I have a question,

In the article http://msp.org/gt/2004/8-3/gt-v8-n3-p13-p.pdf, in Theorem 7.1 at page 1430, the author makes mention of the phrase:

"a limit group can be obtained from abelian and surface groups by a finite sequence of free products and amalgamations over Z".

My question is :

In this decomposition is not necessary the HNN-extensions?. Why?