There's a webservice which allows, given a specific folder, to get all the subfolder trees: it is getCategory.

By setting the parameter "cascade" to "true", you will get the tree of all the subfolders. You can find a php sample in the attachments.

Alternatively, you can use the findByProperties2 web service, setting the properties.childOf parameter with the categoryId or the prettyId of the main folder (only one folder at a time). You will retrieve the detail of all its subfolders:

{
    "client": {
        "clientId": ""
    },
    "properties": {
        "childOf": "categoryId or prettyId here"
    },
    "offset": 0,
    "numberOfResult": 50
}