Despite how cold it feels right now, spring is around the corner and soon dormant bees and plants will rouse themselves. The witch hazels, pussy willows and red maples will bloom, and the bees will get busy collecting pollen and nectar and raising their young. On a bright and dry day, the red maple beside our deck gives off a distinct hum from bees coming and going, singing happy songs all the while. A little later, the wind will get chill and blow, and the deck will get covered in catkins blown down from the overhanging oak, which I will sweep off, grumbling a little. Oak trees look majestic and beautiful when they are massive old Penn’s oaks, but those in my yard are tall and a little skinny and do not seem that special. Their foliage goes brown in the fall (although some red oaks supposedly blaze like maples[1]). Then, I had an awakening: I read Douglas Tallamy’s book on oak trees and about how it is the single most beneficial plant in any garden. Of course, I had known about oak trees’ value as lumber and firewood, but had not recognized how indispensable living oaks are to wildlife.
Figure 1: Red oak leaf vs. white oak leaf. From: https://www.wood-database.com/wood-articles/distinguishing-red-oak-from-white-oak/. Fair use under copyright law.
Tallamy classifies oaks as a ‘keystone’ species. Pennsylvania being the self-declared “Keystone State’, let’s take a minute to review that term. The keystone (also called capstone) of an arch is the wedge-shaped stone that is set in place last, but which supports the entire edifice by locking all the other stones in place, compressing and distributing the weight across the whole arch. The earliest keystone structures were Roman. Romans, of course, built aqueducts and bridges and buildings – and what buildings! The Colosseum in Rome had arched entrances through which people and animals streamed to fight each other to the death, or to watch the spectacle, until they collapsed under the weight of their own decadence.
Figure 2: The Colosseum in Rome. Reproduced from https://commons.wikimedia.org/w/index.php?curid=95579199. Image credit: FeaturedPics. CC BY-SA 4.0.
The term keystone was also generally applied to anything that has a large impact. A keystone species is not necessarily a populous species, but it is one that disproportionately and beneficially impacts its ecosystem. Typically, top predators are considered keystone species because, like the keystone of arches, they exert just the right amount of pressure on the community to keep it in balance. If such predators are eliminated from the ecosystem, their prey (usually herbivores) will increase in population. This will result in decimation of plants and habitat destruction.
Another type of keystone species is one that supports so many communities that removing it causes enormous damage to the community, even to the point of collapse. It turns out that oak trees provide nutritional and habitat support to about 2,300 species, including insects, fungi, animals, worms in the leaf litter below. The oak’s acorns make excellent eating because they are rich in carbohydrates, proteins and fats. Red oak acorns are more bitter than white oak acorns because of their higher tannin content, but last longer in storage and are therefore preferred by gray squirrels. White oak acorns are sweet and are sought after by a variety of animals, including humans and their ancestors since the Pleistocene, 600,000 years ago. But the animal with the ‘special relationship’ with the oak is the blue jay, which has a special ‘oak capping’ hook on its bill with which it can shuck acorns caps and get at the nut. Blue jays also store acorns in caches underground. Apparently, they are such good judges of acorns that 90% of the acorns they select for storage could germinate. Many do – and oaks spring up where jays forgot to retrieve their cache.
You may have seen bees buzzing around the catkins (male stamens) of the oak, which produce an abundance of pollen. Although the female flowers do not produce nectar, once the young acorn is developing, drilling into it, as gall wasps do, releases sugars. The sugars were intended to provide nutrition to the growing acorn, but they are now termed ‘honeydew’ and collected by bees. Sometimes, when oaks are cut down, they gush slightly sweet liquid, but this is true of all plants – it is the product of photosynthesis and would be transported around plants in phloem.
Tallamy estimates that 950 species of caterpillars feed on oaks. Many of these caterpillars themselves become food for birds and other predators which would be counted as depending on the oak. Additionally, about 800 species of tiny, midge-sized cynipid wasps lay their eggs on oaks at various locations: leaves, twigs and trunk.[2] The larvae hatch and produce a chemical which causes the oak’s cells to then grow around them to form cancers called galls. The galls may be of various shapes and sizes, growing to the size of a golf ball, and they may drop off after the animal has pupated and left, indicated by a hole in the ball. It’s been known since Roman times that tannins present in such galls reacted with ferrous iron (Fe2+) to produce an indelible dark brown ink which can not be erased or washed off, and some of the most important documents in the world: one of the oldest complete Bibles—the Codex Sinaiticus—from the 4th century, the Magna Carta, Leonardo da Vinci’s drawings, Shakespeare’s plays, the Declaration of Independence and the Constitution are written with oak gall ink! The following recipe to make the ink is from Craft Invaders[3]. Collect and crush a handful of galls and soak them in water overnight. Strain the pulp through cheesecloth or muslin. Separately stand an iron nail in cider vinegar overnight. Strain through cheesecloth or muslin. Then add drops of iron solution into the gall solution and the dark color should appear almost instantaneously. Keep adding until satisfied with the color. Thickening with Gum Arabic (dried sap of acacia trees) is recommended to make a solution that will adhere to the paper. A few drops of essential oil to extend the ink’s lifetime. Note that this ink is highly acidic and will eat through paper.) The Magna Carta and Bible are still here because they were written on vellum, or calfskin leather. Fun fact: The United Kingdom stopped storing its laws on scrolls of vellum only in 2016.[4]
Oaks originated 56 million years ago and have diversified into over 435 species distributed worldwide. Oak trees in America come in two major groups: white and red.[5] Red oak leaf lobes are sharp with bristles at the tip of each lobe whereas white oak trees have blunt-lobed leaves. White oak acorns develop and are shed the same year the oak flower is pollinated, as opposed to red oaks, whose acorns mature and drop the year following the year in which the flower is pollinated. When their water- or food-conducting vessels are damaged or infected, white oak trees quickly plug them with balloon-like structures called tyloses, preventing leakage or spread of infection, whereas red oaks do not, thereby becoming more susceptible to infection. For their watertight and rot-resisting capabilities, white oak has long been used for making liquor aging barrels and wooden ships. A dugout canoe recovered from Lake Superior is estimated at around 3,000 years old and survived a very long-ago sinking. One other thing, boat building required angled pieces of wood to build out the hull, and oak branches arising from a white oak’s trunk had just the right angles and enormous strength to support the weight of a ship. In England, laws were passed requiring villages to plant an oak on the village green which was to be nurtured until old enough and large enough to cut down for shipbuilding. The oldest floating ship in the US is the USS Constitution was built in 1795 from white oak. She is nicknamed “Old Ironsides” for her thick oaken hulls which endured cannon fire during the War of 1812 without collapsing.[6] She is berthed in Charlestown Navy Yard and, if you happen to be in Boston, I highly recommend a visit! A curious fact is that there is a collection of living oak trees at the Naval Surface Warfare Center in Indiana called ‘Constitution Grove’ to provide replacement wood for the USS Constitution.
A key feature of oaks is that they are both widely distributed but there is so much variation that there can be multiple species within a region due to water and soil preferences. Red oaks and white oaks are therefore found together and within each group, there is no clustering of related species. The benefit of this, of course, is slower disease spread due to diversity. This is a huge boon for animals which depend on the oak for food: when one plant or a few plants die due to age or disease, others are available in the same area to provide habitat and nutrition. Oaks are also helpful: older trees have well-established mycorrhizal networks that permit and support other oaks to establish their root systems.
Many oaks live hundreds of years. In Pennsylvania, Penn Oaks are oaks which were already alive and growing when William Penn landed at Penn’s Landing in 1682, making them over 340 years old now. A Southern Live Oak in Louisiana is estimated at around 1,500 years; the Stelmuže oak is the oldest tree in Lithuania and is estimated at 2,000 years old. But, according to the US Forest Service, the oldest oak trees is the clonal cluster of shrub-like oaks called the ‘Jurupa oak’ in California, estimated at 13,000 years old.[7],[8]
The United States will be 250 years old in 2026. The Jurupe oak has been around 52,000 times the US’s lifetime. How inspiring is that!
Bibliography
[1]. https://www.wikihow.com/Identify-Oak-Trees
[2]. https://thelastgreenvalley.org/the-fascinating-life-of-oak-gall-wasps/
[3]. craftinvaders.co.uk/how-to-make-oak-gall-ink/
[4]. https://www.bbc.co.uk/news/magazine-35569281
[5]. https://www.scientificamerican.com/article/how-oak-trees-evolved-to-rule-the-forests-of-the-northern-hemisphere/
[6]. https://www.nps.gov/guis/learn/historyculture/live-oak-naval-icon.htm
[7]. https://www.nps.gov/articles/species-spotlight-oaks.htm
[8]. https://www.nationalgeographic.com/science/article/the-13000-year-old-tree-that-survives-by-cloning-itself