Again, I need to see the actual study, but this would not be the first time a climate study said “well, we investigated every cause we could think of, and none of them seemed to fit, so it must be global warming.” It’s a weird way to conduct science, assuming Co2 and warming are the default cause for every complex natural process. No direct causal relationship is needed with warming, all that is required is to eliminate any other possible causes. This means that the less well we understand any complex system, the more likely we are to determine changes in the system are somehow anthropogenic.
Yes, that's exactly what it means.
UPDATE: Climate Skeptic has now read the study and has a few more thoughts. Here's the gist of it:
Anyway, the finding of large and spatially diverse increases in mortality of trees is an important finding, and one for which the authors should feel proud to identify. The second part of the study, the hypothesized causes of this mortality, is far far weaker than the first, though this is not atypical of academic studies. Remember, in the press summaries, the authors claimed that global warming had to be the cause because they had eliminated everything else it could possibly be. So here is what their Science article mentions that they considered and rejected as possible causes:
Changes in forest density and fire exclusion policies
Old trees falling and crushing new trees
Ozone levels (they claim they look at “pollution” but ozone is the only chemical discussed)
Activity of fungal pathogen, Cronartium ribicola, in certain pines
Wow, well that certainly seems comprehensive. Can’t think of a single other thing that could be causing it.
I'm pretty sure those last two comments were meant to be sarcastic.