I really can´t answer this... the only way to answer these kind of questions is to actually test the map in MP... I´ve made some maps with a lot of trees with no lag at all, and a few maps which do lag in MP but works fine in SP... It must be tested to be sure that it wont lag. Trees usually (at least from my own experience) doesnt cause that much lag in game, unless someone uses flamethrowers on them... then they are bound to create lag (fire effects).
~Zeke.
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