I decided to make an informational post. Sorry for the long text, but I had trouble finding info on this topic before we used it, so this can be useful for you too. + some nice photos!
We have 9 ha (22 acres) of land in Central-North Portugal and (as the enthusiastic 36-old forester I am) decided to build a robust Agroforestry System and rehabilitate the land. For now, we have some tree plantes for the main long-rotationed-wood-layer.
We planted 5700 trees of over 25 different species. To some of them (1500) we added tree protectors of 1.20m (4 feet) tall. This was to allow for the sheep of the neighbor pasture to graze there and not eat the plants (also other wildlife - hare), and to test the performance of the trees inside and outside.
It is going well for the growth of some of the woody-trees we protected. The micro-climate really seemed to make the difference to the cherry trees, they were planted in November are are now about 90 cm (3 feet) taller - almost poking out of the tube.
Poplar is also enjoing this added humidity quiet well. You can see in Photo 1.
Some trees managed to escape from the tube's side ventilation holes (see photo 2). I then had to cut it open towards the bottom and put it back inside. This didn't happen much, maybe three times.
I think even with branches coming out of the side-holes, you could ignore it, since you'd prune them anyways.
The oaks (Northern Red Oak and English oak) seems to be in the middle. The leaves look healthy, but it looks at times like they are struggling to fit in there. The branching and leafing patters of these oaks might not be the best for these tubes. It didn't feel like it is promoting height growth, more like it is holding them down.
The tubes were also of good support for our Sycamore maple. They were already tall when planted, and now where they are planted we have a lot of wind.
I knew about it before, and this species is very tolerant to wind. But as you can see in photo 3, it started branching from the bottom to compensate for the wind.
The tubes worked for now (photo 4). Let's hold they inhibit these side-branching and promote a straighter, leaner trunk.
When planting, also be attentive to weed seeds and roots in the tube area. I had so many tubes with grass poking out of them, and sometimes the tree was inside almost dormant, with no light.
Most species managed that well, specially if they were already a bit big when planted (like the poplar, the birch and the Sycamore maple) of if they don't mind being dormant in the understory (Oaks).
Regardless, they all benefited from not having competition inside the tube. So if you can clean the area a bit before, this can be avoided, as the tube itself will prevent weds from reaching it after it is installed. See photo 5.
Photo 5 (before):
Photo 5 (after)
Another tree that liked the tubes was Ash (Fraxinus excelsior). Photo 6. It seems to have grown quite quickly and it is not even very firm to stay on its own. Soon it will gain more diameter and firm up.
Some down sides.
The tube is supposed to last about 8 years. If you expect longer, that seems not to be possible.
It is plastic. We will have to collect all of them once no longer used and recycle it. It is a category 5: PP (Polypropylene) plastic, so check if you can dispose consciuously when it's no longer useful.
:( We have found 3 birds dead inside the tubes. All of them European serin. They basically entered the tube to munch of the vast amount of bugs that enjoy the higher humidity and couldn't get out. We checked and opened MANY tubes after finding the first one, but we didn't manage check all.
The wooden sticks are bought sepparatedly. Make sure it is of a type of wood that will outlast the tubes. Otherwide they will rot before you made the best of the tube's life.
[IMPORTANT] Sometimes, if the tree you're plant is too tall, the zip tie you use to fix the tube to the stick can hook it in. It happed to some of our poplar trees. We had people helping plant the tubes and they were not attentive to that. We have time to fix it, but it is something easy to overlook and forget until it is too late.
Specially because you won't see it easily.
Overall, I'm very satisfyed with the tubes. There are many brands out there and I imagine they are not so different so pick whatever fits your budget.
I can keep you posted on how the trees like it over summertime. It is supposed to ventilate due to the holes in the bottom and create a cooler environment. We'll see.