Porcupines are especially visible in spring as they dine on a smorgasbord of tasty tree buds. Photo by Larry Stone.

A dozen adults hunched over tables examining bare twigs with borrowed hand lenses. Back and forth they looked, between the dichotomous key and their specimen. Did it have one or three bud scales? Or more than five bud scales? Was the pith of the twig chambered or solid or hollow? Finally, we arrived at an answer: the smooth, gray twigs with plump, red buds (covered by three bud scales) belonged to a basswood tree. For fun, I had the students each nip off a bud with their teeth and experience the slightly sweet flavor and mucilaginous texture of this wild edible.
We practiced keying out a few more twigs, and then the winter tree identification class headed eagerly out into the warm sunshine to walk the trails at the Northern Great Lakes Visitor Center in Ashland, Wisconsin. We sniffed the glossy, aromatic buds of balsam poplar, admired the snazzy, scarlet stems of red osier dogwood, and commented on the stalked buds of speckled alder. Identifying woody plants by their winter twigs is one of my favorite challenges, and I hoped that the students would gain an appreciation for it, too.
As I drove home on Highway 2, a dark blob high in a roadside tree caught my eye. One quick glance backward confirmed my suspicion. The blob showed a spikey outline against the evening sky. Porcupines have begun their spring feast!
If you want to meet a real expert in winter tree identification, be sure to bring a thick leather glove for shaking their paw. Porcupines survive on low-nutrient bark all winter long, and the lengthening days of spring bring a welcome change in their diet. A few years ago I started noticing that in late winter these bristly roadside silhouettes start appearing with some regularity. As it turns out, porcupines spend their spring eating a rotating smorgasbord of juicy tidbits, and they time their courses to match the period when each entree is the most nutritious. They know their trees well because their life depends on it.
Dr. Uldis Roze, professor emeritus at City University of New York, radio-collared a porcupine he named Rachel, and followed her around the Catskills one spring. By testing the chemical content of the buds and leaves while Rachel fed on them and then analyzing the plants again after she ceased to eat them, Roze discovered that Rachel pinpointed plant parts high in protein and low in toxins.
To get at the most nutritious parts of a twig, porcupines will balance out toward the terminus of a branch and nip off its end using their self-sharpening incisors. Turning the wand around, they nibble off all the most tender twig tips and buds and then discard the rest. Trees on sunny roadsides may be further along in their seasonal sequence than their relatives in a shady forest, so porkies are extra visible now as they satisfy their hunger for spring.
Sugar maple buds are one of porcupines’ first spring snacks. As sap rises, the buds swell. Packed inside the overlapping layers of toffee-colored bud scales, tiny leaves and twigs begin to expand. Their chemistry changes, too. At the height of their nutrition (which happens just as the syruping season ceases), sugar maple buds have more protein than enriched breakfast cereal.
After just a few weeks, once the buds have fully released their tender young leaves, porcupines cease their sugar snacking totally. The protein content might still be the same, but it no longer matters. The tree has now fortified its appendages with toxic tannins. Tannins are chemicals that bind with proteins, making them an effective defense against many types of herbivores.
Sugar maple’s close cousin, the red maple, defends its buds, twigs, and leaves with tannins year-round. It doesn’t seem to have a place in the porcupine’s buffet. Instead, the sweet basswood buds that the students tasted are a porcupine delicacy, as are expanding aspen and willow catkins.
Ash trees are the last to open their buds, and they also represent that last of the spring smorgasbord. Once the juicy expanding packets of ash leaves get too old, porcupines switch to a diet of ground plants for a couple months. When the dandelions, clover, and raspberries dry out, porkies head back up into the trees and return to their winter diet of tree bark and evergreen needles.
Phenology is the study of when specific events happen in nature from year to year in a specific place. Each tree species has its own place in nature’s calendar. Porcupines follow this schedule closely, since each species has an optimum nutritional stage. Amazingly, porcupines are active mostly at night, and so identify their next meal by bud and twig under the cover of darkness. Perhaps my next tree identification workshop should include a pricklier instructor!

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! Our new phenology exhibit: “Nature’s Calendar: Signs of the Seasons” will open May 1, 2016.

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