Normality
How did the Elder Tree detect various creatures and lifeforms that were not just important to its survival in some way or another but also experiencing deep fear when it was almost completely spatially isolated from the rest of the world?
The answer was that its roots were connected to the roots of other surrounding flora, which in turn were in networks of flora spreading across enormous portions of the Beast Domain. It received a massive flood of sensory data pertaining to light, sound, odor, the chemical composition of the air, and several other parameters.