Usually when upper trees are shorter than 15 meters in Real Life, it means that the soil is either too dry (sandy or limestone soil), too wet (bog) or just low on nutrients. It's also common that the forest in that case is sparse. In some media, it seems that entire forests can be very low-lying (and still can be impassably thick). Sometimes even in biomes you wouldn't expect the same to happen in real life. The reason may be due to technical limitations. Sometimes this is a result of things not being strictly to scale. Sometimes it just seems to be Artistic License.