The Great Oak of Valley Grove Church Bur Oaks and the Oak Savanna
By Myles Bakke
“How much longer do you think it might live?”
The question was from a visitor to our annual Country Social, last fall, as our little group stood admiring the beautiful old bur oak tree. I was leading a short natural history walk on the tall grass prairie restoration that surrounds the church property, as part of the day’s festivities.
I shrugged, “Who knows? Five? Ten? Or maybe twenty-five to thirty years or more, but bur oaks don’t often live much longer than this one.”
I smiled, “My plan is to outlive it though, if the doctors can keep me in spare parts, bracing bolts, and chains.”
My reference was to the stabilizing assortment of cables, chains, bolts and other arboreal bling, added to the ancient tree, through the years, to extend its longevity. A large crack extended vertically down through the middle of the trunk, and once on an early spring day before the winter dry tree had slipped its dormancy, I was amazed to see daylight through its nearly five-foot diameter.
My answer, I suppose, was meant to be the glib response of one familiar with the aging process, as if my rank of “senior citizen” somehow also bestowed a kind of equivalency. Now, when I think of that comment, I wince at the callow fatuousness of it; that tree may have been nearly one hundred years old when the nearby stone church was built in 1862.
Several weeks later the Great Oak, which anchored the southwest corner of the Valley Grove Cemetery was dead. It was a casualty of last falls devastating storm, when a spate of tornados and high winds swept through the northern end of Rice County. When my wife Ann and I received the sad news, the next morning, we drove out to pay our respects.
The base of the trunk looked as though it had exploded, its stump shattered into vertical spikes and shards. As it fell, the strong limbs slipped between the headstones, caught the great weight of tree, and eased it into an almost decorous repose doing surprisingly minimal damage on its way down.
Part of what made that tree so special, of course, was what it had endured during its long life: drought, storms, prairie fires, and European settlement. The loss of that particular tree hit me hard, and I mourned its passing as if it were an old friend. As a biologist, I know better than to invest a tree with human qualities. But, this tree was so old, had stood like a guardian on the edge of the cemetery, and been there for decades prior to consecration, that my human nature side had imbued the old bur oak, in my mind, with a certain gravitas. It must be my square headed Viking heritage, but I think all Great Oaks should die in great battles, with great storms.
It is rare to see a bur oak, of that age, that still retains the broad and spreading, open grown shape of a savanna tree. European settlement has drastically changed the landscape: prairie was plowed up, fire suppressed, savannas cut down or grazed by fenced livestock. Without fire other trees encroached on the bur oaks, and their shade intolerant lower branches were lost turning them into canopy trees with an up-swept vase like shape. Their original, iconic open grown, shape was essential in their ability to live in the high wind environment of prairie. Long branches, of high tensile strength, extended horizontally for many feet, from low on the trunk nearly touching the ground at their tips. I once stepped onto such a branch from the top of a hillside, where the oak was growing, and tight-roped a stroll about forty feet into the tree, to reach the trunk. Probably not something I would do any more, but it delighted me at the time, and the memory delights me still when I pass that wonderful old tree.
The cushion-like shape, stout trunk with a deep anchoring tap root, allows the tree to stand up to high wind by hugging the earth, and creates a laminar flow of air over the tree, instead of meeting the force like a wall.
The Great Oak had the good fortune to live in place where only deceased parishioners were planted in large numbers, and trees remained scattered. It allowed the Great Oak to remain as an exemplar of trees that typified those of the oak savanna.
The oak savanna was a widespread, pre-settlement forest type, which occurred as an eco-tone separating large prairie and forest biomes. They encroach on grasslands as scattered trees with disconnected canopies, and act as a transition between those markedly disparate life zones. They may also have a variety of shrubs and scrub trees, which are burned back to the ground, only to re-sprout from the roots after a fire. The flora beneath the oaks was sometimes an odd synthesis of prairie and forest species, and the gentle park-like quality of the place usually gives savanna an ambiance comforting to our anthropoid sensibilities.
Early government land surveyors didn’t have a uniform word for these habitats, and used a variety of terms to describe them, such as: oak scrubland, or oak barrens, and mentioned brushy red oak stands that would re-sprout from massive root systems after a burn, and produce large acorn crops only a couple of years later. Bur oaks are the dominant tree species, of this forest type, and their seedlings and saplings may survive this way, for a number of years, before they escape burning long enough to produce the thick bark that provides some fire resistance.
Oak savanna, on the boarder of forest, had mitigating effects in several ways: scattered trees created turbulence breaking the force of the desiccating wind, that swept unimpeded across hundreds of miles of the great plains. Shade from the oaks produced shorter grasses under them, and shelter for wildlife. Both of which favored grazers and browsers, which in turn, further reduced biomass. Nearer the forest edge clonal trees and brush, like aspen, sumac, and prickly ash dominated, shading out grasses and raising the humidity. All of these buffering effects were extremely important as adjoining, and vastly different ecosystems graded into one another. In pre-settlement Minnesota, oak savanna made up about ten percent of the area of the regions plant communities, only 0.01 percent of that original amount remains. What remnants, of any size, that still exist, are found on sand-plains that could not be farmed effectively, and were mostly used for grazing cattle. As a result, the prairie flora in these habitats was badly damaged or destroyed altogether. Black soil oak savanna is all but extinct in Minnesota. The leading edge of this transition is the bur oak tree, and is, by itself, the foundation of a large community of insects, spiders, mites, birds, mammals and other organisms.
The prairie is a hostile place for a tree. First of all, well-established, perennial plants already occupy nearly all space on a mature prairie, and they have highly evolved systems for collecting and conserving most water available from rainfall. The tree must depend on its deep taproot for permanent water. Next, are the desiccating effects of full sun, high wind, and the accumulating collection of other organisms that make a living at the expense of the oak tree. This includes an annoying array of creatures that eat the leaves, buds, roots, fruit, seedlings, and saplings, that makes survival difficult, even before the whole place burns to the ground with distressing frequency. But, as the old saying goes, “When life gives you lemons…some damn fool will decide to make lemonade out of your fruit.” Maybe not an old saying, but that brings us to acorns and squirrels. The acorn is technically a fruit, and tree squirrels are the major seed dispersers of oak trees, and it is complicated.
Tree squirrels, of the Genus Sciurus,[1] are “scatter hoarders”; meaning that they store large numbers of acorns and other nut species by burying them over a large area, within their territory. In most areas, acorns supply about 75% of the squirrel’s provisions for winter and early spring. Without these stores they could not survive, which also makes them the major seed predator of oak trees. The fox squirrel is the largest, and most common squirrel to live in the original savanna. Its larger size probably reduced the number of predators that it has to deal with, in the more open terrain. Many species depend heavily on acorns for food, including weevils, chipmunks, red squirrels, mice, turkeys, jays, bear, deer and others, but contribute little as seed dispersers. None are as finely co-evolved[2] to benefit each other as oaks and scatter hoarding squirrels. Various mechanisms have developed, between them, to accommodate the inherent cross-purposes of seed dispersal and seed predation. The battle of self-interests, must work for the long term to ensure the survival of both groups of squirrels and oaks, but not necessarily in the short term. First, the oaks: our oaks are divided into two sub-genera[3], the white oak group (WO) generally oaks with rounded lobes on their leaves, and the red oak group (RO) with pointy lobed leaves. Both are “masting”[4] tree species.
Masting, refers to the strategy of an entire species limiting seed production for a series of years resulting in the reduction of seed predators during those lean years. Then, in a mast year, a prodigious seed crop is produced, which exceeds predator satiation to ensure a surplus for tree reproduction. Squirrel reproduction, for example, is highly dependent on the previous fall’s successful acorn crop.
Scientists do not yet have a handle on what cues the sequence of low production years, followed by a mast year, or how synchronicity is achieved among members of a given species. Further complicating matters is the fact that not all species mast at the same time, making environmental cues, such as rainfall, temperature, humidity, etc. difficult to correlate to the phenomenon. Masting interval is also not constant, often occurring between two to five years, making some kind of internal biological clock unlikely. Synchronicity must, it would seem, require a chemical communication system within the species, but nothing has yet been identified.
The fact that the RO group germinates its acorns in the spring, and the WO, which includes bur oaks, germinate acorns in the fall, presents another complicating and interesting factor in the squirrel/oak relationship. Acorns of the RO group have a high tannic acid content, which makes them less palatable then those of white oaks, and better acorn candidates for caching. Several reasons for this fact have been suggested: first, that the presence of high tannins may cue the squirrel that the acorn has a winter dormancy, and second, that by caching the nut it allows for acorn weevils to develop longer, and provide additional insect protein for the squirrel. The weevils are also known to counter the protein binding effects of tannins, making the infested acorn more nutritious in an additional way. This, in turn, benefits the trees by reducing weevil populations, which is a major seed predator.
The greater palatability of WO acorns results in many of those acorns being eaten, on the spot, rather than cached, and would seem to be disadvantageous for the tree. It may have a behavioral effect on the squirrel, or at least some of them, however. The WO acorns are fall germinators, and when they germinate they quickly produce a large taproot, into which they transfer about half or more of the nutrient content of the nut, denying it to the squirrel, and sequestering those resources safely for the winter. It also means that any uncollected acorns will, essentially plant themselves, by quickly growing a large taproot into the soil under the tree. Most of these seedlings that start in the shade will not survive since they require full sun, but those surrounding the drip line at the edge of the tree’s canopy will develop. Fire and browsers would have controlled their numbers, in the past, but a large number of the Great Oak’s offspring still exist on the prairie edge of where the old oak stood.
This fall germination would foil caching by squirrels, and probably produce too many bur oak trees to create an effective and sustainable savanna; if it were not for squirrel counter measures in the escalating battle to balance their co-evolutionary dance of conflicting needs and requirements. Both species of tree squirrel will “notch” the apical end of the WO acorn to excise the nut’s embryo, before burying. The tree has countered by concentrating unpalatable tannins in the lower region surrounding the embryo. This tactic seems a bit futile in response to a determined squirrel, but as I mentioned earlier, it may have enough of a behavioral effect on some squirrels that it is adequately successful.
Adolescent squirrels are successful in embryo excision, only about half the time. Having helped to raise three sons, I find exquisite irony in the idea that Mother Nature has, in a sense, recognized the dependable incompetence, picky eating habits, and inattention to detail of adolescents, thus enabling the long-term survival of both groups of organisms.[5]
Further research is required to determine whether male or female adolescent squirrels are the more competent in this skill, but I’d be willing to give odds on the results.
[1] The grey squirrel, S. carolinensis, and the fox squirrel, S. niger, are the two most important seed dispersers for all oaks in Minnesota.
[2] Co-evolution occurs when two or more species influence each other’s evolution.
[3] White oak group (WO), in the Sub-genera Leucobalanus Engelmann, and red oak group (RO), is in the Sub-genera Erythrobalanus Spach.
[4] Masting is common among temperate forest trees, and many prairie species as well.
[5] In fairness to adolescents, squirrels generally cache more nuts than they use, and the winter mortality of squirrels means their caches go largely uneaten.