Is the Forest Different on the North and South Facing Slopes?

Timberline on north-facing slopes near Hilda Glacier, has looked the same for the last 400 years. However, a detailed survey shows that many seedlings have become established in the last century. Cold conditions on this north-facing slope have slowed seedling growth.		Seedlings grew about 4 times faster on the south-facing slopes of Mt. Wilcox than at the north-facing Hilda site. Many seedlings that began growth in the 1960s are now tall trees.
Source: Hilda and Wilcox photos and graphs were provided by Dr. T. Kavanagh, University of Wisconsin, Oskosh
What is happening at timberline? If conditions are warmer now, are trees growing faster?