Part one of this three-part series, published in our July issue, discussed the distribution of bur oaks and the differences between northern and southern varieties and their naming.
The Local Cosmic Oak
Regardless of provenance, every bur oak (and most every organism) keeps time with the cosmos. When the sun rises on the oaks of my family home, the green magic that feeds countless creatures from turkeys to toadstools—not to mention the oak itself and the soil it grows in—begins. The photosynthetic system and all the cells that make up the symplast (web of living tree cells) function according to a circadian clock of about 24 hours—the time it takes the earth to make one axial revolution. This does not change; the cellular clock always measures the same length of time in every place and season.
There is also a seasonal clock that runs according to the time it takes for the earth to revolve around the sun. The seasonal clock runs according to external cues like day length and temperature and can be influenced by weather events like unseasonable temperatures or drought. These seasonal cues and events are called the zeitgebers, the “time-givers.” The circadian clock is adjusted by seasonal zeitgebers. How an individual oak responds to zeitgebers is determined by the genetic information that it inherited from its parent population. This why the provenance, the geographic origin of an oak (or any tree), is critical in matters of tree planting and care.
It’s all about timing. The internal clock of living cells is the result of living on a spinning planet for millions of years. Even though the cellular circadian clock in each cell is fixed, the phenology of an individual tree (the timing of natural processes) relies on external zeitgebers. How the oak in your backyard responds to these is determined by provenance. Knowing when to open buds, shed leaves, prepare for drought, produce acorns are matters of health and survival to a tree. The common phenology of bur oaks everywhere is part of what makes them distinct as a species; the subtle differences in phenology is what makes individuals and local populations distinctive and unique.
As tree planters and planners we can either work with natural rhythms or against them. But there is something much greater at stake than the well being of individual trees. The phenology of an oak is intimately connected to those of the multitude of organisms with which it has evolved, shares a habitat, feeds and on which it depends. Hardiness is not just a matter of how an oak responds to its environment; it is also a matter of how it responds to other members of its ecosystem, and how they respond to it.
Plants feed the planet; bur oaks are very good at this. I sometimes find one with especially sweet acorns and love to eat them fresh from the branch. Mice, turkeys, fox squirrels, deer, weevils, woodpeckers and other woodsy co-habitants might prefer that I abstain, but bur oaks fed the native peoples of the Plains, and I’m keeping the tradition alive. Every part of an oak is eventually eaten by something, when the time is right. But timing also protects an oak; how and when it uses energy protects it from excessive damage from those whom it feeds. Thus, the phenologies of a multitude of creatures is naturally, cosmically and intimately linked.
Caterpillars make this point. Bur oak leaves are eaten by hundreds of native species of butterfly and moth larvae, and these are eaten by other insects and by rodents and by birds. The heaviest herbivory usually happens fairly late in the summer, when leaves can survive a fair amount of damage and the work of photosynthesis is beginning to slow down. An important principle of sustainable arboriculture is that trees can best resist the creatures that usually eat them.
In order to prevent damage that is too serious, bur oaks (and other trees) manufacture their own chemical defenses. With the proper timing of leaf growth and the concentration of natural pesticides, they can be fed upon by caterpillars and other defoliators without permanent damage. And the hungry birds in attendance are happy to help. But the timing of a planted oak of distant origin might be a little off, making it more vulnerable and more easily stressed. It might also be out of rhythm with the phenologies of the countless creatures that depend on a single bur oak (numbering in the billions, if we include soil organisms) upon which the oak itself depends.
A walk in the tashka oak-hickory community of the Loess Hills above the Missouri River floodplain is like listening to the avant-garde recordings of Miles Davis or the “Rite of Spring” by Igor Stravinsky. There’s so much going on that it’s sometimes hard to make sense of the combination of sensual stimuli coming from all directions. But “In a Silent Way” (1969) by Davis became a jazz classic, and the “Rite of Spring” is considered by many to be one of the essential works of the 20th century—even though it caused an angry riot when it debuted as a ballet in 1915. The audience was put off by the weirdness of the dance, and the primal and dissonant musical score seemed unrefined and offended the ear of the time.
Of course, nature is always primal and unrefined. Even if you pause during your walk to focus on one plant or mushroom or birdsong, the way it connects to everything else around you is not immediately clear. An ecosystem is held together by the movement of energy and messages and the unimaginable complexity of relationships between countless creatures. Syncopated rhythms and harmonious dissonance is the order of the day. It is a strange and fascinating dance. When we plant a tree, we enter into to a great work composed long ago that we may not fully understand. We can either help create something mysteriously beautiful or offend the ear of the cosmos. It worked out fine for Igor, but an alien oak won’t fare so well.