October 13, 2022

Why don't all trees shed their leaves?

Written by TreeUtah
Evergreens in Millcreek Canyon Evergreens in Millcreek Canyon Jem Ashton

When Fall rolls around, many trees begin to change colors and eventually drop their leaves in a process called shedding, a topic we explored in last week’s blog post. This week, we’ll be learning about the trees that keep their leaves, both evergreens and the deciduous trees experiencing marcescence.

Marcescence is the phenomenon of plants retaining plant organs that are usually shed, sometimes referring to mushrooms that dry out and later revive, but most commonly used to refer to trees with leaves that change colors but do not drop until outside forces (like wind) remove them from their branches. Trees that experience marcescence are sometimes called “everciduous.” This group of trees includes species of oak and beech. In fact, all oaks display foliage marcescence for at least a portion of their lives, only becoming truly deciduous after reaching their full maturity. The evolutionary advantage of marcescence is not fully understood, but there are two main theories among many. The first being that retaining the leaves will help protect new leaf buds from winter desiccation. The second is that the leaves dropping later in the winter season or even in early spring provide the tree with a delayed source of moisture-preserving mulch, allowing the tree to take full advantage of the snowmelt.

Perhaps more famous than the “everciduous” trees are the evergreens, a group that includes pines, cedars, hemlocks, and spruces. These trees keep their green leaves (called needles) all year long. Instead of shutting down the production of chlorophyll in the winter months, many evergreens continue to produce and benefit from the chemical, one of the necessary ingredients for photosynthesis. With the energy they produce throughout the winter through photosynthesis, evergreens are able to maintain their needles, which allows them to return to full-capacity energy production immediately when spring rolls around. However, some evergreens do cease photosynthesis during the winter months. These evergreens are usually found at higher altitudes, where the soil isn’t as rich in nutrients. The needles of evergreens are able to store vast quantities of nutrients the trees need to survive. These nutrients are typically already scarce in soils at higher altitudes and even harder to access during the winter months due to the cold temperatures. During this time, evergreens are able to use the nutrients stored in their needled to compensate for the lack of nutrients available in the soil around them. The needle-shaped leaves of conifers also help the tree retain water throughout the season by producing a chemical called cutin, a wax-like substance that traps fluids in the needles and protects them from damage. The needles also produce proteins that act like antifreeze by binding to ice crystals, inhibiting their growth and preventing ice damage. These adaptations are also what allow conifers to succeed in drier climates, like Utah. The cutin they produce to retain water works in cold and hot months, lending itself to the trees’ survival all throughout the year.

There are evolutionary advantages in keeping leaves or in dropping them and the evolutionary pathways of tree species depend on the climate in which they find themselves. Deciduous trees found themselves in situations where it was more appropriate to shed their leaves and wait winters out in a dormant state while evergreens found a different way to pass their winters by. Both are marvelous examples of the natural world at work!