Part of what the researchers were trying to discover was whether the die-off was a linear progression or whether it was when a critical temperature was reached and there was a mass collapse, as with a tipping point.
There will be more research on the subject, but my personal reasoning is that 3C increase in temperature is sufficient change to kill most trees. It may kill many other things but in a world where all the trees die we are in real trouble. Clearly there are holes in that statement, but it is close enough to describe a world that is not as we know it, where the trees are dead and burning and agriculture is under severe threat, and that is not a pleasant thought.
The reasoned explanation for this is that the planet is divided into 180 degrees of latitude, 90 North and 90 South. If the temperature at the Equator is 25C and, at sea level at the Pole, it is -20C, then there is a convenient temperature range of 45C which means that the temperature changes by 1C for every 2 degrees of latitude. A degree of latitude is 60 nautical miles or 111 kilometres so every 222 Kilometres the temperature will change 1C and for 3C we need to travel 666 kilometres North or South.
From Kerikeri in Northland, New Zealand it would take you down to Wellington or, from the South coast of the UK it would get you to Scotland, well North of Glasgow and in the USA from Washington DC down to Augusta in Georgia. Google Earth has a ruler to do instant measurements from near your home so its easy to check it out.
In the few times that I have checked this theory it has worked and, the trees in the new locality are very different from where I started, which simply shows that the trees where you started are not suitable in the new destination. Or, if the trees stays still and the temperature changes 3C the tree will likely die.
Another check is using altitude as with every 1000 metres of altitude gain the temperature drops 6.5 degrees Centigrade. If you go up a hill 500 metres the temperature will drop 3.25C and the vegetation will be different.
The scientist Steve Running does a very good lecture on You Tube https://www.youtube.com/watch?v=LLL7t3tF7z8 which describes how the spruce trees are dying in Montana and helps explain why species of trees are dying from around the World.
Its pretty depressing but that what research shows us.