The title of this highly readable book, published by Island Press, is one of many tantalizing natural history questions that author John Pastor posed to himself and proceeded to answer in a lifetime of studying the complex ecosystems of the North Woods. Pastor is professor of biology at the University of Minnesota Duluth.

The book delves into the geology, biology, and ecology of the natural communities that stretch from northern Minnesota across the continent to Maine and the Maritime Provinces — a landscape that inspired earlier nature writers including Henry David Thoreau, George Perkins Marsh, Sigurd Olson, and Paul Gruchow.

As a teacher, Pastor is a good explainer; the book is readable without sacrificing scientific rigor. His writing ranges from the lyrical (a cadenced description of what happens during the change of seasons has the unity of a Beethoven symphony) to the precisely scientific (a detailed description of how skunk cabbages attract blowflies leads to ecological lessons and further questions). Pastor’s line drawings illustrate key points beautifully

Pastor is a consummate researcher, a passionate lover of the North Woods, an evangelist for close observation of nature by everyone, and so full of questions that the book is liberally spiked with suggestions for graduate students looking for thesis topics.

His starting point is the retreat of the Laurentide Ice Sheet, which he says defined the distribution of water in this wet landscape.  He describes how the assemblage of plants and animals we now know as the North Woods gradually moved in from the south. There is a fascinating description of how beavers build their dams and stock their pantries, and how they barely survived the rapacious demands of 18^th and 19^th century European fashionistas. He analyzes the different strategies of evergreen and deciduous leaves to absorb nutrients and fight off predators. And who knew that in fall, nutrients move from leaves back into twigs, ready to feed bud growth in the spring? That’s certainly convenient for the moose, who need all the food they can get from twigs through the winter! And as to what a clever moose should eat: the question is complicated.  As moose browse on their preferred aspen and birch trees (conifers are less nutritious and harder to digest), they run the risk of stunting the deciduous trees’ growth, which could allow conifers to grow tall, eventually making too much shade for aspen and birch to survive.  But Pastor and students studied moose foraging on Isle Royale and found that it’s very common for moose to move along after eating only 20% or less of the twigs on a given shrub or tree. As gardeners know, nipping a branch often produces two new branches, and this judicious trimming allows moose to leave a landscape for their descendants not much worse or perhaps even better than the one they found.

Anyone who lives in or near the North Woods is familiar with tent caterpillar outbreaks. Every 10 or 15 years in June, unstoppable waves of these slithery creatures swarm over the landscape, denuding whole stands of aspens.  But did you know that the aspens fight back?  Some produce extra bitter tannins and blood-thinning glycosides, which sets up an evolutionary arms race between trees that make nastier chemicals and tent caterpillars that can withstand the chemicals. And what is the effect forest-wide of these periodic defoliations? Some research suggests the extra light falling on the forest floor, along with the extra nutrition provided by caterpillar droppings and dead bodies, provides a boost to understory plants, promoting forest diversity. That may be the bright side of an otherwise disgusting North Woods phenomenon.

Pastor also includes an informative section on how fire shapes the North Woods, and a worried but still curious meditation on how climate change might “disassemble” the ecosystem that we know and love as the North Woods. Anyone who has dipped a paddle in a northern lake–or even dreamed of doing so-will be rewarded by mind-expanding dips into this celebration of curiosity, science, and the nature of a very special ecosystem.