The Holy Trinity of the Southwestern Mountains: tree, squirrel, fungus

by Elizabeth Blaker

You can neatly overlay a map of ponderosa pine forests with a map showing where the tassel-eared squirrels live – the maps are nearly identical. Surely this is no coincidence. There must be a very strong relationship between the tassel-eared squirrels and the ponderosa pine forest.

squirrel eating an acorn

An Abert’s Squirrel munching on an acorn.

When white explorers came to the high mountains of the southwest, they found a vast forest of tall ponderosa pines with yellow-orange trunks. The forest was open, easy to travel through, park-like. After a summer rain the ponderosa pines produced a perfume that lived forever in the memory as the essence of these mountains. Among the trees frolicked a notable squirrel, large, gray-backed, white bellied, the underside of its tail a blaze of white when held proudly over its back, its unusually large ears decked out with fur tassels. In 1851 Samuel Woodhouse, a surgeon and naturalist, collected the first scientific specimen of an American tassel-eared squirrel on the San Francisco Mountains of Arizona. He described the squirrel as ‘elegant,’ and noted that he saw no more of them after leaving these mountains.

There are six types of tassel-eared squirrels, all subspecies of Sciurus aberti. Two of these subspecies are the Abert’s Squirrel which lives in Arizona, Colorado, New Mexico, and Southeastern Utah, and the Kaibab Squirrel, which lives only on the Kaibab Plateau at the north rim of the Grand Canyon. The distinctive ear tassels grow each autumn, reaching their maximum length in mid-winter, and are gradually shed as the weather warms, usually completely gone by July. Perhaps the extra length of the ears requires a bit more thermal insulation in winter than do the smaller ears of other kinds of squirrels.

squirrel standing on hind legs

An Abert’s squirrel poses for the camera.

Tassel-eared squirrels love to eat ponderosa pine seeds. Researchers who first studied the ponderosa pine forests of the Southwest thought the tassel-eared squirrels were destructive to the trees. The squirrels use their scissor-like incisors to clip green cones from the branches. Sitting on its haunches, a tassel-eared squirrel rotates the pinecone bottom up with its hand-like front paws and then neatly snips off the scales one by one. Hidden beneath each scale are two fat seeds. Some species of squirrels, such as the Mt. Graham Red Squirrel, make large food caches in one or two places, but tassel-eared squirrels bury uneaten green pinecones in many locations scattered over a large area. Sometimes they forget to dig them all up. Experiments have shown that green cones can and do germinate and grow into seedlings. It is true that the tassel-eared squirrels eat a great many pine seeds (up to 20% of the crop in some places), but they also plant trees.

pine cones and squirrel pencils

A collection of pine cones and “squirrel pencils”.

If you are standing beneath a tree where tassel-eared squirrels have been feeding, you will see cores of eaten pine cones, but you may also notice many small whittled sticks under the tree. Each stick is about the size and shape of a half-used pencil.  Some Arizona mountain dwellers even call them ‘squirrel pencils’. Almost everyone likes sweets, and the tassel-eared squirrels do, too. Instead of eating candy, they sit high in the branches of a ponderosa pine and snip the growing shoots off the very ends of the branches. Holding the shoot by its twig, they nip off the tip, dropping the bunch of pine needles to the ground. Sometimes there are great drifts of green pine sprigs carpeting the ground under a favorite feed tree. The squirrel holds the twig it has just clipped from the tree in its paws as it peels off the out bark with its teeth.  This is like unwrapping a candy bar. Once the bark is removed, the squirrel turns the twig as it eats the sweet inner bark like a kid eating a cob of corn at Sunday picnic.  The inner bark, called phloem, contains the sugar the tree has made through photosynthesis.

Tassel-eared squirrels are connoisseurs of phloem, it seems, as scientists have found that they prefer to feed on trees with a specific chemical profile. Trees protect themselves by producing more bitter tasting terpenes, but it takes time for a tree to mount this defense. The tassel-eared squirrels eventually react by switching to a new feed tree. The chemical profile preferred by tassel-eared squirrels is different among two of the tassel-eared subspecies that were tested. Scientists suspect that the taste preferences of the squirrels might influence the genetic makeup of trees in different regions.

squirrel with acorn

A tassel eared squirrel with an acorn in it’s mouth.

The trees have also driven the evolution of the tassel-eared squirrel.  In order to survive, the squirrels must maximize the amount of nutrition absorbed from food. The inner bark of pine twigs is not easy to digest, so the digestive system of the tassel-eared squirrel has evolved to break down and absorb the maximum nutrition from this sweet but fibrous material by increasing in length, up to three times longer than other squirrel species examined. This allows a greater surface area for absorbing nutrients and more room for helpful bacteria to live and to work on breaking down the food.

Underneath the soil of the ponderosa pine forest is another forest, a forest of fungi. These fungi, known as micorrhizae, grow a network of filaments which contact the roots of the pine trees. The network of filaments acts like a sponge absorbing rain and snow-melt water and making it available to the tree. The fungi also provide minerals and other nutrients to the tree. The tree pays for these services by providing sugar to the fungi through its roots. The micorrhizae mature and begin to bear fruit. But instead of producing mushrooms above ground that open their caps to disperse spores (their seeds), the micorrhizae produce fruiting bodies (some of which are called false truffles) beneath the ground. As the spores mature, the aroma of the false truffles increases.  Along comes a tassel-eared squirrel, sniffing about on the forest floor, and voila – it has found what it was looking for.Quickly the squirrel digs up the false truffle and eats it.

Researchers who have studied the stomach contents of tassel-eared squirrels learned that fungi comprised up to 80% of their diet in November, but less than 10% in February when snow often covers the forest floor.  If a tassel-eared squirrel digs up more false truffles than it wants to eat at the moment, the excess is carried up into the branches of a pine and left to dry for later.  The squirrel scampers about in the forest depositing billions of fungal spores in its feces. Studies have shown that micorrhizal spores sprout perfectly well after passing through the digestive tract of a squirrel. The tassel-eared squirrel is acting like a taxi service for the spores, distributing them widely.

squirrel-and-nest

An Abert’s squirrel in a tree near it’s nest.

After decades of research, scientists have learned that the tassel-eared squirrel is not a destructive force in the forest. Rather, the tree feeds the squirrel, the squirrel plants the tree, the tree feeds the fungi, the fungi feeds and waters the tree, the fungi feeds the squirrel, the squirrel spreads its seeds all over the forest. Tree, tassel-eared-squirrel, fungi, a holy trinity of relationships that nourish each other and continually regenerate the forest. One can be certain that as more is learned, this trinity will appear to grow as many complicated tendrils as the micorrhizae under the soil, tendrils that form a beautiful and intricate web of ecological relationships.