Return of Life
By ten thousand years ago, when the region was free of ice and relatively dry — what did it look like?
One way to guess is to remember that much of North America was never touched by glaciers. Central Pennsylvania and New Jersey lay south of the ultimate ice margin, and here you’d likely find, 18,000 years ago, a patchwork of tundra and forest. Permafrost gripped some parts of this zone: polygonal and wedge-shaped patterns found today are traces of ice that once lay deep under the surface, thawing and refreezing and cracking the earth. Above permafrost could only be tundra. But there’s also clear evidence of spruce and fir forest, maybe even growing right up against the ice front. It’s possible that the thickness of the ice actually shielded the earth beneath until it ebbed, leaving forest-ready ground behind.
Perhaps this mosaic gradually followed the ice retreat north. So imagine wind blowing over the naked rocks of the Taconics, blowing seeds and soil in from tundra and forest to the south. Where the stony surface was smooth, the in-blow had nothing to grab on to. Study the flanks of the mountains today, and it’s easy to pick out spots of exposed rock. Perhaps those have never known soil: remnants of the old, bare mountainsides.
(An early settler in northwest Connecticut captured the mountains’ texture in rhyme: “Nature, out of her boundless store / threw rocks together, and did no more.”)
But the material left by the disappearing ice dome — till — had enough grain to catch what the wind gave and, crucially, hold water. Likely, scattered pockets of dirt, particularly in ravines, and in the valley between the two Taconic ridges, sheltered the process of life’s return. Hardy pioneers, denizens of tundra such as dryads, crowberries, moss campion (found only in the far north today) might have grown first, along with more familiar varieties of sedges and grasses. Once plants began to live, they died, and their decomposition added to the growth of ground. They also would have served as wind-breaks, helping catch and hold soil in place.
Today, where Alaskan glaciers are disappearing, willows and cottonwoods take hold. But many of them — nitrogen-starved — can barely lift their limbs above the ground. They await the arrival of species like alder, which can pull nitrogen from the air into the soil. I picture the Taconics ten, nine thousand years ago, pocked with struggling plants hungry for nitrogen, and nutrients, and also for a crucial partner: fungus. Once wind brought fungal spores, their growth intertwined with plant roots, allowing more nutrition to flow up the stems.
Eventually, deeper soils permitted forests, probably dominated at first by black spruce. Jack pine, larch and birch arrived early. Within a century of de-glaciation, a kind of parkland probably evolved, similar to the ecosystem that survived south of the ice: patches of forest dotting tundra. But the climate may have actually been warmer than today. The ice dome itself could have blocked those northern wind-blasts that chill the twenty-first century Taconics. Winters may have been relatively warm, summers cool and wet. But all in all, the changes no doubt welcomed wildlife.
Such wild life! Where we stroll today on blazed trails, encountering chipmunks and chickadees, saber toothed tigers may have wrestled two thousand pound ground sloths, dire wolves pursued hundred and twenty pound beavers, and caribou fled from short-faced bears taller than moose and able to run forty miles an hour. Lumbering along, tundra grasses dangling from their mouths: fourteen foot tall mammoths and mastodons. Did I mention the muskoxen, North American horses, and bison? Given time-travel, I might choose a journey to the late Pleistocene (if I could arrive well-armed.)
But if these creatures ever roamed the Taconics and the valleys below, those days were brief. By nine thousand years ago, most of them were extinct. The causes remain one of the most puzzling mysteries of deep history. Climate change? But wasn’t the climate changing in ways that benefitted most species? And what sort of climate change would have wiped out so many and such a variety? Arrival (by at least 11,500 years ago) of a super predator called homo sapiens? But nine thousand years ago, were there enough humans to eliminate a whole continent’s worth of animals? Some catastrophic event? But surely such a phenomenon would have left clear evidence behind.
The answer could be “All of the Above,” or some as yet undetected event. But the survivors of the great extinctions included many familiar beasts: white-tailed deer, beavers (“normal”- sized) wolves (not dire) cougars, foxes, and some, like bison and elk, that disappeared more recently. The warming climate allowed a vast expansion of life.
Forests Travel
South of the ice line, south of the tundra, south of the boreal forest lay lands where more familiar forests grew, even at peak glaciation. Hemlock and white pine found refuge along the mid-Atlantic coast. Near the mouth of the Mississippi, chestnuts and sugar maples grew. As suitable habitat appeared, they moved. Writing this, I picture individual trees lifting their roots like skirts and marching north, but of course what we’re talking about is simply expansion of range. Seeds blown north that died one year, the next year lived.
Pollen counts taken from core samples show when a given species arrived in a given area — their pollen trace distinctly rises. By comparing arrival times in different parts of the continent, it’s possible to posit an average rate of advance:
Pine: 300-350 yards per year
Hemlock: 200-250
Spruce: 270
Maple : 200
Chestnut: 100
Oak: 380
By comparison, the ice sheet moved north (on average) about half a mile per year.
So trees that depended on wind to carry their seeds moved more slowly; birds and small mammals must have helped the oaks move so “quickly.” Why chestnuts spread at such a relatively slow rate is a puzzle. Were conditions too dry and cold? They were among the last to colonize the Taconics, arriving only three to four thousand years ago.
Slope by slope, acre by acre, hardwoods replaced the boreal forest. Yet it’s a mistake to conceive of an entire ecosystem moving in lockstep. Rather, individual species arrived at specific times and, like colors in a paint-by-numbers image, filled in a now-familiar picture. Examine a field-going-back-to-forest in the Taconics today, and you’ll likely see birch and pine sprouting in the sunshine: they embrace light, and can root in dry stony soil. They must have been among the first post-boreal tenants of the mountains.
Hardwoods spread via valleys and basins, led by oaks and elms (about 10,000 years back) and then sugar maple, with its long lifespan and prolific seeding, and the shade and drought tolerant beech. By 8000 years ago, the neighborhood was well-settled: birch, pine, oak and hemlock joined by black cherry, white ash, elm, basswood. (The boreal forest wasn’t entirely evicted however; remnants today include wild raisin, mountain holly, and hobblebush.)
The nut-bearing laggards came last: walnut, hickory, chestnut. (Some propose they may have been aided by those gardeners of the forest, Native Americans.). Along with them came my least favorite denizen of the mountains: the ubiquitous, dense-growing, (almost) impassable mountain laurel; also blueberry and hackberry. This colonizing forest settled itself by elevation and soil type, sugar maple and beech and red oak on the lower slopes, chestnut oak and birch on the stony uplands, alder and willow in bogs and swamps.
As an amateur student of forests, I almost unconsciously imbibed the idea that, left alone, a forest eventually reaches a stable “climax” state, in sync with climate and soil, and somehow the “right” and “proper” sort of ecosystem. But a closer look suggests, first of all, that forests are never left alone. Even if the disturbers-in-chief, humans, were to vanish tomorrow, tornadoes, high winds, drought, ice storms, all would continue to shatter stability. Many species depend on those events to clear ground and give them access to sunlight. All these processes — climate change included — were in play before humans entered the arena, and of course will continue when we’re gone.
The pollen records show that, 4800 years ago, the region’s hemlock population crashed. Did some sort of species-specific pest invade? Was this a time of drought that hemlocks were ill-adapted to? Whatever the reason, the population rebounded within a couple thousand years, but never to its original size. So if my account suggests a smooth and steady forestation of the Taconics, that’s misleading. The story is full of jerks and retreats: a warming trend from nine to five thousand years ago allowed some species to expand their range north of today’s; a “Little Ice Age” from 1350-1870 A.D. re-shaped the range of others. When I walk in Sage’s Ravine and admire today’s dense, shade-spreading hemlocks, I have to imagine a Taconics where they spread much further, and one where they almost disappeared.
