Hi Wolfie,
I have had the same issues with considering what to use as realistic vegetation in 1:72 scale. Pegasus hobbies make some reasonable good palm trees and banana trees in 1:72, which might be useful, depending upon your subject.
Obviously, the type of trees you'll need depends upon the type of jungle you're modelling. I am a geologist and have spent a lot of time in various "jungle" terrains in northern Australia, Papua New Guinea and SE Asia and I can assure you that the jungle varies considerably from location to location.
The jungle in PNG generally has a very thick and high canopy, which means that the undergrowth is usually confined to stunted saplings, small fens, lichen, moss and fungi. Unless the canopy has been recently broken (eg. by a falling tree, or clearing for a village garden), the undergrowth is usually very sparse, simply because insufficient light penetrates to allow growth. Conversely, where the canopy is broken, undergrowth is very dense, usually comprising grass, vines, young trees, palms, ferns, etc. The jungle floor is usually dark, damp and has a thick layer of humus, which is mostly decaying leaves and fallen timber. The floor is usually also quite uneven.
The northern Australian jungle is generally very similar to PNG but is usually not quite so dense, probably as a result of the lower rainfall. Because the canopy is not quite as thick, nor as high, more light reaches the jungle floor and the undergrowth is a little thicker, mostly with ferns and palms.
Here in Laos, the jungle has been subject to generations of hunting, clearing, slash and burn and logging, so it's a whole different story. Also, the Lao/Vietnam border region was heavily bombed and defoliated by the US during the Indochina war, so that has left a legacy also. I have not been to Vietnam, but the jungles in Thailand are much the same as Laos, so I would guess that most of SE Asia is similar to what I have seen. Although there are isolated pockets where the jungle is relatively untouched (sometimes reminiscent of PNG), most of the jungle here is quite sparse, consisting mostly of dense bamboo, vines, ferns and leafy "weeds" and is in a constant state of regrowth following some form of human disturbance. Palms are present but their distribution is sparse. Banana trees are very common through the Lao jungle also. The floor is usually quite dry in the scrubby regrowth jungle, but damp in the denser areas.
As for the Amazon and Congo jungles, I can't help you there.
Hope these comments help...