All Leaves Don’t Fall

By Paula Ogilvie:

As Fall approaches, trees’ once bright green leaves change to reds and yellows. The season likely gets its name from falling leaves but you may have noticed that some trees hold onto their leaves throughout the winter. Let’s begin by understanding the process by which leaves drop, then look at the unique instances where leaves remain.

The green in leaves comes from the pigment chlorophyll. Other pigments are present too but masked by the green chlorophyll. The red to bluish color seen in stems and fruits as well as fall leaves is called anthocyanin. Yellow is from the pigment carotene (also present in carrots). These secondary pigments vary with the type of tree. As the days get cooler chlorophyll production stops and the green color fades allowing other pigments to become visible. With the drop in temperature and shorter days trees start a process called abscission to “cut off” the leaves. A layer of cells forms between the branch or main stem and the leaf stalk (petiole). This abscission layer cuts off the flow of water and nutrients to the leaf. The leaves dry up and fall, blowing off in the wind. Curiously, a few trees and shrubs will retain their dry, brown leaves until spring.

Trees start a process called abscission to “cut off” the leaves...but a few will retain their dry, brown leaves until Spring.

Some oak trees in the city and especially the Scrub Oak or Gambel’s Oak in the foothills retain their dead brown leaves all winter. The petiole is still present though no nutrients are delivered to the leaves. In the spring the abscission layer is completed and the new leaves push off the old dead ones. Winter and spring winds also help to remove them. Beech (oaks are in the beech family), hornbeans, and witch hazels are some of the other 20-plus trees that may retain their leaves. A sudden cold snap may also abort the abscission process so that leaves that remain on branches. The retained leaves are “marcesent” or withering leaves, and the deciduous trees and shrubs with this trait are called everciduous. Marcesent leaves occur throughout the US and Europe.

While the process is not completely understood a few reasonable theories are suggested and it’s likely some combination of these. Marcesent leaves are often found on young or smaller trees growing in poor soil in dry locations. This is true of Scrub Oaks in Colorado foothills as well as the closely related beech trees on the East Coast. In the foothills deer graze on trees for winter food. Dry brown leaves are neither tasty nor nutritious. They hide the dormant buds and young stems, and deter deer from eating them. The dead leaves also shade the ground so that rain and snow will evaporate and melt more slowly helping replenish the roots. Pushed off in the spring by the developing new growth, the dead leaves mulch the ground around the trees providing humus for the soil as they break down. Marcesent leaves also shelter birds during the winter.

It is important to remember that trees begin the dormancy process as days start getting shorter soon after the summer solstice. Day length is consistent year to year while temperatures, freezes and snowfall varies. Plants need to be dormant to best survive winter. In Spain, which has climate areas similar to our region, it is thought that green leaves remaining longer on the trees allows for photosynthesis (how plants nourish themselves) over a longer period. This also occurs on oaks in the Midwest where leaves on upper branches stay green longer before eventually becoming marcesent. The downside is more surface area to hold heavy snows and branches more likely to break.

When winter finally arrives listen to the rustling of marcesent leaves on a windy day.

Paula Ogilvie is a former instructor of botany and biology in Denver.