It might be an import / units / rescaling issue as when using any "scaled surface(s)" Forest will inherit this scale (and that's not always desirable). In these cases, I would suggest you to apply the Reset XForm
utility over that surface to reset its local coordinates & scale.
Generally, if you are using an imported surface, usually it's handy to apply the Reset XForm
utility on it to "reset" it's local coordinates.
Hope that helps, but if the problem persist, please feel free to send us
your scene to analyse it here. Thanks in advance.