'Topography' implies variation in altitude. A flat rain forest, such as most of the Amazon basin has very little variation in altitude, and therefore doesn't constitute a topographic barrier. By contrast, the topography of the Andean mountain chain, further west, has a huge altitude variation, and hence constitutes one of the classic topographic barriers in respect of air flow, rainfall, salt balance, etc.
So any mountain range or plateau may be a topographic barrier. Forests may or may not be topographic barriers, according to whether or not they have significant topographic variation. Hence the forests of Papua New Guinea constitute a t.b., but most lowland forests do not.