Next refer to site index curves available from forestry agencies. Find the average tree age, say 40 years, on the horizontal axis. Then move vertically up the graph and locate the average tree height, say 70 feet. Here these lines intersect on the curve that indicates site index is That would be a very good quality site. Aspen should be grown for commercial timber production only where the site index is 60 or higher. This means that aspen should be able to grow 60 feet tall in 50 years.