Just like people and animals, plants throughout the world all interact with each other in various ways. Some produce specific nutrients needed by others, some provide support, some provide protection. Many compete with each other and some even prey upon other plants. While many of the interaction are not as obvious, they are all there and sometimes extremely complex.
First, there are general relationships between plants in the rainforest. The canopy is comprised of millions of species, some are minute and some are arboreal vines that run for hundreds of yards and produce large leaves. These plants are getting more sunlight and providing shelter for all the plants below them. The canopy plants also break the force of the wind and rain, preventing erosion, and trap in the moisture, stopping the soil from drying out. Their leaves and such fall to the ground were numerous things, plants, insects, and bacteria, decompose them, adding nutrients to the surface floor of the rainforest. This is important because most of the trees have roots that are very shallow, and the trees are needed by the canopy to raise it off the floor so nothing eats it!
Then there are more specific interactions. One tree in the Caribbean Rainforest has craggy bark which provides surfaces for numerous small plants to grow on. Those plants keep wood -boring insects away as well as help retain moisture. Those smaller plants may also provide nutrients that the big tree has no other way of getting in sufficient quantity. Then the branches spread out from that large tree, resting on the smaller trees underneath while it shades them.
There has also been some indications that similar plants can "communicate" with each other. If one plant is attacked by a strange disease or creature, it produces a chemical that causes the nearby plant to actually change and become more resistant. While it has been shown to occur in palms and aspens, the causes are questionable.
Then there is plant competition and plant parasites, the less favourable side of plant interactions. The competition occurs when to plant species try to fill the same niche and using the same resources, and there isn't enough for both. It can be two vines or even trees. Eventually one overshadows or out grows the other, or an equilibrium is established, in a few cases they both die! With parasite, one plant can actually kill or destroy the other, Stranglers Figs is the best known example!
Plants, like all living things do interact with their environment. The interactions are complex, often overlooked, but essential to the rainforest ecosystem.