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Mar232017

The Eastern U.S.: Just Gotta Be a Forest?

By JOE RANKIN

Maine is the most forested state in the nation. About 90 percent.

It wasn’t always that way, of course. European settlers in eastern North America initially confronted what they considered a fearful wilderness of trees. But when settlement ramped up, farmers expeditiously cleared millions of acres of trees to grow fodder for their animals and food for themselves. In Maine, the agricultural tide began to ebb in the mid-1800s and the trees came back.

You’ve probably heard it said that Maine just wants to be a forest. But it’s not just Maine. The whole eastern U.S. “wants” to be a forest. It’s apparently the region’s ecological default setting, though the tree species vary from loblolly pine in the south to tulip poplar to white pine to shagbark hickory and yellow birch, to spruce and fir.

But, why?

It comes down to a handful of fortuitous conditions — a climate that’s not too cold, adequate and consistent moisture and favorable soils, said Andrew Barton, a professor of ecology at the University of Maine at Farmington and the author of the book The Changing Nature of the Maine Woods.

The most important is moisture.A Typical Maine Forest

“If we look at our entire globe we can understand how plant communities are different from place to place. Temperature and moisture variability are the top two things. The next two things that determine that variation are fire and fertility of the soil,” Barton said.

Average annual precipitation for the lower 48 states totals just over 30 inches. But the East gets vastly more moisture than the midsection of the country. Louisiana gets some 60 inches and parts of the Southern Appalachians are technically a temperate rainforest, getting upwards of 80 inches of precipitation a year. Most of the rest of the South gets 50 to 60 inches. The northeast gets 40 to 50 inches, and most of the Upper Midwest 30 to 40 inches. The coastal Pacific Northwest, of course, is awash in moisture, getting more than the Eastern U.S.

To see how water is the defining factor driving forests in North America, lit helps to look at the tropical rainforests of Central and South America. They get huge amounts of rainfall. Those forests are among the most diverse on the planet and their trees can grow very, very large, even though many rainforest soils — in the Amazon Basin, for instance — are nutrient poor.

To see what happens when moisture is severely limited, you have only to look to the High Plains states in the U.S. The precipitation drops off drastically west of the 100th meridian. To 20 to 30 inches a year in South Dakota, Nebraska and Kansas; 10 to 20 inches in the Rocky Mountain states and only 10 inches in Nevada. Forests in that region are largely confined to the river valleys and mountains, where there is more moisture. Across that expanse, grasslands or even deserts were the default setting.

“If you asked what made the prairie before European colonization, why it is a prairie and not something else, one, it’s dry and two, it’s prone to fire that would help keep out woody plants and three, it had a lot of big grazers. The prairie adapted to all those three conditions, but it mainly comes back to moisture, which drives everything,” said Barton.

Maine's State Seal: The Pine Tree StateMaine averages some 45 inches of precipitation a year, spread through the seasons. Snow builds up through the winter and that moisture is released with the spring melt just at the time when trees need it. There is rainfall in the summer and fall and, though summer droughts occur (one did last year), they are usually not common and not so protracted.

Partly because of adequate precipitation the forests of the Maine and the rest of the northeast have not been as heavily influenced by fire as other parts of the continent, including the boreal forests of northern Canada, where fires are fairly frequent and widespread. Except in a few spots, such as Deblois and some areas of southern Maine, Indians apparently did not use fire to mold the woods to their liking as tribes in Southern New England did.  And while there are some big browsers in Maine — moose and deer — big grazers like bison were absent. In fact, the biggest influence on the Maine woods since the last glaciers receded has been, well, humans.

Because it wasn’t so long ago that much of the eastern U.S. wasn’t growing trees. It wasn’t growing anything. It was under ice.

The ice sheets from the last great glaciation blanketed Canada with ice a mile thick: covered much of the eastern U.S. including the midwest, New England and the Northern Rocky Mountain states; and reached as far south as Long Island. It was only when the ice began retreating about 20,000 years ago that the trees began recolonizing the giant sandbox the ice left behind.

It was a slow process, said Andrew Whitman, a scientist at the Manomet Center for Conservation Sciences in Brunswick, Maine. “It was first built up by forbs (herbaceous flowering plants) and herbs and low lying shrubs,” then trees, said Whitman. Research on retreating glaciers in the western U.S. shows that “It takes a couple of decades to 50 years before they start getting trees. And they won’t be big trees at first, either.” In Maine, he said one of the first species to gain a toehold was probably alder, which fixes nitrogen that can aid establishment of other tree species.

One crucial aspect of the re-establishment of the forest in the northeastern U.S. is the existing forest below the glacier’s reach. It provided a ready seed source.

But even across the eastern U.S., there were variations in how the forest came back. Some areas as far east as Ohio and Kentucky, for instance, became grasslands, notes Barton.

Over time — thousands of years — the forests worked to create a deep, organic loam across the region, and trees thrived, in most places. In New England, soils on tops of the highest mountains were not conducive to a typical forest, though around the edges they supported tonsures of dwarfed trees that were incredibly old for their size. Bogs were too wet and acidic to support trees, and in southern Maine rocky ledges were too dry and their soils too sparse to harbor large trees.

Vegetation Growing in Unity Gravel Pit“In Maine, I’m not aware of a place that the soils are so nutrient deficient that you don’t get trees. Even in old gravel pits you get quaking aspen coming in. They usually get five or six inches in diameter and maybe make it to 50 years old. But they manage,” Whitman said.

Scientists point out that climate across the eastern U.S. has changed, several times, in the millennia since the retreat of the glaciers, with some species disappearing for thousands of years, then returning. Remember the Little Ice Age? About 1300 to 1850? With those changes there have been corresponding changes in the makeup of the eastern forests. But it remained forest.

What will the current round of human-caused climate change bring?

Whitman and Barton say projections don’t call for alterations in rainfall patterns and temperatures that would favor a savannah ecosystem rather than a temperate forest. Barton said that in the east what is likely to happen is more southern tree species — such as oaks and hickories — will be working their way north and northern species such as spruce and fir will gradually move even farther north.

Refugia of oak trees from a previous warm period might also help spread species like oaks across Maine, said Whitman. He noted that big old oaks are still found in northern Maine’s Big Reed Pond Preserve and that locally harvested oaks provided flooring for houses still standing in Aroostook County.

But tree species will probably migrate individually, not en masse, thus creating new forest communities.

“We should have trees in Maine,” agrees Whitman. But the mixing of species could be complicated. More frequent droughts, invasive plant species and exotic insect pests could lead, in some cases, to “some forest community dystopias” where the trees wouldn’t reach large sizes, Whitman said.

Barton notes that the forests of New England might be better off than some other areas of the eastern U.S. simply because it’s cool to begin with. “This region will surely change. But it’s a little better buffered against climate change,” he said.

 

Joe Rankin lives in New Sharon and writes about forests and nature.