About the length making the arrows less easily deflected by vegetation, I would like to make a correction... It's not quite as simple as that.
The longer arrows would have more mass on impact and out of the powerful longbows would penetrate very well... but on the other hand they will have more paradox as they bend and straighten out initially once leaving the bow... Meaning, they need a large clear area initially to not be deflected. Having said that, most of these bows are very narrow and stiff, so the amount of bend the arrow must make to get around the bow is probably not that much more than a conventional arrow.
I think a parallel can be drawn between Amazonian archery gear and papua New Guinean. PNG bows and arrows follow similar designs... however PNG arrows are typically heavy and lack fetching. I feel the advantages of long arrows combined with the need for forward weight due to the last of fetching has led to the development of what we recognize as PNG style arrows.
...just more thoughts
