Science Tuesday: Why Mesophication Matters
Authors: Lenya Quinn-Davidson
No matter where I am, I always find myself subconsciously looking for two things: fire and oaks. I start looking for clues, trying to uncover the stories and processes that I know are there, and wondering where the Quercus is to round out the picture. (I remember being elated to find oaks in the cloud forests of Chiapas, Mexico, and having the feeling that something was missing when I’ve visited my sister in Alaska.) I know—by default, I seem to have a fairly narrow focus–but alas, I come from a fire-loving state with more than 20 species of oaks. It’s in my nature.
So when I touched down in New Hampshire for the first time earlier this month, I found myself busily scanning the roadside in my usual fashion. Even after an overnight flight with a toddler and less than three hours of sleep, I was eager to understand the place—and, of course, to see some oaks.
On my second day there, my friend took me on a short walk from her house to a massive red oak, which was standing alone in a sea of eastern hemlock, red maple, beech and smaller red oaks. Local rumor has it that this oak is one of the largest in New Hampshire, and given its size, I don’t doubt it. It had a big fire scar stretching along one side, and the crown was well out of reach. As I gazed at the tree, I felt conflicted. In some ways, this place seemed very foreign to me. I could feel the history—the centuries of heavy-handed management—and sense that this was a very different landscape than it once was. And yet some things were so familiar: the huge oak, the thickets of shade-tolerant trees in its understory, the obvious lack of fire.
After my trip, I looked back at a paper that I’ve cited many times, but more as a theoretical framework for the issues I work on here in northern California’s oak woodlands. This time I read it with new eyes, thinking about the specific landscapes and forests of the North Atlantic and the eastern United States. The 2008 paper by Nowacki and Abrams, which coined the term “mesophication,” gave us a new way to think about the ecological impacts of fire exclusion. The paper describes the widespread conversion of fire-maintained open forests, woodlands, and prairies to closed-canopy forest, and the pervasive shift from diverse, fire-adapted plant communities to shade-loving, fire-sensitive communities. These changes jumpstart a positive feedback cycle, wherein species shifts and forest densification move the microclimate toward cooler, wetter, and more shaded conditions, which then further promote shade-loving species and the conditions within which they thrive. The process of mesophication is usually associated with changes in fuelbed flammability (from light, flashy herbaceous fuels to dense, compact woody fuels), which further preclude fire and inhibit the success of fire-adapted species.
In their paper, Nowacki and Abrams discuss the historical role of fire in different forest types across the eastern United States, and they estimate the extent and magnitude of changes in fire regimes. They show that across most of the eastern United States, there has been a temporal shift toward less fire (see figure). That’s no surprise, but the cycle of biotic and microclimatic shifts that have accompanied these fire regime changes is striking—an insidious threat to biodiversity and fire resilience, often overlooked when the focus is on fuels and fire hazard.
And this is where oaks come in. Throughout their range, oaks (especially deciduous oaks) are signatures of fire adapted communities—an arboreal expression of a long, mutually beneficial relationship between humans, plants and fire. And ironically, it’s this quality that makes oaks even more vulnerable to the effects of fire exclusion. This is true in California, and it’s true on the other side of the country. It’s the reason that the majestic red oak in New Hampshire seemed so familiar to me, with its swarm of hemlock and other mesophytic tree species; it could have easily been a big California black oak adrift in a sea of Douglas-fir. When I travel, I look for oaks because they tell a story—usually a fire story—and they make me feel at home.
Nowacki, G. J., & Abrams, M. D. (2008). The demise of fire and “mesophication” of forests in the eastern United States. BioScience, 58(2), 123-138.