Dangers of Deforestation
Despite the fact that we're dealing with a work of fiction, The Overstory nonetheless makes many sound scientific observations, the main one being the danger of deforestation to the planet and to humanity. Logging companies are given more credence than scientists, which is ironic because they are logging in order to make room for new construction for a population that will not be able to grow without trees and other elements to provide clean air. The story is very much anti-deforestation and shows the devastating consequences of prioritizing development of concrete jungles over maintaining the environment.
The Psychology of Groups
One of the main characters in the novel, Douglas, is a constant joiner of groups, most likely because he is an orphan and does not feel attached to a family of his own. He uses these different groups as a family substitute. This theme is actually seen through the lens of Adam's thesis, which is about the way in which living in a group of environmentalists and protesters can affect the human psyche.
The book also has a kind of "Lord of the Flies"-like descent into violence that is produced when a group mentality overrides that of an individual. We see the individual sincere, law-abiding and passionate citizens descend into violence and hate, and culminate in arson which kills one of their group. This is as a result of the psychological breakdown that they begin to experience when they live only with each other and feed off of each other's frustrations rather than having those outside the group also contributing to their view of humanity. Throughout the novel the theme of group psychology is one of the central ways in which we see how the group is affected by the deforestation that continues apace despite their hard work and best efforts.
Hope
Admittedly, it doesn't seem like there's much to invest hope in—for either the characters or the readers. Deforestation and climate change and its concomitants have wrecked and ravaged the planet to the extent that there doesn't seem to be any going back. Capitalism and corporate interests, coupled with the human propensity for greed and desire to dominate, are too formidable of foes for regular people to go up against. In The Overstory, acts of protest and even those of eco-terrorism bring about very little meaningful change; another example is that Patricia's expert witness testimony garners an injunction that is quickly removed. So, is there any hope at all? Surprisingly, Powers does seem to suggest that humans can do things to change hearts and minds, mitigate some disasters, replant and rebuild, or work to change their own nature. It isn't easy, and maybe it is too late and maybe certain things will vanish or have to change, but if humans and trees develop a "mutualism" rather than humans being "parasitical," then there could be a future in which they coexist.
The Nature of Trees
Trees emerge as characters in their own right in this novel. On the one hand, they behave in ways that are familiar to us: they collaborate, socialize, assist, live and die, fight, and more. However, Powers makes sure not to anthropomorphize them too much; after all, they aren't exactly like us, and if we are really to face what we've done to the planet and find out ways to behave better, we have to see trees for what they are. Their age, their experience of time, their abilities—all of these things are hard for humans to comprehend, but we must in order to procure any sort of sustainable future.
Human Nature
Powers displays human nature in all of its beautiful, doomed (Patricia's sentiments!) messiness. He shows how we are capable of remarkable acts of sacrifice, tenderness, bravery, and compassion. He also shows, of course, how prone we are to ignorance, a belief in exceptionalism, greed, a desire to dominate, and myopia. We focus, as Adam notes, on ourselves and our own, which is at the expense of the larger community of people and other species. Our cognitive biases hamper us and it takes a great deal of work to move beyond them. Nevertheless, Powers does seem to suggest that we are capable of change, but that it is not easy.
Time
Powers reveals that humans just don't seem to have the best grasp on time. We live in the present, incapable of comprehending that there are, for example, some pines that have been dying before humans invented writing. We cannot conceive of the impact our choices have on the future, and we cannot manage to set aside what the short-term has for us in order to prepare for that future. Trees flummox us in their ancientness and different experience of time. Patricia states that a dead tree has more life on it than a living one; this obviously boggles the mind, as the tree has stepped out of time in one respect, but is still very much a part of it in another. Loggers cut down trees as old as Jesus, or the founding of America, or the Civil War, not knowing or caring that the "time" embodied in them is now dispersed and never to be recovered. Old-growth forests repel and scare people, so they "clean them up" and cut them down, again literally erasing history as well as part of the future. Powers wants us to experience time as ancient peoples saw it—at the speed of wood.
Death
Death is an inevitable part of the experience of every living creature, and Powers weaves it into his novel to demonstrate this fact. His main characters know people close to them who have died, and by the end, many of them are nearing death or have died. As for the trees, they too are dying, but their death at human hands is not like their natural death. In fact, humans barely understand death as it exists in the arboreal world. In The Overstory, Powers wants us to see death as an opportunity for rebirth and renewal—to not be afraid of it. He tells an interviewer, "Contemporary consumer/humanist culture is convinced that if we just hold out long enough and surround ourselves with the best state-of-the-art technologies and biomedical interventions (from apple cider vinegar all the way up to the uploading of souls), then we will never have to die. Consequently, the prospect of death has never been more debilitating. We are all rushing around in a state of hysterical denial, because our central conviction—that meaning is personally generated—is utterly incompatible with the central truth of existence: Everything dies." The best way to think about individual death, he suggests, is not an annihilation; rather, it is a return to a way of existing that ultimately lives with the world, not against it.
