Combining my interest in history with botany, I have sometimes wondered what my area looked like when the earliest Europeans arrived - especially the plants. How were the plants different, and how were they similar? Well, based on reports from native and European sources, and scientific analysis of old pollen and plant remains, one can make some guesses.
Apart from the obvious lack of modern cities, highways, roads, housing subdivisions, shopping malls, etc., etc., one of the most striking differences would have been in the characteristics of the forests themselves. Much of the forest would have much larger trees than we are used to seeing in wooded areas today. There were large areas of forest that had never been cleared at all, truly old-growth forests that would be full of widely-spaced trees with trunks that were 3, 4, 5, or even 6 feet in diameter at the base, and 10 to 20 feet in circumference, trees that had reached this size because they were several hundred years old. There is very little forest like this left in New England today - almost every piece of land has been cleared or logged at least once in the last 300+ years. Almost all wooded areas today are much younger overall, ranging from forests of numerous young saplings 10 or 15 years old to those with much larger, older trees, perhaps 100 years old, that look impressive in size but which are actually still skinny youngsters compared to the massive, several-century old trees that were fairly common when the first Europeans came.
Of course, not all of the land was covered with old-growth forest. The American Indians in southern New England had been clearing land for agriculture for centuries, and there was quite a bit of second-growth forest in areas that they had once farmed but later left. They also kept some hunting areas clear of woods for many generations by repeatedly setting small fires that would kill young trees but leave grasses and other non-woody plants to grow back. Because of this controlled use of fire to create terrain more favorable for hunting, Europeans were sometimes surprised to find large open areas of fields, extending many miles, on land where they expected to find dense forest. The use of fire extended to the old-growth forests themselves. Small fires did no permanent damage to the older trees, but they helped to clear out vines and thorny shrubs and other undergrowth, which made tracking and chasing animals in the forests easier. Again, early Europeans sometimes remarked on how open the forest floor was in the old-growth forests. A man could easily ride on horseback along the forest floor, with little dense underbrush to impede him. One early English explorer in Virginia noted that in some areas, the spaces between the big old trees were large enough that one could have driven a full carriage with its team of horses right between the trees with no trouble! Not all of the wooded areas were this clear, however. In areas where there were fewer animals to hunt, such as steeper hills and mountains, or areas with poor soil or areas just not frequented by many animals, the Indians didn't practice their forest management through controlled burning, and the underbrush was much thicker and took some effort to penetrate.
The majority of the types of trees most common in forests were the same as today, but their proportions were different. Old growth forests, not surprisingly, tended to be dominated by the trees that had the longest lifespans, which usually meant various oaks, various pines, sugar maples, and beeches. Trees that are common in the relatively young forests found today, such as red maple, black cherry, and birches, were a lot less common. They would be found mainly in the second-growth patches where there had been Indian farms and towns, or major forest fires or storms had cleared an area of its old trees. They were also often found in areas where the more dominant tree types would not grow, like swamps and steep, rocky hillsides. One consequence of this was that the autumn foliage was probably not nearly as impressive, since the brightest colors - the reds and oranges and the most brilliant yellows - often come from maples, which were less common in the 17th century forests.
There was one tree that was quite common in these forests that is only occasionally seen anywhere today, and that was the American Chestnut. They were once common enough that many New England towns have a "Chestnut Street" along with an "Oak Street", "Maple Street", "Elm Street", "Pine Street", but the Chestnut Blight fungal disease killed virtually all of them in the early 20th century, and the only thing that kept some of them alive was their tendency to re-sprout from the roots after the main part of the aboveground tree has died. Elm trees have suffered a major blow from Dutch Elm disease at the same time, but they were not quite as common as Chestnuts before the diseases came, and Dutch Elm disease isn't quite as universally destructive as Chestnut Blight is.
Many of the other plants one could see at various times of the year would have been familiar to people today - wild grapevines and Virginia creeper climbing up trees, skunk cabbage poking through the ground in swamps before the snow is even gone, short-lived spring wildflowers blooming before the trees above them have grown their leaves in the early spring, milkweeds producing their seedpods full of seeds with feather-like tufts that allow the wind to catch them, goldenrods blooming in late summer, along with the much less welcome ragweed. There would, however, have been some very familiar plants that would be missing. Some of these missing plants wouldn't be missed very much, since they are notoriously invasive, fast spreading, and tough to kill. There would have been no clumps of Japanese Knotweed, no bittersweet vines climbing up trees, no purple loosestrife forming dense swathes in wetlands. Some less invasive, but very common weeds would also have been lacking. The best known of these is probably the common dandelion. Other common weeds seen today that would have been completely lacking at the time of the first Europeans would have been chenopodium, aka lamb's quarters or goosefoot, common plantain, shepherd's purse, and common mullein. Some common wildflowers would have been lacking as well, including Queen Anne's lace, common or ox-eye daisies, Chicory, and Black-eyed Susans. Almost all of these plants, whether considered weeds or wildflowers (or both), are originally from Europe and Asia, and were introduced to the eastern part of North America. The one exception is the Black-eyed Susan, which is native to the prairies of the midwest and was not brought east of the Appalachians until the 19th century.
There is a lot more stuff that I could add, but the short version is that the forests and fields of Massachusetts would have looked considerably different from today in the 16th or early 17th century, even apart from the obvious and blatant changes like clearing of forests, creation of farms and villages and towns, and later cities, factories, highways, malls, etc. Just the changes in how the forest was handled and used by the people living in the area, and the plants that were introduced, made major changes. Of course, there were many other changes to the environment caused by changes in the animal life, but that is a story that I don't have time for right now ....