(Click to enlarge)
Looking at this photo you can practically hear the little bur oak seedling sighing, "Ahhhh." I took this photo yesterday. Nice sunny day, but the wind was howling with gusts up to 35mph and more.
An un-tubed bur oak seedling - or any seedling - would have been under severe moisture stress as the wind continually stripped away the moist envelope of air surrounding the leaves. Not this guy. All he "felt" was the gentle swaying of the tree tube thanks to its PVC tree tube stake. No wind. No stress. Just perfect conditions for growth.
Why do trees grow so much faster in tree tubes? Two reasons (well it's a lot more than two reasons, but we'll concentrate on two for today). One a windy day like yesterday the leaves on un-tubed trees close their stomata - the pores through which they exchange gases and transpire moisture. This is a good strategy for conserving limited moisture resources, but it's a bad strategy for photosynthesis, which slows dramatically. In other words, it's a survival strategy, not a growth strategy. By contrast the leaves of a seedling in a tree tube keep their stomata open and photosynthesis continues full bore.
Since the leaves of the seedling in a tree tube are not exposed to stressful, windy conditions they can optimize their morphology and structure for growth: High surface to weight ratio to optimize light absorption and gas exchange, bright glossy surface. The leaves of a seedling outside a tree tube are smaller, thicker, darker and duller. They are survival leaves, not growth leaves.
To put it another way (and to use a Memorial Weekend analogy), the leaves in a tree tube are an Indy car, while the leaves of an un-tubed seedling are a 1974 Pinto.