0
Three or more uses1
Five or more uses2 (Maximum)
  1. Modified from Tiwari et al. (1993)