Is Natural Vegetation Getting Depleted Because Of Air Pollution?

Your question is not specific enough to get a good answer on it's own.

There are various types of pollution, and only some actually affect plants in a negative way.

For instance, carbon dioxide emmissions do not hinder plants at all. Plants need carbon dioxide for photosynthesis, their method of obtaining energy to grow/live (a byproduct being oxygen).

Certain chemicals will affect it though- such as chemical pollution in the air manifesting as acid rain, chemical spills (acids, poisons)

The main reason plants are "getting depleted" is because humans are destroying them and their environments. Trees are used for lumber and crop lands, fields are used for crop-lands, and other areas are destroyed due to roads, cities, and other things like that (which probably doesn't account for much in the big picture though).