Ask Mr. Smarty Plants

Mr. Smarty Plants has talked about your concerns in a previous answer. Part of that answer is worth repeating here because the belief that "cedar trees" which is what we here in central Texas call Juniperus ashei (Ashe juniper) somehow "poison" the soil is so widely held (see the Mark Twain quote at the end). Here's a quote from the article  Biology and Ecology of Ashe Juniper by F. E. Smeins and S.D. Fuhlendorf:

"There is little evidence that the accumulated litter of Ashe juniper in anyway alters the chemical nature of the soil as it relates to growth and development of other plants (Yager 1993). Soil chemical and physical properties are in fact "improved" by the presence of the juniper litter (Marshall 1995). No allelopathic effects have been shown to be produced by the litter. The major impact of the litter seems to be its physical presence and its alteration of hydrologic properties of the area under the canopy. The thick litter layer is a difficult physical medium for seeds of other species to germinate and grow in mainly because moisture either runs off (due to the hydrophobic nature of the litter) or it dries out very quickly after a precipitation event, which prevents seedling growth from reaching the mineral soil (Yager 1993)."

The wording of your question indicates you understand a very important point when you say, "Junipers create an environment under their canopy." They do indeed. That environment includes dense shade and a lot of shed material like leaves, berries and bark (aka mulch).  Add to this that the shed material is hydrophobic ( it sheds water rather than absorbs it) and I ask you what could be less conducive to existing plants or new seedlings?"

To sum up, Mr. Smarty Plants says don't worry about what grew there before. You need to till the litter from the junipers and oaks into the soil and plant natives appropriate for you area, soil, light, drainage and enjoy.

It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so. - Mark Twain.