There are a few published papers on sumac allelopathy. The most extensive work was done by Petranka and McPherson in Oklahoma with Rhus copallina (Ecology, 60: 956-965; 1979). A Google search using the search terms “Rhus” and “allelopathy” will bring up more papers, including a number published (in English) in China.

The toxins involved are supposed to be tannins, although detailed chemistry has apparently not been done. From an ecological point of view, the chemistry and plant physiology are not too critical. What is important is how long the substance remains active in the soil. My experience is that a former dense sumac clone will be colonized with some native species within a year, and with careful restoration work native prairie will be recovered within two years at most.

Sumac is a really bad invader. Once it takes a hold of an area, it is a four to five year job to get rid of it. See this post for details.