Trees: What does "highly tapering stem" mean?
A "highly tapering stem" is simply that the trunk of the tree is less cylindrical and more conical in profile. It's a problem because while you may have a lot of timber volume you don't have as much timber that is usable because you'll end up with a higher proportion of sawmill waste.

I'm English and have a degree in forestry. Regardless of comments elsewhere, a "highly tapering stem" is perfectly good terminology, although I agree that it could be clearer. I would tend to say "a high degree of tapering" or a "more tapered stem" for clarity's sake.

A full description of this can be found in Forest Mensuration: a handbook for practitioners by Matthews and Mackie on page 114. (They refer to a tapering log, because they are discussing felled timber).