yellow maple leaf

"In the fall, a sugar maple tree may appear yellow or red. These colours are caused by two other types of pigments that are more stable than chlorophyll. These come to dominance in the leaf as the chlorophyll breaks down - yellow light is reflected off carotene molecules while red light is reflected off anthocyanin molecules. Carotene is another pigment that helps the leaf trap sunlight and anthocyanin is a pigment that is formed in the sap when sugar concentrations get quite high.

Autumn leaf colour is a function of both the concentration of these two leaf pigments and the weather. As the chlorophyll breaks down secondary pigments that are more stable dominate. Bright sunshine and temperatures that are just above the freezing mark promote the development of anthocyanins within the sweet sap. Temperatures that drop below zero will reduce the amount of anthocyanin produced and cause the leaf to appear yellow."
from Why do maple leaves change colour in the fall?

Sunny Acres Park, Guelph