But if warmth or CO2 kills trees, how'd they get to be 200-1000 years old?

globeandmail.com: "Study: Global warming decimating old-growth forests at stunning rate"

CALGARY — The death of old-growth forests in the western United States and Canada is increasing at a stunning rate, a troubling trend linked directly to global warming that could soon transform forests into carbon dioxide emitters rather than much-needed carbon sinks, a new study warns.
...
The report, published Friday in the journal Science, examined 76 undisturbed stands in Oregon, Washington, California, Arizona, Colorado, New Mexico and British Columbia, which were at least 200 years old. Some were more than 1,000 years old.

Researchers, who have been counting trees since 1955, found that mortality rates increased in the vast majority of areas studied. They also found that the death rate doubled in the Pacific Northwest in a period of just 17 years and doubled every 29 years in the U.S. interior.

What's worse, the stands surveyed were considered healthy and resilient...