I need to place an exterior deck in close proximity to my very old oak trees and I don't want to damage them with any toxins that may leach from pressure treated lumber. I am planning on using posts and joists as pressure treated lumber and the decking/planks as 'untreated' douglas fir. Will the pressure treated lumber cause any potential damage to my oak trees? How long do you think the 'untreated' douglas fir will last? Are there any building techniques (e.g., placing tar paper under bearing surfaces, putting spacers under bearing surfaces) that may extend the life of my douglas fir decking?

Background: I may need to remove this deck in a few years for another construction project but this is not definite - so, I don't want to spend any more than necessary on materials. I live 30 miles east of San Francisco where we have rain weekly for 4 months - rest of time it is fairly dry - plus the deck will be in the shade. Thanks.