The most conspicuous goldenrods in many parts of North America are those growing in fields that once were used for farming, so-called old fields (Catherine Keever was one of the first ecologists to carefully document old-field succession). Some of them (perhaps most) grew well in the grasslands and prairies that preceded European-style agriculture in the middle of the continent, places with lots of sunlight because woody vegetation was grazed and/or burned away, or couldn’t grow in the dry conditions. With plenty of light, rainfall, and good soil, some goldenrods can grow two meters tall, while other species are shorter.
When Europeans began to colonize the east coast of North America, most of the land they encountered was forested. As they spread agriculture and cities westward, up to 90% of the forest was cut in any one location. We can see the results of deforestation in the sediments of lakes and other bodies of freshwater. Most trees and many weeds are pollinated by wind, not animals, and their pollen blows all over the place, including into the water. The outer covering of the pollen grains is highly resistant to decay, and the grains remain identifiable (to some degree) for thousands of years. Researchers (Margaret Davis being one of the most notable) can extract, identify and determine the age of the pollen laid down in these sediments. What do they find? When settlers removed the trees, tree pollen decreased precipitously. At the same time, ragweed pollen shot up like a rocket, so much so it the Ambrosia pulse (Ambrosia is the genus of ragweeds) is now used to mark the arrival of European-style agriculture in any part of eastern North America. Ragweed thrived in the sunlight and exposed soil of farms. Before that, it was extremely rare.
Goldenrod pollen is heavy and sticky, great for glomming onto passing insects, but terrible for blowing in the wind. There is no pollen record for goldenrod. But the sun-loving goldenrods might have behaved much like ragweed, their sun-loving herbaceous neighbors. Or not, because their seeds disperse differently. Goldenrod seeds (fruits, technically) disperse in the wind, while ragweed fruits have no hairs (pappus, technically) but its fruits contain some oils that can attract birds.
But where were these sun-loving species before the land was cleared, before there was abundant available sunlight? We don’t find them today in the deep shade of the forest (with few exceptions), nor was there more than a trace of ragweed pollen for most of the time the forest was present after the ice age. These weeds must have had difficulty finding places to live when the forest was intact.
But their lives were not impossible. There are always some places where trees die and light reaches the forest floor for a few years until other trees fill in the gap. Sometimes, the gaps are no larger than a single tree. Other times, they are huge when a storm is intense (a hurricane, or a tornado, or just a nasty regular storm). Or maybe a pest (disease, insect, etc.) had its way with some trees, maybe a lot of trees.
One of these enemies might have been beavers. These busy rodents build dams where there isn’t already a pond, and the resulting flood kills trees that were living near the stream. Beavers also gnaw down trees to reach the twigs that they store for winter. In short, beavers can make major modifications of the landscape.
After some number of years, beavers are likely to deplete the supply of preferred trees within easy access of the pond. Then they move. Or they might starve in a severe winter, or some predator might manage to catch them. However it happens, when the beavers are gone, the dam will fail, the pond will drain, and the land will revert to meadow.
I’m just speculating here, but could the areas thinned by beavers, and/or the meadows that grow when beavers were gone, have been habitat for those goldenrods that thrive in lots of sunlight? There is no pollen record to test this hypothesis, so for the moment, I haven’t been proven wrong.
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