The surest sign of winter in my Arizona mountains is the fall change of colors. The summer’s greens are replaced by greys and browns, as ferns die back, sunflowers set seed, and the trees lose their last leaves. The change is remarkably synchronous. It feels like in a single day, summer is replaced by winter.

This rapid change could be achieved in a few different ways. One is that species evolve a ‘fixed program’ where the time for autumn dieback is fixed. Another is that species evolve a ‘fixed response’ where the time is a fixed response to a certain environmental signal. The second option is more robust if environmental change doesn’t occur at a fixed time each year. There are a few such signals plants can recognize as indicative of the time of year – changes in temperature, changes in day length, changes in the peak wavelength of sunlight. Plants are exquisitely sensitive to the environment.

On my mountain the late afternoon temperatures have just begun to drop down below the freezing point, and the plants know it. The aspens have just lost their last leaves, and the forests will keep a wan shade of white-gray for the coming months. The forest floor is carpeted in a rainbow of colors – a visual transition from the growth of summer to the quiet of the long and coming winter.