How Does Weather Affect Fall Foliage? (Credit: Harvard Forest)

​‘Tis the season to travel north and bask in nature’s glory upon viewing the changing colors in the forests. Peak fall foliage season is roughly from late September to end of October, depending on the location. Year-to-year variability definitely plays a role in the exact timing of peak colors. Much like any other planned event, it is always dependent upon the weather. Whether it be a warm or cold summer, a wet or dry summer, plays a key role in how quickly or slow the leaves change color. Here, we will discuss how weather affects fall foliage.

First off, what changes leaf color overall? There are three main factors: leaf pigments, length of night, and of course weather. Leaf pigments such as carotenoids and anthocyanin present leaves with colors other than green. Chlorophyll, which is most familiar, presents leaves with their classic green color. As the year goes on and night time hours drift longer and longer, chlorophyll production slows down and eventually stops. This opens the door for carotenoids and anthocyanin to be unmasked and show their colors. Each tree has their own particular color: Oaks tend to have red or brown leaves, Hickories tend to have golden bronze, Aspen have golden yellow and Dogwood have a purplish red, to name a few. With longer and cooler nights, these colors begin to be more prominent in leaves, thus presenting us with these vibrant fall colors.

Where does weather come in? Temperature and moisture are the main influences on fall foliage. The perfect “recipe” for most prominent color displays have been found to be warm, sunny days and cool, crisp nights. During these days, sugars within the leaves are produced which help anthocyanin to form. If nights become too cold and near freezing temperatures, then these sugars cannot be produced and the veins within the leaves start to close off. Sugar and light spur production of anthocyanin pigments, which tend to produce the red, purples and crimson pigments. Carotenoids are more so unfazed by temperatures because they appear in leaves year round. So yellow and gold colors, commonly produced by carotenoids, are seen fairly constant year-to-year.

Soil moisture also plays a role in these colors as well. A late spring, for instance, can cause fall foliage to be pushed back a few weeks due to the water budget in the ground to be thrown off. A severe summer drought with essentially choke the soil and cause the fall foliage, later on in the year, to be not as vibrant. Also, drought may cause an abscission layer, reduction in photosynthesis, to form in leaves which will cause them to fall earlier than usual.

Truly, there is nothing like the vibrant colors that autumn presents us. The changing of colors are here to stay, but the timing? That’s up to the weather. 

©2019 Weather Forecaster Alec Kownacki