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Normal mapping all those tree's would [most likely] create a big slowdown...
Maybe.. but do you really know? Dont make assumptions about performance unless you have done tests. Games like Farcry and Oblivion for instance use normal mapping on all the tree trunks (not the leaves)...and performance is fine.
There is a such a thing as shader LOD as well, and one can turn off the normal mapping shader at a certain range..
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