The abstract says:
Greenhouse gas emissions have significantly altered global climate, and will continue to do so in the future. Increases in the frequency, duration, and/or severity of drought and heat stress associated with climate change could fundamentally alter the composition, structure, and biogeography of forests in many regions. Of particular concern are potential increases in tree mortality associated with climate-induced physiological stress and interactions with other climate-mediated processes such as insect outbreaks and wildfire. Despite this risk, existing projections of tree mortality are based on models that lack functionally realistic mortality mechanisms, and there has been no attempt to track observations of climate-driven tree mortality globally. Here we present the first global assessment of recent tree mortality attributed to drought and heat stress. Although episodic mortality occurs in the absence of climate change, studies compiled here suggest that at least some of the world's forested ecosystems already may be responding to climate change and raise concern that forests may become increasingly vulnerable to higher background tree mortality rates and die-off in response to future warming and drought, even in environments that are not normally considered water-limited. This further suggests risks to ecosystem services, including the loss of sequestered forest carbon and associated atmospheric feedbacks. Our review also identifies key information gaps and scientific uncertainties that currently hinder our ability to predict tree mortality in response to climate change and emphasizes the need for a globally coordinated observation system. Overall, our review reveals the potential for amplified tree mortality due to drought and heat in forests worldwide.The methodology of the paper is basically to do a literature search on various combinations of the search terms "tree," "forest," "mortality," "die-off," "dieback," "decline," and "drought". They assessed the resulting literature based on whether a) there was documentation of the extensiveness of the die-off, and b) there was clear attribution of the problem to some combination of higher temperatures or less precipitation. They then present a big table of all the resulting incidents they found around the globe, along with maps.
The number of papers meeting their criteria has been increasing over time:
Of course, this cannot distinguish between "forest die-offs because of drought are increasing", and "forestry scientists are becoming more interested in drought induced die-offs". Intuitively, to me, it seems most likely that there is some of both going on. But that's only a guess - their data cannot distinguish (and the paper is careful not to say that they have documented a trend of increasing forest die-offs.
The most striking thing is the spatial pattern of incidents. In North America, the bulk of incidents are in the Western US, especially the interior:
which of course is generally much drier and more mountainous than the Eastern half of the country and the immediate coastal environment.
And in Europe, they are mainly in the drier Mediterranean region and the Alps:
(There are maps of all the other continents in the paper, but I think I'd be exceeding my fair-use limit to post them all and you'll have to go pay to look at them, as well as all the other interesting figures and tables).
If there were a generalized increased in forest die-offs associated with rising global temperatures, we might expect it to hit first and hardest in places that are already closer to the edge of viability for trees due to not very much water. So the geographical pattern is consistent with that idea. Of course, one might argue that, since the paper was specifically looking for "drought" related die-offs, rather than all-causes tree mortality, even if the problem was due to natural variability it might be expected to show up in this paper mainly in more arid places.
In western North America, in particular, the problems are mainly disease/insect mediated, and there is now a fair amount of attributional literature suggesting they are climate change related:
The Bentz reference is a short book, Bark Beetle Outbreaks in Western North America: Causes and Consequences, that you can order from Amazon, so I did.
It's tough to know how to talk about this stuff. On the one hand, it's impossible to read this paper without the obvious apocalyptic potential on one's mind - if half a degree Celsius of warming can cause a fairly substantial beginning to broad-scale die-off of forests, at least in drier regions, then what will a few degrees do? In particular, the potential for nasty positive feedbacks is obvious as the carbon in dead and rotting trees is released into the air. There are some references in the paper suggesting this is happening regionally in some of the worst affected regions of bark-beetle infestation, with the forest going from a net carbon sink to a net carbon source. However, that is not a globally significant phenomenon yet (the fraction of carbon emissions taken up by the land does not have a significant trend).
On the other hand, there's no question, as an issue of scientific logic, that the case is unproven on a global scale by this paper (and the authors freely admit that). They just don't have a time series of incidents that doesn't have obvious biasses and so would allow a firm conclusion to be drawn. Apparently no such thing exists at present. So really it's a call for forestry scientists to study the issue as intensively as possible (while the rest of us wait to discover whether we are already screwed or not).