Organisms always choose ways to sustain and nourish themselves with the available resources instead of demanding for the ideal resource, it is evolutionary process.. Through natural calamities and human errors every organism has gone through this often. Also initially when we start taking chemically grown food, our body shows symptoms, but when we keep repeating it, it stops showing the symptoms as we are suppressing the symptoms with those foods. We need to do our best in cleaning it up before consuming.

So even the trees might have initially shown the discomfort. But for decades since we have continuously forced chemicals on them, may be gradually it stopped showing outwards.