Ecology question

[Explorer-8]

There was an ecology clue in a local crossword, for which the answer was "LAYER", but I have not been able to find anything about it on the internet.

The clue was "One of four levels of vegetation defined in ecological studies". I don't know what "layer" means and I don't know what the other three levels are either because I can't find anything about it.

Does anyone here know what it means?

[ll_billym]

I think it means the different layers in height.

Tree Layer (Canopy) - Oak, Beech
Shrub Layer - Hawthorn, Holly
Herb Layer - Flowers, Ferns
Ground Layer - Mushrooms, Moss

[Explorer-8]

Thank you, ll Billym. It looks as though the clue could have been better worded.