Summer Oaks: Survival of the Fittest
Note: This is part 2 of a multi-part series on Oregon oaks and their relationship with native pollinators.
By Arti Kirch, PPRV board member emerita and Plant Sale “Ask-an-Expert”
For many people, summer is a time of leisure and pleasure: sunny, warm days spent playing in the water and plucking juicy fruits from the vine.
But for oaks, summer means surviving a hot, dry season. Simultaneously, oaks are engaging in the demanding processes of producing the next generation of trees and setting the stage for their own growth next spring while also (inadvertently) providing food and shelter for pollinators. Let’s look in more detail at what exactly is happening with oaks and pollinators in summer.
Because oaks are wind pollinated they don’t need pollinators but pollinators need them. In fact, oaks support more insect species than almost any other tree in the Pacific Northwest as well as numerous birds and mammals. Oaks provide pollinators with key resources such as gentle shade that protects young flowering plants which produce nectar and pollen. Oak leaves are especially critical, being a common food source for dozens of caterpillars like the western oak tussock moth.
Speaking of leaves, they stop growing once summer arrives. Leaves emerged in spring as delicate things but have then toughen to prevent water loss. They do this by thickening what is called the leaf cuticle, a protective, waxy outer layer covering the leaf surface to retain water. The cuticle also defends the plant against environmental stress like the powerful UV rays from the sun.
Another tactic the leaves perform to prevent dehydration is to close their microscopic pores (stomata) during the hottest parts of the day, shutting down photosynthesis to preserve moisture.
Which brings us to the all-important subject of water and finding it when it hasn’t rained in months and there is no irrigation. This is an art perfected by oaks.
Mature oak trees build a lateral root system which can easily reach 40 to 90 feet away from the trunk. This is what pulls water from the ground, but moisture may exist only in scattered pockets during summer. To help find and fetch those water troves, the trees collaborate with a type of beneficial soil organism called mycorrhizal fungi. These micro-organisms are composed of tiny thread-like structures called hyphae which grow past standard root limits to find hidden water and nutrients. The mycorrhizal fungi form an extended network that helps plants absorb moisture from tiny soil spaces that roots cannot reach.
As if survival during drought was hard enough, oaks have to support the next generation of trees as well as their own continued existence. Oaks develop acorns in summer from flowers pollinated in the spring. This is an extremely complicated and demanding process, requiring the tree to pump sugars, fats, and starches into the acorns and hardening the outer shells. The white oak lineage (like the Oregon White Oak) completes this process in a single summer, while red oaks (like the Kellogg oak) are mid-way through a two-summer cycle for their acorns.
While performing this feat of reproduction, oaks also get them-selves ready for next year. Tucked into the junctions of current leaves, the tree forms next year's leaf and flower buds. These tiny buds are encased in protective scales to keep them safe through the upcoming fall and winter, waiting for the spring trigger to begin their growth anew.
The photosynthesis that does occur during cooler summer mornings is sent down to the root system to be stored as starch, fueling the sudden burst of growth next spring.
Now, summer is ending ...... stay tuned for what's next.
Want to learn more about oaks right here in Southern Oregon? Check out the exciting oak habitat restoration project led by Klamath Siskiyou Oak Network, sponsored by multiple organizations such as the Klamath Bird Observatory!
Title photo: Oregon State University
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