The writer of the story, Lynda Mapes, could not have been more explicit:
The cause of death was climate change: steadily warming and drier summers, that stressed the tree in its position atop a droughty knoll.
So, lets check the data and determine the truth. My first stop was the nice website of the Office of the Washington State Climatologist (OWSC), where they have a tool for plotting climatological data. Here is the summer (June-August) precipitation for the Seattle Urban Site, about a mile away from the tree in question. It indicates an upward trend (increasing precipitation) over the period available (1895-2014), not the decline claimed by the article.
Or lets go to the Western Region Climate Center website and plot the precipitation for the same period, considering the entire Puget Sound lowlands (see below) using the NOAA/NWS climate division data set and for June through September. Very similar to the Seattle Urban Site. Not much overall trend, but there is some natural variability, with a minor peak in the 70s and 80s.
Related:
Ben Rhodes rhetoric, media and governance
Madison’s English 10 experiment.