As Aristotle wrote: “Nature abhors a vacuum.” Vermont’s first ecosystems arrived right on the heels of the retreating glaciers about 14,000 years ago. With the glaciers gone, the land was finally – for the first time in over 70,000 years – exposed to the elements (and bacteria, spores, pollen, and seeds). In the next few newsletters, we’ll look at the shifting communities of plants in Vermont through these 14,000 years of changes in the climate.

More on the history of trees in Vermont

Enter Plants

Ecologists working in glaciated areas around the world have been using layers of pollen and ostracods (small crustaceans called seed shrimp) preserved in ponds to reconstruct the timeline of plant colonization and infer details about paleoclimates (see image below).

Pollen percentage diagram for Knob Hill Pond showing selected taxa; for the Pinus graph, line with symbols = total % of Pinus pollen, area with horizontal drop lines = % pollen identified as Pinus subgenus Strobus, and open area = % pollen identified as Pinus subgenus Pinus. Organic content (percent weight loss-on-ignition) is also plotted (Oswald & Foster, 2012)

Many plants, particularly in colder climates, are wind pollinated, and their pollen grains are typically small and winged. Wayward pollen that misses the mark can instead land on the surface of a nearby pond, usually not all that far from the parent plant (not too far, at least, for our purposes; pollen from genetically modified crops has been found 21 km away from its source: link). Laurie Grigg at Norwich University has been using the sequential layers of pollen at field sites like Twin Ponds in Brookfield (source) to reconstruct the progression of plants from those early stages of tundra to the first fully formed boreal forests to the mixed hardwood forest we enjoy today. The relative abundance of pollen for the different species also allows us to reconstruct what these forests would have looked like.

White pine pollen grain with inflated wings, which facilitate wind dispersal


Open Tundra: beginning 13,500 years ago

The exposed, desolate, till-derived soils were – at least initially – devoid of organic material. Still they served as a receptive, if not fertile, ground for the spores of lichen and mosses and the seeds of sedges, grasses, and other herbaceous plants (relic populations of these tundra species, like Bigelow’s sedge and diapensia, can be found in the alpine meadows at the summits of our tallest mountains). Where there are plants, so too are there herbivores, and eventually came the elk, caribou, helmeted muskox (Bootherium bombifrons), wooly mammoths (Mammuthus primigenius), and others. They roamed Vermont’s tundra at lower elevations in the shadows of the Green Mountains, scraping away snow in the winter to graze on the low lying vegetation. 

A small family of woolly mammoths, with steppe bison in the background and a saiga antelope in the foreground and yak off to the side, all of which were found in North America at the end of the Pleistocene (diorama at the American Museum of Natural History).

These large roaming herbivores were hunted by gray wolves (Canis lupus), saber-toothed cats (Smilodon fatalis), American cave lions (Panthera atrox), bears (grizzly, polar, and possibly the now extinct short-faced bear)**. The carcasses of the hunted were scavenged by dire wolves (Aenocyon dirus) and California condors. As the climate continued warming, new shrubby species of plants arrived (e.g. shrubs like dwarf birch (Betula glandulosa), sweet fern (Comptonia peregrina), sweetgale (Myrica gale), and other stubborn plants of the northwoods), which ultimately pushed these open tundra species north into Canada (during this time period many more species went extinct than migrated north, but more on that below). 

If you’re like me and pine to experience firsthand this late-Pleistocene, post-glacial Vermont, you can either hike to Vermont’s highest summits or drive north to Hudson Bay in Canada. Until Pleistocene Park opens (link), sadly there’s nowhere today that you can be chased by dire wolves or feel the ground shake under the footsteps of wooly mammoths. But still in these beautiful, cold, open areas you can feel winter’s grip and imagine some small taste of what the first humans would have seen upon their arrival to Vermont at the dawn of our forests.

** While the fossil record is far less complete for animals than it is for plants, there are fossils from around the Champlain Sea of mammoths, polar bears, and other large mammals (plus marine mammals like walruses, belugas, and harbor seals). We can also infer animal communities based on fossils found farther from Vermont’s borders and from modern ecosystems. 

Meltwater & the Younger Dryas: 12,900 to 11,700 years ago

While the glaciers may have already disappeared from the lower elevations in Vermont, alpine glaciers were still caught up in higher elevations and sheltered valleys, and the ice sheet was still blanketing much of Ontario, Quebec, and Newfoundland. As plants struggled to get a foothold, seasonal melting from the residual glaciers meant that erosion of our till-derived soils was a constant. Silts and clays drained down into our murky glacial lakes, giving them a milky blue-green glow; the heavy sediment load made these waters inhospitable to aquatic plant and animal life. Today, Lake Champlain drains north into the St Lawrence River by way of the Richelieu. But 13,500 years ago, the retreating glaciers up north blocked that flow of water into the St Lawrence. The basin filled with fresh water (forming a lake known as Glacial Lake Vermont) until the water level reached the next lowest point drainage, about 600 to 700’ in elevation, where it connected with the Hudson River valley. About 12,000 years ago, the wall of ice blocking up the Champlain Valley melted far enough north that the glacial dam burst. According to UVM geologist, Stephen Wright, within hours or days, Glacial Lake Vermont dumped 300 to 400’ of fresh water out into the Atlantic Ocean. Meltwater from the eastern seaboard (and the epically large Lake Agassiz – which was larger than the 5 Great Lakes combined) poured relentlessly into the oceans and eventually disrupted the North Atlantic conveyor (which brings warm water from the tropics thousands of kilometers north to Greenland and Iceland). With the conveyor shutdown, we entered a sharp period of cooling, called the Younger Dryas (named after the 8-petaled mountain-avens, Dryas octopetala, whose pollen count drastically increased during this period).

In the next issue, we bid the tundra farewell and welcome in our firsts forests!

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