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.

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