Even then, a high-latitude tree would probably be skinny, like the pipecleaner spruce of Alaska's interior. The tendency for a tree to have a narrow span in comparison to its height is energy-efficient at high latitudes. The British report included an elegant diagram showing sun angle compared to tree shape, using a stylized conifer to make the image clear. Near the northern tree line, the low-angle sunlight promotes a more vertical form. There, the optimal proportions for a conical tree crown are 8 units in height for 1 in radius.
Northern spruce stand tall and skinny because that's their most efficient shape.
The permafrost part is utter speculation. You see a lot of trees when you drive that far through Alaska, and some barely earn the name. Wow, bad permafrost here, we said. The interesting thing is that nearby trees on better soil look fine, generally a bit broader than their kin growing near Fairbanks. That's to be expected, given the slightly higher sun elevation south of the Alaska Range.