With many old trees, there arises the question, can they live forever? In principle, yes. There is little evidence supporting the existence of senescence in conifers, and all really old trees (discussion) live many times longer than the oldest living cells in the tree. In practice, though, there are certain environmental factors that can kill trees. When a tree is resistant to those factors, and/or when the factors are expressed weakly or rarely, then the tree may attain a great age.

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Redwood fire ecology includes not only resprout potential, but also fire tolerance mechanisms including highly fire-resistant bark, and an ability to produce epicormic branches in the aftermath of fire. Canopy researchers have found evidence that large trees might lose live foliage throughout a substantial portion of the crown in response to a crown fire, and then replace it all within the span of a century or so via epicormic branching.

http://www.conifers.org/cu/Sequoia.php