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Piggy back, please!” squealed Kylee, my five-year-old niece. Because she is adorable, and I don’t get to see her very often, and she only weighs thirty-eight pounds, I squatted down to let her climb on for about the fiftieth time that day. Only Auntie Em is such a pushover. So with the little monkey on my back, we galloped through mowed paths of my parents’ restored prairie in northeast Iowa. Down to the old barn, past the woodpile, and then back toward the old apple tree.

“Whoa, check that out!” I said as we rounded the side of the tree and noticed that the trunk was only half there. From ground level up to about Kylee’s chin, the tree was only a partial cylinder of wood with a hollow center. Kylee slid to the ground and we both stuck our heads inside the cavity to look around.

Blue sky peeked through two holes higher up in the trunk. I circled around to look down in, and by positioning myself at just the right angle, Kylee and I could see each other through the tree, her looking up and me looking down. We giggled.

 “Is it still alive?” Kylee asked. “Yes, it is,” I replied, “and isn’t that amazing?” This old apple tree still produces a bumper crop of the tastiest apples every few years. Since the tree was already here, and already hollow, when my parents bought the land in 1979, we have no idea what variety of apples they are, or when it became hollow.

Just by the shape of the tree – its straight bottom and leaning crown – I might guess that it was a double-trunked tree that split, or perhaps had a large lower limb that peeled off in its middle age. Once an injury like that opens up, bacteria, fungi, and invertebrates move in to the heartwood and slowly decompose it away. The heartwood is already dead, so all the tree loses is a little bit of structural stability.

All of the growth and water transport continues in the outer shell of the tree–the sapwood–even as the center of the tree rots away. Xylem cells in the sapwood transport water and minerals up from the roots to the rest of the tree. Phloem cells transport sugars from the leaves to the roots, and into the growing apples. As new rings of sapwood grow on a tree, the innermost sapwood cells fill with resins and become heartwood.

Losing one’s heartwood seems like a devastating prospect, but it actually brings new life to the forest. Tree cavities provide homes for scores of critters. Bluebirds, kestrels, wood ducks, raccoons, flying squirrels, gray squirrels, woodpeckers, owls, wrens, chickadees, bats, mice, porcupines, bears, invertebrates, playful kids, and many more of our neighbors find shelter in the cozy space of a hollow tree.

In order for a tree to become hollow, though, it must start the process while it is still alive. Trees that are not already hollow will not become hollow after they are dead. It is the contrast between living sapwood and dead heartwood that sets the stage for a great hidey-hole. Cavity trees must get their start early in the life of the forest, and have plenty of time to develop. They are a rare and valuable resource.

This hollow old apple tree, with its unique flavors and incredible longevity, is highly regarded among our family. We’ve grafted its twigs onto two little baby apple trees to try and perpetuate its legacy into the future. We’ve canned and dried its fruits to feed us through the winter. I’m sure that it’s housed a few families of rascally raccoons. And today it inspired wonder and giggles in two little girls. I’d say that it still has plenty of heart.

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! The current exhibit, “Nature’s Superheroes—Adventures with Adaptations,” opens in May 2014 and will remain open until March 2015.

Find us on the web at www.cablemuseum.org to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot, http://cablemuseumnaturalconnections.blogspot.com.