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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 character Parameters Pattern of assessment Grade allotment (point)
Tolerance Air Pollution Tolerance Index Less than 12 1
Between 10 and 16 2
Above 17 3 (Maximum)
Morphological Type of tree Deciduous, Semideciduous 0
Evergreen 1 (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 structure Sparse, irregular, globular 0
Spreading crown, open, semi dense 1
Spreading dense 2 (Maximum)
Laminar structure  
 (a) Leaf size Less than 225 mm2 1
225–2025 mm2 2
2025–4500 mm2 3
4500–18,225 mm2 4 (Maximum)
 (b) Texture Smooth 0
Rough, coriaceous, leathery 1 (Maximum)
 (c) Hardness Soft, delicate, fragile 0
Hardy, robust, sturdy 1 (Maximum)
Socio-economic value Food, fodder, timber, medicinal, raw material, erosion control, shade/shelter, ornamental, reclamation Less than three uses 0
Three or more uses 1
Five or more uses 2 (Maximum)
  1. Modified from Tiwari et al. (1993)