By David Wagner

The wind was blustering across the top of Gillespie Butte when I pulled my wife close and gave her a sudden kiss. “What brought that on?” she asked. “Look up,” I replied, “we’re under the mistletoe.”

 The old oak trees on the top of Gillespie Butte have gnarly branches with mistletoe. The branches stand out strongly this time of the year, naked nearly two months now and to remain exposed like this until spring is well under way. Oak trees with swollen, twisted branches are admired as icons of toughness and durability. One on Gillespie Butte has been designated a legacy tree by the Eugene Tree Foundation. However, there are oaks much older than this legacy tree that don’t show this kind of tortured branching, but instead show rather graceful, wavy, upward forking patterns.

 It is the mistletoe that causes the gnarly character of our oaks. Mistletoe is a stem parasite, sending probing, absorptive organs called haustoria into the living tissue of the oak branches. These haustoria take the place of roots for mistletoe, sucking water and nutrients from the oak host. The oak responds by attempting to smother the roots in masses of woody scar tissue. The lumps and twists in the branches grow wherever the infection turns, squeezing off its nutrient supply. The very few vestiges of mistletoe that occasionally persist on old infection burls prove that smothering works. Only near the tips of branches, where mistletoe infections are young, do you see large clumps of vigorous mistletoe.

David Wagner is a botanist who has worked in Eugene for more than 30 years. Every year he makes the Willamette Valley Nature Calendar, available this month at Down to Earth and the UO Museum of Natural and Cultural History. Reach him at fernzenmosses at me.com