Our local forests are generally susceptible to fire each year during two very short, dry periods. The first is when mud season has ended and the flush of green understory plants has yet to break ground from spring rains. The leaf litter from the previous fall can be as dry as paper. The second is right now, when the trees stand naked after the leaves have fallen and the leaf peepers have all gone home in late fall. In a dry year, the newly fallen leaves can easily kindle a major fire.
But fires in the northern hardwood forests of New Hampshire and Vermont are relatively rare, as evidenced by the fact that the forest is not fire-adapted like those in the southern or the western portion of the continent. Even in the southern New England hardwood forests, many of the trees, such as oak, hickory, and red maple, sprout vigorously from burned stumps and trunks after a fire. But the northern hardwood forest is comprised of more fire-sensitive species such as hemlock, beech, and sugar maple. Anything more than a quick ground fire can kill a mature tree in the northern hardwood forest.
How often do forest fires naturally roll through a typical northern hardwood forest? Recently, researchers from the Harvard Forest in Petersham, Massachusetts, studied ancient charcoal and pollen for the answer. The scientists sampled four lakes in northern hardwood forests using a tool that sinks into the sediment and retrieves a solid core. Each layer of sediment was examined for microscopic fossil charcoal and pollen. Ages of each layer were determined using radiocarbon dating, with the timing of European settlement noted by decreases in the amount of tree pollen and increases in pollen from plants such as ragweed and grasses that were rare or absent before.
Comparing the northern hardwood forest cores to those from other New England forests showed that fire is quite rare in northern hardwood forests, with a given area being burned only once every 1,000 years or more prior to European settlement. Our cool, moist northern hardwoods are not easily ignited. When they do burn, it is usually a light burn through the relatively moist leaf litter.
Three of the sites had significant increases in charcoal after European settlement, ranging from a 5- to 10-fold increase between pre-settlement and post-settlement times. In fact, today, over 95 percent of all forest fires locally are caused by human activity.
We often hear that Native Americans burned large tracts of forest regularly In New England. In 1632, Thomas Morton, living near Boston, famously wrote, “The Savages are accustomed, to set fire of the Country in all places where they come; and to burne it, twize a year, vixe at the Springe, and the fall of the leafe.” However, that was in southern New England, close to the coast. There is little evidence that fire was ever used frequently, if at all, in our local northern hardwood forests, where Native American populations were much lower.
Evidence of fire can be found in a forest for many years afterward, and some clues are relatively easy to identify. Hot, slow fires leave two obvious signs: basal trunk scars and multiple-trunked trees. But be careful – so may past logging operations. With a fire, basal scars will be more randomly distributed than logging scars in flat areas or, on hillsides, will be found only on the uphill side of the trunk. Why only on the uphill side? Think about leaf litter and gravity; the leaves and twigs slide slowly down the slope only to be stopped by a tree trunk, where they gather like water behind a dam. As fire passes and wraps around the trunk, it encounters the deeper leaf litter, which burns slower and hotter than the thin cover below the trunk, biting through the bark of the tree and leaving a triangular scar.
Finding charcoal as a clue to a past forest fire is much more difficult. Charcoal breaks down and is generally not visible after just a few years. Dead wood, especially beech and maple, often has a smooth, black coating that has the appearance of being charred but isn’t. This is actually a fungus and is part of the natural decaying process. But it’s still worth a try to find evidence of one of our local forests’ rarer phenomena.
Given the extraordinarily wet fall we’ve had, the chance of a forest fire happening here this year, either from a lighting strike or careless match, is very low. Which is just as well. Unlike many of the forests out West, our forest here in northern New England does not depend upon fire for its ecological health. And there’s already been enough excitement for one year from the flooding.