Oaks, Part 1

by Arti Kirch, PPRV Board Emerita

The first in a multi-part series on Oregon oaks and their relationship with native pollinators.

Winter: the Busy “Hush”

Winter in the northern parts of our hemisphere is frequently a time when the landscape looks almost lifeless.  We call this dormancy, which many people define as a temporarily inactive state.  But nothing could be further from the truth. 

Dormancy in a native oak woodland is more of a strategic pause, not a shutdown.  It is triggered mainly by decreasing daylight and cooler temperatures.  For the oak, growth stops above ground and leaves are shed in the case of our big Oregon white oaks (Quercus garryana).  

Likewise, the approximately 500 species of caterpillars, moths, and butterflies, along with various beetles and wasps supported by oaks shift modes.  They use the tree and everything around it as shelter, pantry, and time capsule to survive winter.

Let’s look at what the trees and insects are doing during this important period.

What’s Happening Above Ground

Oaks actively shed their leaves by forming an abscission layer at the base of each leaf stem. Think of it as a tiny scar which reduces water loss, prevents snow and ice damage and limits disease and pest pressure.

That crunchy layer under oaks isn’t waste.  It is prime winter habitat for:

• Butterfly and moth pupae wrapped in cocoons  

• Native bees overwintering as larvae (i.e., bees-to-be) or adults

• Ground beetles, spiders, and springtails, i.e., predators that become active on warmer days

This beautiful Io moth started life under ...

Credit: Richard Crook/Flickr

…this cluster of leaves

Credit: Jeffrey Popp

Leaf litter holds moisture and stays warmer than open air, making it a winter lifesaver for animals sheltering in it.

Next year’s leaves and flowers are already formed inside winter buds. These buds are wrapped in tough, waxy scales that protect delicate tissues from freezing temperatures and drying winds.

Dormant oak leaf buds

Credit: A. Kirch

Oak bark is full of tiny cracks, plates, and crevices that act like insulated apartments for insects.  Oaks also host the larvae of teeny wasps that specialize in getting the tree to produce amazing structures on their branches called galls, each of which is different according to the wasp species.

Typical oak gall, AKA oak "apples".  Taste like cardboard!

Credit: Jen Radueg

Oaks end up protecting insects from wind and temperature swings and from many predators (sorry, hungry birds!).

What’s Happening Inside the Tree 

Throughout the growing season, oaks have stored sugars (mainly starch) in their roots, trunk, and branches. During dormancy some sugars act as natural antifreeze, protecting cells from freezing damage.  Other internal chemicals keep their buds from opening during mid-winter warm spells and change the cell membranes so they won’t rupture in a freeze.

Many insects ALSO produce their own antifreeze, called cryoprotectants which are sugars and alcohol-like compounds that lower the freezing point of their body fluids and prevent ice crystals from damaging cells. Some insects can even survive being partially frozen. In addition, most insects dramatically reduce their metabolic rate, burning stored energy slowly to last months without feeding.

What’s Happening Below Ground  

 Even when the soil is cold, oak roots often continue to grow slowly—especially during our mild Oregon winters. They absorb the winter’s precipitation and the minerals dissolved by the influx of water.  Winter is also a time for the tree to perform some repair on their finer roots.  Contrary to popular belief, it’s the small roots of trees that are the most important for sustaining the life of the tree, conducting water and nutrients throughout the plant and communicating with beneficial fungi in the soil.

The soil also hosts a bevy of insects overwintering underground, especially solitary ground nesters like the fabulous sweat bees.  These charismatic cuties, which come in metallic green, bronze, or black, create tunnels in the ground. 

Sweat bee

Credit: Wikicommons

They do so because soil is one of the most stable winter environments -- even during cold snaps, soil temperatures change slowly compared to air.

So, winter is a hive of activity, a time of planning: spring is around the corner.  But the coming explosion of life only happens because winter worked. 

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Arti Kirch is a long-time PPRV volunteer and a regular Plant Sale “Ask-an-Expert.” She loves oak trees and all they do for our planet.