How about creating a line of hills with the spike-forest to hide the horizon?

The problems: the spike forest looks best as a conifer forest. It takes a lot of polygons to look really good. Check "Heart's Ease Lake" in my gallery. The background forest had about 67 million polygons.

OTOH, you can download finished Xfrog models from Xfrog public plant repository. Fir or something, and use them for closer work. For more distant trees, you should postion them and create fake masks to create layers. but it will take time.

Or you could download the demos of Xfrog and Xfrog Tune from Greenworks and then create trees with much smaller level of detail in Tune.

-- erlik