Why So Many Hollow Trees?

According to legend, Robin Hood and his Merry Men hid from their pursuers in a hollow tree in Sherwood Forest during the harsh reign of King John (1199-1219). The tree still stands, albeit courtesy of props and wires, and called the Major Oak. Another veteran had hollowed out enough by the 18th century to provide room for seating 20 people with a plank floor and door. Thought to be over 1,000 years old, the Bowthorpe Oak survives in Lincolnshire. Across the Atlantic ocean in the same century, Jacob Marlin and Stephen Sewell were the first white pioneers to settle in the Greenbrier Valley of WV. When they fell out over a religious argument, Sewell separated to make a home in a hollow sycamore tree (later he was killed by Indians).

Southern red oak
Hollowed Southern red oak

A financial loss to lumber companies, hollow trees win our curiosity, even affection, as natural works of art and have inspired storytellers. Albert Bigelow wrote the children’s book, The Hollow Tree and Deep Woods Book (1898), which introduced as major characters Coon, Possum and Old Black Crow who struggled to live together amicably in the same tree.

My curiosity has been stirred by trees in our yard. If the angle of a side branch creates a cup for water and debris to accumulate or if a limb is torn down by a storm or sawn off doesn’t heal over, the rot starts as specialized fungi invade the heartwood to digest cellulose and lignin. They are followed by beetle and insect grubs that nibble channels and chambers, and they in turn become the menu for woodpeckers (7 species here) that carve out larger holes for nests. A growing menagerie includes invertebrate (called saproxylic), bird, bat, racoon, and even human occupiers.

Red maple
Hollow filled in a red maple

Trees that are not stressed by storms to breaking point can reach great ages from the strength of their shell of living sapwood, but they are often felled in parks for public safety. I minimize rot and hope to save mine by filling hollows with polyurethane foam, and apply a thin coat of tar to blend with the bole and provide waterproofing. Regrettably, some of the critters I like are thereby made homeless, or would be if I didn’t make nestboxes (next post)

A Swedish study of oak trees (Quercus robur) found <1% had hollows at under 100 years old, rising to 50% at 200 to 300 years, and all were affected at greater than 400. I recall few hollow trees in England and Scotland, except for rare veterans. Yet, here in Virginia I see them everywhere, and often in relatively young trees. A local dendrologist (academic tree expert) told me it is impossible to tell the age of the oldest trees because the inner rings of heartwood always rot out, although hardly a single specimen is more than 200 years old.

Why are hollows more prevalent here than in northern Europe? If a greater abundance of fungi and bugs exists in our warm and moist climate, trees with a natural insect repellent ought to be better protected. I asked a friend in Melbourne about Australian gum trees because eucalyptus oil is a powerful repellent (I spray it to deter insects and ticks). She replied, “I’ve seen lots of old dead trees with high up hollows where birds and possums nest, and quite a few living giants with a hollow at the base (such that a person can stand inside them, how the trees remain  standing is a miracle), and plenty of fallen trees/logs with hollows in them.” There dies a beautiful hypothesis. But I have another.

The same Swedish researchers noticed the fastest growing trees were most vulnerable, much more than oaks which age slowly and seldom have hollows until past middle age, like the famous “great trees” listed by the Woodland Trust. Trees grow faster in the mid-Atlantic climate, which probably means that less lignin, a hydrophobic, cross-linked polymer, fills spaces in cell walls. I’d be glad to hear a better explanation.

Girdling the Maple

Consider the consequences if your largest blood vessels were coursing under surface of the skin instead of deep inside the body. I know it’s hard to imagine. But if the aorta ran the length of your back, branching out to limbs and major organs, the pulse wave might be visible and certainly palpable over the spine. The vena cava would return blood along the midline from belly to chest where it would plunge inside to the heart.  In a lean body this great vein would look like a bluish canal through the skin. Such a vascular anatomy offers no obvious advantages but there are several disadvantages, including a greater risk of hemorrhage. It wouldn’t take a deep wound to release a fatal gusher from either of these vessels; even a bruise might cause enough swelling to seriously affect returning venous blood.

sapsucker bore holesMy crazy musing began when I noticed rows of shallow, neatly-drilled holes encircling the trunk of our prized Japanese maple tree. This was the unmistakable signature of a yellow-bellied sapsucker which visits us in winter. Besides licking the sap, the bird finds insects that are attracted to the weeping holes so that its work generates carbohydrates and proteins for its diet. By springtime the sapsucker had left for its northern breeding grounds and the holes had dried up, but the fresh foliage was far less luxuriant than in previous years. Since I couldn’t find any signs of the tree being attacked by insects, molds or viruses I assumed the woodpecker had damaged its conducting vessels which, like the vascular anatomy in my imaginary person, lie just under the surface.

Yellow-bellied sapsucker by John James Audubon: Birds of America
Yellow-bellied sapsucker by John James Audubon: Birds of America

Although there is no circulatory system in plants the vascular tubes are somewhat analogous to blood vessels. Xylem carries water and minerals up from the root system, while phloem shifts the products of photosynthesis in the foliage to other parts. Between these two great highways lies the cambium consisting of a stem cell type (meristem) which generates new xylem and heartwood on one side and new phloem and tree bark on the other.

Trees could not have evolved conducting vessels deep in their boles because the inert heartwood would prevent their girth from increasing. But the price paid for this superficial distribution under the bark is a greater vulnerability to traumatic injury and infection. Where the sapsucker had been drilling the xylem and phloem was permanently damaged because cambium is not replaceable. Fortunately the harm to our tree is not fatal or as serious as lesions to

Frost damage in tree
Beech tree with frost lesion

others in our yard caused by frost or insect borers which expose heartwood to the elements and disease. Only by complete girdling, as American pioneers often did when clearing the eastern forests for farming, is a tree condemned slowly to an early death.

One day while hiking in l’Estrie when I lived in Quebec I came across a tiny shack with a lopsided metal chimney poking out of its roof.  I could have easily missed it deep in the maple forest, but curiosity forced by legs to follow the snow tracks of someone who had branched off from the beaten trail. I ended up at the front door, and because it was unlocked I stepped inside.

It was a dream house for a child. Every surface I touched felt sticky like cotton candy (candy floss). It was a sugar shack where maple syrup was still being made in the traditional way. There was a huge boiler in the corner standing over an open fireplace from which a chimney pipe ascended to the ceiling. This was where sap would soon be slowly evaporating to produce one gallon of amber syrup from every 40 gallons of thin fluid tapped from maple trees outside.

I didn’t have to look far to find those trees. Many of those more than nine inches in diameter had holes now vacant but which in previous seasons had held a spile from which sap would drain into a bucket. Some of them had multiple holes a few inches apart and often arranged in a spiral pattern. The harvesters had been more careful than my sapsucker to avoid harming the trees, but I can’t blame it when pancake days come round.maple syrup

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