Fall Leaves

Have you ever wondered why the leaves change color? 

In order to understand this process, we have to think about what leaves do for the plant. Leaves are the site of photosynthesis – the chemical process that allows plants to create sugar (which is their energy or food source). Plants harvest the energy from light and the carbon from CO2 to create food for themselves. They do this with a molecule called chlorophyll, which gives leaves their green color.  

 

 

As the days get shorter, there is less and less light available for the plant to use. The leaves have aged since they emerged in spring and soon enough, it costs the plant more energy to maintain its leaves than it makes from photosynthesis. This is where the plant begins readying itself for winter.  

In the winter, the plant will rest dormant (not actively growing) and wait out the colder months. It needs to drop its burdensome leaves. But rather than just shedding them quickly, the plant instead slowly breaks down the important nutrients in its leaves and draws them back into its trunk. This process can take weeks and it is the breakdown of chlorophyll which creates the spectacular colors that we associate with fall.  

In springtime, when the plant awakens, those same nutrients will be put back into the new leaves and will help them collect more light (and therefore food) for the tree throughout the summer months! 

 

 

Of course, not every plant in the forest drops all of their leaves in fall. Evergreen trees, which retain their leaves, are known as coniferous trees (those with needles and cones). Less commonly – broad leafed evergreens, like Oregon grape or the madrone tree, can be spotted in winter with full leaf canopy.  

Want to learn more about this topic?

ESF: Why Leaves Change Color
Washington Trails Association: Spectacular Fall Hikes