Lightmapping shows lighter areas cast by tree shadows

When I attempt to bake a lightmap on my terrain I keep getting lighter shadows from where my trees cast shadows onto an already shadowed area. So basically, imagine the landscape has a raised mountain casting a shadow, any trees in that shadow cause a lighter patch within the shadow.

At first I thought it was something to do with bounces, so I set this to 0, then I thought maybe an issue in the Dual Lightmaps, so I've tried with Single Lightmaps, it's like the trees cast lighter shadows (because of the transparent leaves) and the lightmap does not take the existing shadow cast by the mountain into account.

Anyone come across this before or have a suggestion to work around?

Thanks,

Jon