Family disagreement!

 Ever since I was a tiny little kid growing up in the Alps, I was told that frost was causing leaves to change colors in the Fall and I suspect still many of my contemporaries and even younger inhabitants of my remote valley of origin have maintained that belief. 

My big sister, who is 9 year my elder, not only has lived through the German occupation of France, but has seen and learned many things during her long life. Yet, since she stayed in my hometown all of her life, she still subscribes to the popular belief that that deciduous trees change colors because the frost makes them. 

The other day as we were having our regular phone conversation, I challenged her belief by saying that this change in color was because the photosynthesis. Naturally, she disagreed vehemently and forced me to go into some basic research to build up my argument. 

I found out that both of our perspectives are partially correct, but the primary reason for deciduous trees changing color in the fall is actually due to a combination of factors related to photosynthesis, which as you can guess, made me feel very good.

Still, to be fair, my sister's observation about frost is partially accurate. Frost can indeed contribute to the browning of leaves, as it can damage the cells and cause them to die, however, it's not the primary reason for the color change. My understanding about photosynthesis is closer to the mark. As the days get shorter and temperatures drop in the fall, trees begin to prepare for winter dormancy. 

Gradually their photosynthetic activity is reduced, which is the process by which they use sunlight to produce food. As photosynthesis slows down, the production of chlorophyll, the green pigment that gives leaves their color, also decreases. 

When chlorophyll production declines, other pigments that were present in the leaves all along become more visible. These pigments include carotenoids (which give leaves their yellow and orange hues) and anthocyanins (which give leaves their red and purple colors). 

The combination of these pigments creates the stunning fall foliage we see. So, while frost can play a role in the browning of leaves, the primary reason for the color change is the reduction in photosynthesis and the visibility of other pigments. Both my sister and I have valid observations, but I’d (humbly) conclude that I won that family debate!