Plants can turn sunlight into food, and so provide the basis for animal life on earth. The massive increase in carbon dioxide in the atmosphere helped plants proliferate in the centuries following the collapse of civilization. That increase in carbon dioxide also created changes in climate that unfolded far more quickly than plants could adapt to on their own. Fortunately, all along the way, some humans consciously moved seeds and saplings poleward faster than those plants could have moved on their own, and other animals likewise carried plants on their poleward migrations. Of course, the mass extinction that civilization caused claimed many, many species of plants, but many other survived, and many others still even flourished. The Fifth World has a diminished diversity of plant species, but their biomass has increased significantly. From pole to pole, tropical rainforests cover most of the land surface of the Fifth World.
Carnivorous plants like the Venus fly trap evolved in poor soil conditions. With soils worldwide terribly diminished, and tropical forests spreading across the earth (which tend to pull more nutrients out of the ground and into the trees), carnivorous plants have proliferated. With insects growing larger, these carnivorous plants have had to grow larger as well. And with more mammals getting smaller, many have found small mammals an easier catch than some insects. While a carnivorous plant large enough to eat a person remains a tall tale, as they get bigger and bigger more and more of them have begun to make a regular diet of mammals, making it a much more believable story.
Plant communities follow a process called succession. When a catastrophe like a fire or a flood destroys an area, pioneer species like grass move in first. These species spring up quickly, provide some cover for the soil, and help to slow erosion as they lay down roots. Next, brushes and small trees begin to grow. They take longer to grow but provide greater stability and begin to cast some shade on the ground. This helps trees to take root and grow, forming first a woodland and then a young forest when their branches grow close enough together to form a canopy. This provides the conditions for other species of tree to begin growing, which often can only begin growing in the shade of an existing forest, but will eventually grow up taller than the existing trees, creating a much taller, thicker canopy. These trees can live for many centuries. When they reach maturity, we refer to the forest as a mature or old-growth forest.
The specifics of this process vary greatly from one forest to the next, depending on local conditions and the species involved, but this forms the broad outline of succession. Pioneer species begin to take hold very quickly. Essentially, with just one year of neglect, one can expect to find them growing in almost any area that has sunlight and water. In the most urban areas, a few generations of pioneer plants may need to live and die to build up soil for later species to take root, but in as little as a decade nearly any building might become home to shrubs, bushes, or small trees. Four hundred years provides just enough time for old-growth forests to emerge across the Fifth World.
Civilization grew out of the ability to cultivate cereal grains specifically, using an array of methods that typically worked by resetting the process of domestication annually. This created the conditions for their favored cereal grains, as pioneer species, to grow. This emerged from the specific climate conditions of the Holocene interglacial, and when that climate changed crop yields collapsed. Throughout the Rusting Ages many survivors struggled to continue practicing agriculture but the soil proved so diminished, and the climate so hostile to their crops, that yields kept falling. After two centuries, through a combination of starvation and abandonment, it had disappeared.
Another cultivation method has survived it, sometimes called horticulture, permaculture, or simply gardening. These methods typically work with succession and promote it, creating ecosystems that also supply human beings with food. The “food forest” might provide the best example of this, a forest that humans help create which includes a great variety of plants that provide food, medicine, and other material for human beings, while also providing those things, as well as habitat, for other animals. The families of the Fifth World fall on a spectrum of gardening and hunting and gathering; no one practices one to the exclusion of the other, though many lean heavily to one side.
In the old world, humans had effectively terraformed the planet using methods such as these, creating the Amazon rainforest and the Great Plains, among others. In the Fifth World, humans have created continent-spanning ecosystems like these once again.
A network of fungi connect plants at the roots, ultimately connecting whole continents of plants together into a single network. Many species use this network to communicate as well as to transfer sugars and other chemicals. Many plants know when a member of their community suffers damage and routes sugar to them to help keep them alive. Stumps of trees may live for decades or even centuries without any leaves to photosynthesize, supported utterly by the plants around them. Through these root connections, plants can recognize their kin and even make friends. Of course, a social life does not mean that all plants treat their fellows with benevolence and respect. Sometimes the competition for light becomes quite vicious, and plants will often vie with one another to reach the sun — and putting their competitors to wither in their shadow.
Plants also communicate through chemical signals released into the air. Some plants learn to eavesdrop on these conversations. For example, some species will raise their own defenses when they detect the “alarm” signal from a nearby plant. Of course, releasing chemicals into the air means that other organisms besides plants can react to them. Some plants appear to know not only that an insect has begun eating their leaves, but can identify that insect. They will then release a specific chemical that will attract the predator of that specific insect as a defense mechanism.
The animal sense of smell listens to these signals, too. For human beings it can present a challenge, but many people in the Fifth World claim to have mastered the subtle art of listening to plants. If one sits in an area, taking the time to take in the scents that the plants produce, and can listen attentively to the emotions that those scents elicit, one can hear plants. This takes great care, patience, practice, and discipline, since one can so easily “shout” over the plants' more subtle messages with one's own thoughts and feelings.
Humans also try to hear what plants have to say by consuming entheogens. Though the messages here may seem “louder” they take no less care, patience, practice, and discipline to effectively sort out.