Yes Dan, walnuts produce growth inhibitors in the leaves, and possibly the roots, they are allelochemical. From what I observe the effect is similar to what spruce trees exhibit, just in the drip zone of the tree. It will have an effect on understory plants mostly, which could still be a significant environmental impact.
I am observing the same kind of climate instability. Every plant variety has a fixed minimum temperature it can withstand. Many others seem to only withstand certain maximum temperatures, or need a specific dormant period. Climate zones are defined largely by the winter low temperatures, which are gradually rising, here we have changed 2 climate zones over 30 years. But this winter is more severe, which will further complicate things for native and introduced species.
There’s also the shifts in summer weather, too much or too little rain at the wrong time has to put stress on otherwise well adapted forest communities. For the last 25 years I’ve been observing a fungal bracket attacking the green ash, previously unknown. However, the emerald ash borer will soon wipe them out anyways, once it arrives.
I fear there could hardly be a worse scenario for forests globally, recent studies have shown that every forest zone in North America is under stress and most are losing carbon instead of taking it in. Faced with all that, I think relocating species might be beneficial for the environment and for preservation of the species themselves, but it’s a bad scenario.