I've noticed in the permaculture and plant-talk world a lot of folks talking about allelopathy. There seems to be some value judgments present in this term allelopathy, and I wonder if this perspective conceals more than it reveals.
Presently, I'm human and I pick my relationships. When I meet people I don't jive with, I'm present and attentive when we meet but I don't make room for them in my life. It's no biggie, it's probably a mutual disinterest and we go our separate ways. But when I meet someone I like and we hit it off, well, I'm going to do what I can to support that relationship.
In the more-than-human world, I think there is something similar going on. Plants, particularly Trees, through a complex of factors including microclimate and soil community, tend to settle into a spot and choose their relationships. A black walnut won't jive with a number of plants, but really does have a good lot of friends who, from what I can tell, the black walnut gives attention to. And that doesn't even say anything for all the other critters who love -- L-O-V-E -- black walnuts. Watching the squirrels everyday eat walnut after walnut tells me that these trees are nourishing a whole lotta life not discouraging it as a term like "allelopathy" might suggest (I"ve heard locals here talk about how walnuts "kill everything", what blindness!). Conifers, too, another derided group have swell relationships with all sorts of other creatures, be they plants a-la the ericaceaes or fungal associations with the amanitas and probably many more mushrooms, oh and their relationship with humans which could also be swell -- All that medicine and gifts from one tree! Not to mention the more invisible things these trees do for the ecosystem at large.
I want to suggest that plant-soil communities -- eco-tones -- exist and that these range and change through the ecosystem to combine to produce lovely eco-melodies.