I find it difficult to apply a one-measure-fits-all to determine whether plants or trees are capable of suffering; they are different life forms than us, vertebrates possessing a nervous system.
We also don't judge a fish by its ability to climb a tree, right?
As a matter of fact, trees, for instance, while lacking the capability to immediately flee from imminent danger, are able to give each other a heads up about potential future risks.
They do that by means of an underground fungal network (called a mycorrhizal network) that is intertwined with their root tips.
Researcher Yuan Yuan Song and his team have demonstrated that: if a tree is under attack either by humans or insects, it predominately depends on that belowground hub to transmit chemical signals, i.e. defensive enzymes, to other connected trees so that they can hedge themselves against possible future attacks.
Read more about this in "The Gravitational Beauty of Trees":