Some people ask “Why native trees?” and “Why bur oaks?” It’s a little bit like asking “What’s the big deal with sandhill cranes?” True, the conservation of all native birds and trees is important. But the bur oak, like the sandhill crane, is in desperate need of protection and is symbolic of the unique and fragile ecosystems of the Great Plains. Without careful and enlightened seed collection, germination and planting, bur oaks and other local, native trees will become nostalgic curiosities limited to a few protected preserves. The impact on countless creatures—including ourselves—would be detrimental, serious and lasting.
One of the best native trees to plant in the Great Plains is the bur oak (Quercus macrocarpa). They natively grow in an amazing range of habitats and have astonishing tolerance for extremes of weather and the wearing passage of time. Because of their wide native range, bur oaks are quite variable in form and individual hardiness. When selecting bur oaks for planting projects, the provenance (geographic location of the parent population) is critical. For example, bur oaks of southern provenances tend to open buds and grow leaves earlier in the spring and hold green leaves later in the fall than their northern kin, which makes them more susceptible to cold injury when planted up north. Likewise, northern bur oaks planted in the south tend to open buds relatively late, thus missing out on spring weather that favors early growth.
Every bur oak keeps time with a phenological clock. Phenology (the timing of natural processes) relies on external cues like day length, temperature, rainfall and other seasonal factors. Knowing when to open buds, shed leaves, prepare for drought, grow roots, produce acorns is based on season clues; how an individual interprets and responds to these clues is determined by provenance. 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.
The bur oak phenology is intimately connected to those of the multitude of organisms with which it has evolved. Phenology 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. 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.
The interplay between provenance and phenology is illustrated by herbivores. Bur oaks have “high apparency.” According to this concept, long-lived species host a large number of herbivores—everything from leaf suckers, chewers, gall formers, borers and a wide range of fungi. The defensive chemicals, often lumped under the category of tannins, make certain tissues less palatable and less nutritious at various times of the year by making proteins less digestible. With the proper timing of leaf growth and the concentration of tannins, they can be fed upon without permanent damage. An important principle of sustainable arboriculture is that native trees can best resist the creatures that usually eat them, and timing is an important aspect of this principle.
The surprising numbers and variety of species and populations living on a single oak, each with distinct and interwoven phenologies, together create dynamic equilibrium. However, the timing of an oak of distant provenance can make 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 and upon which the oak itself depends. If you want to plant a bur oak with the best potential for long life and ecological contribution, then plant one that originates from a wild population close to where you live.
For trees and for tree planters, timing is everything. This is true for root growth and function, and therefore for planting. When the world above ground slows down in late fall and slowly begins to stir in early spring, life in the root zone is very active. Roots flush and produce new roots tips in a seasonally bimodal pattern, around the autumnal and the vernal equinoxes. New root tips will emerge and grow through the winter season in soil that remains above freezing. In healthy soil root tips are infected by mycorrhizal fungi almost immediately. These fungus-root organs connect to a vast and vital fungal web in soil. The goal of planting is to connect a new tree to the soil web; small root tips are the connection points. Oaks can be successfully planted throughout the year, but when we plant oaks at a time when they would naturally make new connections, we work with their natural rhythm. That’s why autumn is a good time to plant.
It is well worth the trouble to find local provenance bur oaks. Tree growers across the Great Plains have come to realize the importance of producing local provenance bur oak saplings with an abundance of fine, soft roots with tiny root tips. Choose a small sapling that has a relatively large root mass and make sure the roots aren’t circling in the bag, burlap or container. Bur oaks are sturdy by nature, so start with a small sapling with low branches that doesn’t need help to stand up straight. I prefer a sapling grown in a root-production system that is 3–4 feet tall that has been pruned very little or not at all. The root mass should be 12–18 inches wide and at least 12 inches deep.
Plant it so the root collar (the point at which the largest, primary roots connect to the trunk) is slightly above grade. This imitates natural germination and early growth. If you can’t see the top of the primary root collar immediately after planting and mulching, then the tree has been planted too deeply. Good tree planting requires good trees and good planters. That’s good advice for any native tree.