How do I go about stopping trees from appearing where I don't want them to, see attachment, I don't want them over that ridge. Is the a relatively easy way of doing this, maybe a tutorial, I don't know what to search for really as I haven't done anything with adding objects yet.
Your going to want to use a distribution shader in the density shader slot of the populator as seen in the image attachment.
There are other ways to mask out populations too like using a bitmap mask, or the painted shader, but for restricting populations by elevation the distribution shader is probably the easiest way.
Some links I found while doing some searching:
https://planetside.co.uk/forums/index.php/topic,8846.msg94767/topicseen.html#msg94767
https://planetside.co.uk/forums/index.php/topic,21687.msg218145/topicseen.html#msg218145
https://planetside.co.uk/forums/index.php/topic,8468.msg90224.html#msg90224
https://planetside.co.uk/forums/index.php/topic,5157.msg53375/topicseen.html#msg53375