Plants as Sensors - What Is It?

The role that plants and trees play, did it just build and you just got to a point of realizing how important it was? Was it a gradual thing or was it something you started from? I think there was an interesting moment when I really became aware of plants as sensors. They were sensing what plants were doing as they were responding to their environments. So this was a way to understand rainfall and heat levels and climate change even with data sets that were collected over time.