Ok, a lot of words have been written marveling at the fast growth and high survival rate of seedling trees in tree tubes.
Not nearly as many - or nearly enough - words have been written about WHY trees grow better and survive at a much higher rate inside of tree tubes. Maybe that's because much of what we thought we knew about tree tubes 20 years ago turned out to be wrong. Maybe because there are several mechanisms at play, and it's always difficult to explain such a multi-faceted phenomenon.
So let's keep it simple. What kills tree seedlings, or reduces their growth rate? Moisture stress, animal damage, and weed competition.
1. First and foremost tubes tubes accelerate growth and increase survival by reducing moisture stress. Every minute of every day during the growing season wind sweeps moisture away from leaf surfaces. Leaves lose moisture, and then close their stomata - their pores - in order to conserve the moisture that remains. Of course when they do that they also stop growing.
Tree tubes shield leaves from wind, conserving moisture (which increases survival rate even under dry conditions), keeping stomata open, so the trees grow more actively more of the time.
Yes there are other, more complicated mechanisms at work to enhance growth, but in the end it all comes down to water conservation and keeping the stomata open so the tree's growth engine keeps chugging along.
2. It's tough to grow if you're being eaten by a deer or rabbit, or gnawed by a mouse or vole. The simple act of guarding seedlings from animal damage of course increases survival rates, but what's often overlooked is that it also accelerates growth.
3. Trees and grass are mortal enemies. The prairie and forest constantly battle each other for supremacy. Grass's biggest asset is that it can absorb and use the site's resources more quickly. In other words, it steals moisture and nutrients from the trees. Trees' ally in the battle against grass used to be fire. Fire would kill the grass, at least temporarily, giving the trees a chance to gain a foothold and get established.
In most cases these days the use of fire is not an option. That means that we, the tree planters, need to simulate the fire by providing complete weed control. Not mowing; mowing helps reduce rodent lairs but does nothing to prevent the below-ground battle for resources - and in fact invigorates grass to make it a fiercer competitor. It means chemical or mechanical weed control, or the use of weed barrier fabric.
How do tree tubes help with weed control? First - and this is a benefit that is under appreciated until you try tree tubes for the first time - you can actually find your trees amidst the sea of weeds and grass. Trust me, this is no small thing. Second, the tree tubes shield your seedlings from chemical weed control or mechanical cultivators.
You can see the problem - I set out to keep in simple and end up writing for a long time. But that's the "down side" of a product with so many benefits!
Think of it this way. If you grew a seedling in a greenhouse environment, safe from animals and with complete weed control it would grow as fast as if it was in a tree tube. But if you did that, it would be in a tree tube - just a very big, very expensive tree tube!