Plant biologist Lori Kozlowski explains in this interview how she helped create the plants and the plant science we see in Avatar. She went as far as creating scientific names for all the species involved. She worked with creator James Cameron and others to develop a believable plant communication system (based in a new field of plant science on Earth, called "signal transduction").
She explains the environment this way: "...the atmosphere is thicker than on Earth, with higher concentrations of carbon dioxide, as well as xenon and hydrogen sulfide. Gravity is weaker. And there is a strong magnetic field. Given the role of the environment in plant evolution, one would therefore expect to see gigantism, less of a gravity response (which makes stems grow up and roots grow down), and possibly a response to magnetic fields, which I named "magnetotropism.""